CRACKS AND RADIATING STREAKS

The lunar features next in order of conspicuity are the mountain ranges,
peaks, and hill-chains, a class of eminences more in common with
terrestrial formations than the craters and circular structures that
have engaged our notice in the preceding chapters.

In turning our attention to these features, we are at the outset struck
with the paucity on the lunar surface of extensive mountain systems as
compared with its richness in respect of crateral formations; and a
field of speculation is opened by the recognition of the remarkable
contrast which the moon thus presents to the earth, where mountain
ranges are the rule, and craters like the lunar ones are decidedly
exceptional. Another conspicuous but inexplicable fact is that the most
important ranges upon the moon occur in the northern half of the visible
hemisphere, where the craters are fewest and the comparatively
featureless districts termed “seas” are found. The finest range is that
named after our Apennines and which is included in our illustrative
Plate, No. IX. It extends for about 450 miles and has been estimated to
contain upwards of 3000 peaks, one of which—Mount Huyghens—attains the
altitude of 18,000 feet. The Caucasus is another lunar range which
appears like a diverted northward extension of the Apennines, and,
although a far less imposing group than the last named, contains many
lofty peaks, one of which approaches the altitude assigned to Mount
Huyghens while several others range between 11,000 and 14,000 feet high.
Another considerable range is the Alps, situated between the Caucasus
and the crater Plato, and reproduced on Plate XIV. It contains some 700
peaked mountains and is remarkable for the immense valley, 80 miles long
and about five broad, that cuts it with seemingly artificial
straightness; and that, were it not for the flatness of its bottom,
might set one speculating upon the probability of some extraneous body
having rushed by the moon at an enormous velocity, gouging the surface
tangentially at this point and cutting a channel through the impeding
mass of mountains. There are other mountain ranges of less magnitude
than the foregoing; but those we have specified will suffice to
illustrate our suggestions concerning this class of features.

[Illustration: PLATE XV.
MERCATOR & CAMPANUS.]

We remark, too, that there is a prevailing tendency of the ranges just
mentioned to present their loftiest constituents in abrupt terminal
lines, facing nearly the same direction, the reverse of that towards
which they are carried by the moon’s rotation; and as they recede from
the high terminal line, the mountains gradually fall off in height, so
that in bulk the ranges present the “crag and tail” contour which
individual hills upon the earth so frequently exhibit.

Isolated peaks are found in small numbers upon the moon; there are a few
striking examples of them nevertheless, and these are chiefly situated
in the mountainous region just alluded to. Several are seen to the east
(right hand) of the Alpine range depicted on Plate XIV. The best known
of these is Pico, which rises abruptly from a generally smooth plain to
a height of 7000 feet. It may be recognized as the lower of the two long
shadowing spots located almost centrally above the crater Plato in the
illustration just mentioned. Above it, at an actual distance of 40
miles, there is another peak (unnamed) about 4000 feet high; and away to
the west, beyond the small crater joined by a hill-ridge to Plato, is a
third pyramidal mountain nearly as high as Pico.

It seems natural to regard the great mountain chains as agglomerations
of those peaks of which we have isolated examples in Pico and its
compeers, and thus to consider that the formation of a mountain chain
has been a multiplication of the process that formed the single
pyramid-shaped eminences. At first thought it might appear that the
great mountain ranges were produced by bodily upthrustings of the crust
of the moon by some subsurface convulsions. But such an explanation
could hardly hold in relation to the isolated peaks, for it is
difficult, if not impossible, to conceive that these abrupt mountains,
almost resembling a sugarloaf in steepness, could have been protruded en
masse through a smooth region of the crust. On the contrary it is quite
consistent with probability to suppose that they were built up by a slow
process somewhat analogous to that to which we have ascribed the piling
of the central cones of the great craters. We believe they may be
regarded as true mountains of exudation, produced by the comparatively
gentle oozing of lava from a small orifice and its solidification around
it; the vent however remaining open and the summit or discharging
orifice continually rising with the growth of the mountain, as indicated
in the annexed cut, Fig. 36. This process is well exemplified in the
case of a water fountain playing during a severe frost; the water as it
falls around the lips of the orifice freezes into a hillock of ice,
through the centre of which, however, a vent for the fluid is preserved.
As the water trickles over the mound it is piled higher and higher by
accumulating layers of ice, till at length a massive cone is formed
whose height will be determined by the force or “head” of the water.
Substitute lava for water and we have at once a formative process which
may very fairly be considered as that which has given rise to the
isolated mountains of the moon.

[Illustration: Fig. 36.]

[Illustration: Fig. 37.]

[Illustration: Fig. 38.]

[Illustration: Fig. 39.]

There are upon the earth mountainous forms resembling the isolated peaks
of the moon, and which have been explained by a similar theory to the
above. We reproduce a figure of one observed by Dana at Hawaii (Fig.
37), and a sketch of another observed on the summit of the Volcano of
Bourbon, (Fig. 38); we also reproduce (Fig. 39) an ideal section of the
latter, given by Mr. Scrope, and showing the successive layers of lava
which would be disposed by just such an action as that manifested in the
case of the freezing fountain; and we quote that author’s words in
reference to this explanation of the formation of Etna and other
volcanic mountains. “On examining,” says Mr. Scrope,[11] “the structure
of the mountain (Etna) we find its entire mass, so far as it is exposed
to view by denudation or other causes (and one enormous cavity, the Val
de Bove penetrates deeply into its very heart), to be composed of beds
of lava-rock alternating more or less irregularly with layers of scoriæ,
lapillo and ashes, almost precisely identical in mineral character, as
well as in general disposition, with those erupted by the volcano at
known dates within the historical period. Hence we are fully justified
in believing the whole mountain to have been built up in the course of
ages in a similar manner by repeated intermittent eruptions. And the
argument applies by the rules of analogy to all other volcanic
mountains, though the history of their recent eruptions may not be so
well recorded, provided that their structure corresponds with, and can
be fairly explained by this mode of production. It is also further
applicable, under the same reservation, to all mountains composed
entirely, or for the most part, of volcanic rocks, even though they may
not have been in eruption within our time.”

To these illustrations furnished from Scrope’s work we add another,
copied from a photograph by Professor Piazzi Smyth, of a “blowing cone”
at the base of Teneriffe (Fig. 40), which is but one of many that are to
be found on that mountain and which has been formed by a process similar
to that we have been considering, but acting upon a comparatively small
scale. Professor Smyth describes this cone as about 70 feet high and of
parabolic figure, composed of hard lava and with an upper aperture still
yawning, “whence the burning breath of fires beneath once issued in fury
and with destruction.”

[Illustration: PLATE XVI
TYCHO,
AND ITS SURROUNDINGS.]

Reverting now to the moon, we remark that, if the foregoing explanation
of the isolated lunar peaks be tenable, it should hold equally for the
groups of them which we see in the lunar Apennines, Alps, Caucasus and
other ranges of like character. There occur in some places intermediate
groups which link the one to the other. Just above the crater
Archimedes, on Plate IX., for instance, we see several single peaks and
small clumps of them leading by successive multiple-peak examples to
what may be called chains of mountains like many that are included in
the contiguous Apennine system. And, in view of this connexion between
the single peaks and the mountain ranges formed of aggregations of such
peaks, it seems to us reasonable to conclude that the latter were formed
by the comparatively slow escape of lava through multitudinous openings
in a weak part of the moon’s crust, rather than to suppose that the
crust itself has been bodily upheaved and retained in its disturbed
position. The high peaks that many mountains in such a chain exhibit
accord better with the former than the latter explanation; for it is
difficult to imagine how such lofty eminences could be erected by an
upheaval, and we must remember that the moon has none of the denuding
elements which are at work upon the earth, weather-wearing its mountain
forms into sharpness and steepness.[12]

[Illustration: Fig. 40.
SMALL VOLCANIC MOUNTAIN AT THE END OF A STREET AT TENERIFFE.]

[Illustration: Fig. 41.]

And we have ground for believing the mountain-forming process on the
moon to have been a comparatively gentle one, in the fact that the
mountain systems appear in regions otherwise little disturbed, and where
craters, which have all the appearances of violent origin, are few and
far between. Evidently the mountain and crater-forming processes,
although both due to extrusive action, were in some measure different,
and it is reasonable to suppose that the difference was in degree of
intensity; so that while a violent ejection of volcanic material would
give rise to a crater, a more gradual discharge would pile up a
mountain. In this view craters are evidences of _eruptive_, and
mountains of comparatively gentle _exudative_ action.

We can hardly speculate with any degree of safety upon the cause of this
varying intensity of volcanic discharge. We may ascribe it to variation
of _depth_ of the initial disturbing force, or to suddenness of its
action; or it may be that different degrees of fluidity of the lava have
had modifying effects; or on the other hand different qualities of the
crust-material; or yet again differences of period—the quieter
extrusions having occurred at a time when the volcanic forces were dying
down. There is an alliance between lunar craters and mountains that goes
far to show that there has been no radical difference in their origins.
For instance, as we have previously pointed out, craters in some cases
run in linear groups, as if in those cases they had been formed along a
line of disruption or of least resistance of the crust; and the mountain
chains have a corresponding linear arrangement. Then we see craters and
mountain chains disposed in what seem obviously the same arcs of
disturbance. Thus Copernicus (No. 147), Erastothenes (No. 168), and the
Apennines appear to belong to one continuous line of eruption; and it
requires no great stretch of imagination to suppose that the Caucasus,
Eudoxus (No. 208) and Aristotle (No. 209) form a continuation of the
same line. Then around the Mare Serenetatis we see mountainous ridges
and craters alternating one with the other as though the exuding action
there, normally sufficient to produce the ridges, had at some points
become forcible enough to produce a crater. Again, upon the very
mountain ranges themselves, as for instance among the Apennines, we find
small craters occurring. We see, too, that the great craters are in many
cases surrounded by radiating systems of ridges which almost assume
mountainous proportions, and which are doubtless exuded matter from
“starred” cracks, the centres of which are occupied by the craters. The
same kind of ridges here and there occur apart from craters (see for
instance Plate XVIII., below Aristarchus and Herodotus) and sometimes
they occur in the neighbourhood of extensive cracks, to which they also
seem allied. We must indeed regard a linear crack as the origin either
of a ridge (if the exudation is slight) or of a mountain chain (if the
exudation is more copious) or a string of craters (if the extrusion
rises to eruptive violence). But the subject of cracks is important
enough to be treated in a separate chapter.

We alluded in Chap. III. to the phenomena of wrinkling or puckering as
productive of certain mountainous formations; and we pointed out the
striking similarity in character of configuration between a shrivelled
skin and a terrestrial mountain region. We do not perceive upon the moon
such a decided coincidence of appearances extending over any
considerable portion of her surface; but there are numerous limited
areas where we behold mountainous ridges which partake strongly of the
wrinkle character; and in some cases it is difficult to decide whether
the puckering agency or the exudative agency just discussed has produced
the ridges. The district bordering upon Aristarchus and Herodotus, above
referred to, is of this doubtful character; and a similar district is
that contiguous to Triesnecker (Plate XI.) There are, however, abundant
examples of less prominent lines of elevation, which may, with more
probability, be ascribed to a veritable wrinkling or puckering action;
they are found over nearly the whole lunar surface, some of them
standing out in considerable relief, and some merely showing gentle
lines of elevation, or giving the surface an undulating appearance. A
close examination of our picture-map (Plate IV.) will reveal very
numerous examples, especially in the south-east (right-hand-upper)
quadrant. Some of these lines of tumescence are so slightly prominent
that we may suppose them to have been caused by the action indicated by
Fig. 6 (p. 28), while others, from their greater boldness, appear to
indicate a formative action analogous to that represented by Fig. 9

We have hitherto confined our attention to those reactions of the moon’s
molten interior upon its exterior which have been accompanied by
considerable extrusions of sub-surface material in its molten or
semi-solid condition. We now pass to the consideration of some phenomena
resulting in part from that reaction and in part from other effects of
cooling, which have been accompanied by comparatively little ejection or
upflow of molten matter, and in some cases by none at all. Of such the
most conspicuous examples are those bright streaks that are seen, under
certain conditions of illumination, to radiate in various directions
from single craters, and some of the individual radial branches of which
extend from four to seven hundred miles in a great arc on the moon’s
surface.

There are several prominent examples of these bright streak systems upon
the visible hemisphere of the moon; the focal craters of the most
conspicuous are Tycho, Copernicus, Kepler, Aristarchus, Menelaus, and
Proclus. Generally these focal craters have ramparts and interiors
distinguished by the same peculiar bright or highly reflective material
which shows itself with such remarkable brilliance, especially at full
moon: under other conditions of illumination they are not so strikingly
visible. At or nearly full moon the streaks are seen to traverse over
plains, mountains, craters, and all asperities; holding their way
totally disregardful of every object that happens to lay in their
course.

The most remarkable bright streak system is that diverging from the
great crater Tycho. The streaks that can be easily individualized in
this group number more than one hundred, while the courses of some of
them may be traced through upwards of six hundred miles from their
centre of divergence. Those around Copernicus, although less remarkable
in regard to their extent than those diverging from Tycho, are
nevertheless in many respects well deserving of careful examination:
they are so numerous as utterly to defy attempts to count them, while
their intricate reticulation renders any endeavour to delineate their
arrangement equally hopeless.

The fact that these bright streaks are invariably found diverging from a
crater, impressively indicates a close relationship or community of
origin between the two phenomena: they are obviously the result of one
and the same causative action. It is no less clear that the actuating
cause or prime agency must have been very deep-seated and of enormous
disruptive power to have operated over such vast areas as those through
which many of the streaks extend. With a view to illustrate
experimentally what we conceive to have been the nature of this
actuating cause, we have taken a glass globe and, having filled it with
water and hermetically sealed it, have plunged it into a warm bath: the
enclosed water, expanding at a greater rate than the glass, exerts a
disruptive force on the interior surface of the latter, the consequence
being that at the point of least resistance, the globe is rent by a vast
number of cracks diverging in every direction from the focus of
disruption. The result is such a strikingly similar counterpart of the
diverging bright streak systems which we see proceeding from Tycho and
the other lunar craters before referred to, that it is impossible to
resist the conclusion that the disruptive action which originated them
operated in the same manner as in the case of our experimental
illustration; the disruptive force in the case of the moon being that to
which we have frequently referred as due to the expansion which precedes
the solidification of molten substances of volcanic character.

Our illustration, Plate XIX., is a photograph from one of many glass
globes which we have cracked in the manner described: a careful
comparison between the arrangement of the divergent cracks represented
in the photograph and those seen spreading from Tycho and other lunar
craters will, we trust, justify us in what we have stated as to the
similarity of the causes which have produced such identical results.

The accompanying figures will further illustrate our views upon the
causative origin of the bright streaks. The primary action rent the
solid crust of the moon and produced a system of radiating fissures
(Fig. 42): these immediately afforded egress for the molten matter
beneath to make its appearance on the surface simultaneously along the
entire course of every crack, and irrespective of all surface
inequalities or irregularities whatever (Fig. 43). We conceive that the
upflowing matter spread in both directions sideways and in this manner
produced streaks of very much greater width than the cracks or fissures
up through which it made its way to the surface.

[Illustration: Fig. 42.
ILLUSTRATIVE OF THE RADIATING CRACKS WHICH PRECEDE THE FORMATION OF
THE BRIGHT STREAKS.]

In further elucidation of this part of our subject we may refer to a
familiar but as we conceive cogent illustration of an analogous action
in the behaviour of water beneath the ice of a frozen pond, which, on
being fractured by some concentrated pressure, or by a blow, is well
known to “star” into radiating or diverging cracks, up through which the
water immediately issues, making its appearance on the surface of the
ice simultaneously along the entire course of every crack, and on
reaching the surface, spreading on both sides to a width much exceeding
that of the crack itself.

[Illustration: Fig. 43.
ILLUSTRATIVE OF THE RADIATING BRIGHT STREAKS.]

If this familiar illustration be duly considered, we doubt not it will
be found to throw considerable light on the nature of those actions
which have resulted in the bright streaks on the moon’s surface. Some
have attempted to explain the cause of these bright streaks by assigning
them to streams of lava, issuing from the crater at the centre of their
divergence and flowing over the surface, but we consider such an
explanation totally untenable, as any idea of lava, be it ever so fluid
at its first issue from its source, flowing in streams of nearly equal
width, through courses several hundred miles long, up hills, over
mountains, and across plains, appears to us beyond all rational
probability.

[Illustration: PLATE XVIII.
ARISTARCHUS & HERODOTUS.]

It may be objected to our explanation of the formation of these bright
streaks, that so far as our means of observation avail us, we fail to
detect any shadows from them or from such marginal edges as might be
expected to result from a sideway-spreading outflow of lava from the
cracks which afforded it exit in the manner described. Were the edges of
these streaks terminated by cliff-like or craggy margins of such height
as 30 or 40 feet, we might just be able at low angles of illumination
and under the most favourable circumstances of vision, to detect some
slight appearance of shadows; but so far as we are aware, no such
shadows have been observed. We are led to suppose that the impossibility
of detecting them is due not to their absence but to the height of the
margins being so moderate as not to cast any cognizable shadow, inasmuch
as an abrupt craggy margin of 10 or 15 feet high would, under even the
most favourable circumstances, fail to render such visible to us.
Reference to our ideal section of one of these bright streaks (Fig. 45),
will show how thin their edges may be in relation to their spreading
width.

The absence of cognizable shadows from the bright streaks has led some
observers to conclude that they have no elevation above the surface over
which they traverse, and it has therefore been suggested that their
existence is due to possible vapours which may have issued through the
cracks, and condensed in some sublimated or pulverulent form along their
courses, the condensed vapours in question forming a surface of high
reflective properties. That metallic or mineral substances of some kinds
do deposit on condensation very white powders, or sublimates, we are
quite ready to admit, and such explanation of the high luminosity of the
bright streaks, and of the craters situated at the foci or centres of
their divergence is by no means improbable, so far as concerns their
mere brightness. But as we invariably find a crater occupying the centre
of divergence, and such craters are possessed of all the characteristic
features and details which establish their true volcanic nature as the
results of energetic extrusions of lava and scoria, we cannot resist the
conclusion that the material of the crater, and that of the bright
streaks diverging from it, are not only of a common origin, but are so
far identical that the only difference in the structure of the one as
compared with the other is due to the more copious egress of the
extruded or erupted matter in the case of the crater, while the
restricted outflow or ejection of the matter up through the cracks would
cause its dispersion to be so comparatively gentle as to flood the sides
of the cracks and spread in a thin sheet more or less sideways
simultaneously along their courses. There are indeed evidences in the
wider of the bright streaks of their being the result of the outflow of
lava through _systems of cracks_ running parallel to each other, the
confluence of the lava issuing from which would naturally yield the
appearance of one streak of great width. Some of those diverging from
Tycho are of this class; many other examples might be cited, among which
we may name the wide streaks proceeding from the crater Menelaus and
also those from Proclus. Some of these occupy widths upwards of 25
miles—amply sufficient to admit of many concurrent cracks with confluent
lava outflows.

We are disposed to consider as related to the fore-mentioned radiating
streaks, the numerous, we may say the multitudinous, long and narrow
chasms that have been sometimes called “canals” or “rills,” but which
are so obviously _cracks_ or chasms, that it is desirable that this name
should be applied to them rather than one which may mislead by implying
an aqueous theory of formation. These cracks, singly and in groups, are
found in great numbers in many parts of the moon’s surface. As a few of
the more conspicuous examples which our plates exhibit we may refer to
the remarkable group west of Treisnecker (Plate XI.), the principal
members of which converge to or cross at a small crater, and thus point
to a continuity of causation therewith analogous to the evident relation
between the bright streaks and their focal craters. Less remarkable, but
no less interesting, are those individual examples that appear in the
region north of (below) the Apennines (Plate IX.), and some of which by
their parallelism of direction with the mountain-chain appear to point
to a causative relation also. There is one long specimen, and several
shorter in the immediate neighbourhood of Mercator and Campanus (Plate
XV.); and another curious system of them, presenting suggestive
contortions, occurs in connection with the mountains Aristarchus and
Herodotus (Plate XVIII.). Others, again, appear to be identified with
the radial excrescences about Copernicus (Plate VIII.). Capuanus,
Agrippa, and Gassendi, among other craters, have more or less notable
cracks in their vicinities.

Some of these chasms are conspicuous enough to be seen with moderate
telescopic means, and from this maximum degree of visibility there are
all grades downwards to those that require the highest optical powers
and the best circumstances for their detection. The earlier
selenographers detected but a few of them. Schroeter noted only 11;
Lohrman recorded 75 more; Beer and Maedler added 55 to the list, while
Schmidt of Athens raised the known number to 425, of which he has
published a descriptive catalogue. We take it that this increase of
successive discoveries has been due to the progressive perfection of
telescopes, or, perhaps, to increased education, so to speak, of the
eye, since Schmidt’s telescope is a much smaller instrument than that
used by Beer and Maedler, and is regarded by its owner as an inferior
one for its size. We doubt not that there are hundreds more of these
cracks which more perfect instruments and still sharper eyes will bring
to knowledge in the future.

While these chasms have all lengths from 150 miles (which is about the
extent of those near Treisnecker) down to a few miles, they appear to
have a less variable breadth, since we do not find many that at their
maximum openings exceed two miles across; about a mile or less is their
usual width throughout the greater part of their length, and generally
they taper off to invisibility at their extremities, where they do not
encounter and terminate at a crater or other asperity, which is,
however, sometimes the case. Of their depth we can form no precise
estimate, though from the sharpness of their edges we may conclude that
their sides approach perpendicularity, and, therefore, that their depth
is very great; we have elsewhere suggested ten miles as a possible
profundity. In a few cases, and under very favourable circumstances, we
have observed their generally black interiors to be interrupted here and
there with bright spots suggestive of fragments from the sides of the
cracks having fallen into the opening.

In seeking an explanation of these cracks, two possible causes suggest
themselves. One is the expansion of subsurface matter, already suggested
as explanatory of the bright streaks; the other, a contraction of the
crust by cooling. We doubt not that both causes have been at work, one
perhaps enhancing the other. Where, as in the cases we have pointed out,
there are cracks which are so connected with craters as to imply
relationship, we may conclude that an upheaving or expansive force in
the sublunar molten matter has given rise to the cracks, and that the
central craters have been formed simultaneously, by the release, with
ejective violence, of the matter from its confining crust. The nature of
the expansive force being assumed that of solidifying matter, the wide
extent of some chasms indicates a deep location of that force. And depth
in this matter implies lateness (in the scale of selenological time) of
operation, since the central portions of the globe would be the last to
cool. Now, we have evidence of comparative lateness afforded by the fact
that in many cases the cracks have passed through craters and other
asperities which thus obviously existed before the cracking commenced;
and thus, so far, the hypothesis of the expansion-cracking is supported
by absolute fact.

It may be objected that such an upheaving force as we are invoking,
being transitory, would allow the distended surface to collapse again
when it ceased to operate, and so close the cracks or chasms it
produced. But we consider it not improbable that in some cases, as a
consequence of the expansion of subsurface matter, an upflow thereof may
have partially filled the crack, and by solidifying have held it open;
and it is rational to suppose that there have been various degrees of
filling and even of overflow—that in some cases the rising matter has
not nearly reached the edge of the crack, as in Fig. 44, while in others
it has risen almost to the surface, and in some instances has actually
overrun it and produced some sort of elevation along the line of the
crack, like that represented sectionally in Fig. 45. It is probable that
some of the slightly tumescent lines on the moon’s surface have been
thus produced.

[Illustration: PLATE XIX.
GLASS GLOBE CRACKED BY INTERNAL PRESSURE.]

[Illustration: Fig. 44.]

[Illustration: Fig. 45.]

We have suggested shrinkage as a possible explanation of some cracks. It
could hardly have been the direct cause of those compound ones which are
distinguished by focal craters, though it may have been a co-operative
cause, since the contracting tendency of any area of the crust, by so to
speak weakening it, may have virtually increased the strength of an
upheaving force and thus have aided and localized its action. We see,
however, no reason why the inevitable ultimate contraction which must
have attended the cooling of the moon’s crust, even when all internal
reactions upon it had ceased, should not have created a class of cracks
without accompanying craters, while it would doubtless have a tendency
to increase the length and width of those already existing from any
other cause. Some of the more minute clefts, which presumably exist in
greater numbers than we yet know of, may doubtless be ascribed to this
effect of cooling contraction. In this view we should have to regard
such cracks as the latest of all lunar features. Whether the agency that
produced them is still at work—whether the cracks are on the increase—is
a question impossible of solution: for reasons to be presently adduced,
we incline to believe that all cosmical heat passed from the moon, and
therefore that it arrived at its present, and apparently final,
condition ages upon ages ago.

Besides the ridges spoken of on p. 140, and regarded as cracks up
through which matter has been extruded, there are numerous ridges of
greater or less extent, which we conceive are of the nature of wrinkles,
and have been produced by tangential compression due to the collapse of
the moon’s crust upon the shrunken interior, as explained and
illustrated in Chap. III. The distinguishing feature of the two classes
of phenomena we consider to be the presence of a serrated summit in
those of the extruded class, while those produced by “wrinkling” action
have their summits comparatively free from serration or marked
irregularity.

Speaking generally, the details of the lunar surface seem to us to be
devoid of colour. To the naked eye of ordinary sensitiveness the moon
appears to possess a silvery whiteness: more critical judges of colour
would describe it as presenting a yellowish tinge. Sir John Herschel,
during his sojourn at the Cape of Good Hope, had frequent opportunities
of comparing the moon’s lustre with that of the weathered sandstone
surface of Table Mountain, when the moon was setting behind it, and both
were illuminated under the same direction of sunlight; and he remarked
that the moon was at such times “scarcely distinguishable from the rock
in apparent contact with it.” Although his observations had reference
chiefly to brightness, it can hardly be doubted that similarity of
colour is also implied; for any difference in the tint of the two
objects would have precluded the use of the words “scarcely
distinguishable;” a difference of colour interfering with a comparison
of lustre in such an observation, though it must be remembered that he
observed through a dense stratum of atmosphere. Viewed in the telescope,
the same general yellowish-white colour prevails over all the moon, with
a few exceptions offered by the so-called seas. The _Mare Crisium_,
_Mare Serenetatis_, and _Mare Humorum_ have somewhat of a greenish tint;
the _Palus Somnii_ and the circular area of Lichtenberg incline to
ruddiness. These tints are, however, extremely faint, and it has been
suggested by Arago that they may be mere effects of contrast rather than
actual colouration of the surface material. This, however, can hardly be
the case, since all the “seas” are not alike affected; those that are
slightly coloured are, as we have said, some green and some red, and
contrast could scarcely produce such variations. The supposition of
vegetation covering these great flats and giving them a local colour is
in our view still more untenable, in the face of the arguments that we
shall presently adduce against the possibility of vegetable life
existing upon the moon.

It appears to us more rational to consider the tints due to actual
colour of the material (presumably lava or some once fluid mineral
substance) that has covered these areas; and it may well be conceived
that the variety of tint is due to different characters of material, or
even various conditions of the same material coming from different
depths below the lunar surface; and we may reasonably suppose that the
same variously-coloured substances occur in the rougher regions of the
lunar surface, but that they exist there in patches too small to be
recognized by us, or are “put out” by the brightness to which polyhedral
reflexion gives rise.

Seeing that volcanic action has had so large a share in giving to the
moon’s surface its structural character, analogy of the most legitimate
order justifies us in concluding not only that the materials of that
surface are of kindred nature to those of the unquestionably volcanic
portions of the earth, but also that the tints and colours that
characterize terrestrial volcanic and Plutonian products have their
counterparts on the moon. Those who have seen the interior and
surroundings of a terrestrial volcano after a recent eruption, and
before atmospheric agents have exercised their dimming influences, must
have been struck with the colours of the erupted materials themselves
and the varied brilliant tints conferred on these materials by the
sublimated vapours of metals and mineral substances which have been
deposited upon them. If, then, analogy is any guide in enabling us to
infer the appearance of the invisible from that which we know to be of
kindred nature and which we have seen, we may justly conclude that were
the moon brought sufficiently near to us to exhibit the minute
characteristics of its surface, we should behold the same bright and
varied colours in and around its craters that we behold in and about
those of the earth; and in all probability the coloured materials of
lunar volcanoes would be more fresh and vivid than those of the earth by
reason of the absence of those atmospheric elements which tend so
rapidly to impair the brightness of coloured surfaces exposed to their
influence.

Situated as we are, however, as regards distance from the moon, we have
no chance of perceiving these local colours in their smaller masses; but
it is by no means improbable, as we have suggested, that the faint tints
exhibited by the great plains are due to broad expanses of coloured
volcanic material.

But if we fail to perceive diversity of colour upon the lunar surface,
we are in a very different position in regard to diversity of brightness
or variable light-reflective power of different districts and details.
This will be tolerably obvious to those casual observers who have
remarked nothing more of the moon’s physiography than the resemblance to
a somewhat lugubrious human countenance which the full moon exhibits,
and which is due to the accidental disposition of certain large and
small areas of surface material which have less of the light-reflecting
property than other portions; for since all parts seen by a terrestrial
observer may be said to be equally shone upon by the sun, it is clear
that apparently bright and shaded parts must be produced by differences
in the nature of the surface as regards power of reflecting the light
received.

When we turn to the telescope and survey the full disc of the moon with
even a very moderate amount of optical aid, the meagre impression as to
variety of degree of brightness which the unassisted eye conveys is
vastly extended and enhanced, for the surface is seen to be diversified
by shades of brilliancy and dullness from almost glittering white to
sombre grey: and this variety of shading is rendered much more striking
by shielding the eye with a dusky glass from the excessive glare, which
drowns the details in a flood of light. Under these circumstances the
varieties of light and shade become almost bewildering, and defy the
power of brush or pencil to reproduce them.

We may, however, realize an imperfect idea of this characteristic of the
lunar surface by reference to the self-drawn portrait of the full moon
upon Plate III. This is, in fact, a photograph taken from the full moon
itself, and enlarged sufficiently to render conspicuous the spots and
large and small regions that are strikingly bright in comparison with
what may in this place be described as the “ground” of the disc. As an
example of a wide and irregularly extensive district of highly
reflective material, the region of which Tycho is the central object, is
very remarkable. We may refer also to the bright “splashes” of which
Copernicus and Kepler are the centres. So brilliant are these spots that
they can easily be detected by the unassisted eye about the time of full
moon. Still brighter but less conspicuous by its size is the crater
Aristarchus, which shines with specular brightness, and almost induces
the belief that its interior is composed of some vitreous-surfaced
matter: the highly reflective nature of this object has often caused it
to become conspicuous when in the dark hemisphere of the moon,
unilluminated by the sun, and lighted only by the light reflected from
the earth. At these times it appears so bright that it has been taken
for a volcano in actual eruption, and no small amount of popular
misconception at one time arose therefrom concerning the conditions of
the moon as respects existing volcanic activity—a misconception that
still clings to the minds of many.

The parts of the surface distinguished by deficiency of reflecting power
are conspicuous enough. We may cite, however, as an example of a detail
portion especially remarkable for its dingy aspect, the interior of the
crater Plato, which is one of the darkest spots (the darkest well
defined one) upon the hemisphere of the moon visible to us. For
facilitating reference to shades of luminosity, Schroeter and Lohrman
assorted the variously reflective parts into 10 grades, commencing with
the darkest. Grades 1 to 3 comprised the various deep greys; 4 and 5 the
light greys; 6 and 7 white; and 8 to 10 brilliant white. The spots
Grimaldi and Riccioli came under class 1 of this notation; Plato between
1 and 2. The “seas” generally ranged from 2 to 3; the brightest
mountainous portions mostly between degrees 4 and 6; the crater walls
and the bright streaks came between these and the bright peaks, which
fell under the 9th grade. The maximum brightness, the 10th grade, is
instanced only in the ease of Aristarchus and a point in Werner, though
Proclus nearly approaches it, as do many bright spots, chiefly the sites
of minute craters, which make their appearance at the time of full moon.

In photographic pictures produced by the moon of itself, there is always
an apparent exaggeration in the relation of light to dark portions of
the disc. The dusky parts look, upon the photograph, much darker than to
the eye directed to the moon itself, whether assisted or not by optical
appliances. It may be that the real cause of this discrepancy is that
the eye fails to discover the actual difference upon the moon itself,
being insensible to the higher degrees of brightness or not estimating
them at their proper brilliance with respect to parts less bright. On
the other hand, it is probable that the enhanced contrast in the
photograph is due to some peculiar condition of the darker surface
matter affecting its power of reflecting the actinic constituent of the
rays that fall upon it.

The study of the varying brightness or reflective power of different
regions and spots of the lunar disc leads us to the consideration of the
relative antiquity of the surface features; for it is hardly possible to
regard these variations attentively without being impressed with the
conviction that they have relation to some chronological order of
formation. We cannot, in the first place, resist the conviction that the
brightest features were the latest formed; this strikes us as evident on
_primâ facie_ grounds; but it becomes more clearly so when we remark
that the bright formations, as a rule, overlie the duller features. The
elevated parts of the crust are brighter than the “seas” and other
areas; and it is pretty clear that the former are newer than the latter,
upon which they appear to be super-imposed, or through which they seem
to have extruded.[13] The vast dusky plains are in every instance more
or less sprinkled with spots and minute craters, and these last were
obviously formed after the area that contains them. One is almost
disposed to place the order of formations in the order of relative
brightness, and so consider the dingiest parts the oldest and the
brightest spots and craters the newest features, though, in the absence
of an atmosphere competent to impair the reflective power of the surface
materials, we are unable to justify this classification by suggesting a
cause for such a deterioration by time as the hypothesis pre-supposes.

As we have entered upon the question of relative age of the lunar
features, we may remark that there are evidences of various epochs of
formation of particular classes of details, irrespective of their
condition in respect of brightness, or, as we may say, freshness of
material. As a rule, the large craters are older than the small ones.
This is proved by the fact that a large object of this class is never
seen to interfere with or overlap a small one. Those of nearly equal
size are, however, seen to overlap one another as though several
eruptions of equal intensity had occurred from the same source at
different points. This is strikingly instanced in the group of craters
situated in the position 35-141 on our map, the order of formation of
each of which is clearly apparent. The region about Tycho offers an
inexhaustible field for study of these phenomena of overlapping or
interpolating craters, and it will be found, with very few exceptions,
that the smaller crater is the impinging or parasitical one, and must
therefore have been formed after the larger, upon which it intrudes or
impinges. There are frequent cases in which a large crater has had its
rampart interrupted by a lesser one, and this again has been broken into
by one still smaller; and instances may be found where a fourth crater
smaller than all has intruded itself upon the previous intruder. The
general tendency of these examples is to show that the craters
diminished in size as the moon’s volcanic energy subsided: that the
largest were produced in the throes of its early violence, and that the
smallest are the results of expiring efforts possibly impeded through
the deep-seatedness of the ejective source.

Another general fact of this chronological order is that the mountain
chains are never seen to intrude upon formations of the crater order. We
do not anywhere find that a mountain chain runs absolutely into or
through a crater; but, on the other hand, we do find that craters have
formed on mountain chains. This leads unmistakably to the inference that
the craters were not formed _before_ their allied mountain chains; and
we might assume therefore that the mountains generally are the older
formations, but that there is nothing to prove that the two classes of
features, where they intermingle, as in the Apennines and Caucasus, were
not erupted cotemporaneously.

[Illustration: PLATE XX.
OVERLAPPING CRATERS.]

Upon the assumption that the latest ejected or extruded matter is that
which is brightest, we should place the bright streaks among the more
recent features. Be this as it may, it is tolerably certain that the
cracks, whose apparently close relation to the radiating streaks we have
endeavoured to point out, are relatively of a very late formative
period. We are indeed disposed to consider them as the most recent
features of all: the evidence in support of this consideration being the
fact that they are sometimes found intersecting small craters that, from
the way in which they are cut through by the cracks, must have been _in
situ_ before the cracking agency came into operation. It is in
accordance with our hypothesis of the moon’s transition from a fluid to
a solid body to consider that a cracking of the surface would be the
latest of all the phenomena produced by contraction in final cooling.

The foregoing remarks naturally lead us to the question whether changes
are still going on upon the surface of our satellite: whether there is
still left in it a spark of its volcanic activity, or whether that
activity has become totally extinct. We shall consider this question
from the observational and theoretical point of view. First as regards
observations. This much may be affirmed indisputably—that no object or
detail visible to the earliest selenographers (whose period may be dated
200 years back) has altered from the date of their maps to the present.
When we pass from the bolder features to the more minute details we find
ourselves at a loss for materials for forming an inference; the only map
pretending to accuracy even of the larger among small objects being that
of Beer and Maedler, which, truly admirable as it is, is not very safely
to be relied upon for settling any question of alleged change, on
account of the conventional system adopted for exhibiting the forms of
objects, every object being mapped rather than drawn, and shown as it
never is or can be presented to view on the moon itself. This difficulty
would present itself if a question of change were ever raised upon the
evidence of Beer and Maedler’s map: it may indeed have prevented such a
question being raised, for certainly no one has hitherto been bold
enough to assert that any portion or detail of the map fails to
represent the actual state of the moon at the present time.

In default of published maps, we are thrown for evidence on this
question upon observations and recollections of individual observers
whose familiarity with the lunar details extends over lengthy periods.
Speaking for ourselves, and upon the strength of close scrutinies
continued with assiduity through the past thirty years, we may say that
we have never had the suspicion suggested to our eye of any actual
change whatever having taken place in any feature or minute detail of
the lunar surface; and our scrutinies have throughout been made with
ample optical means, mostly with a 20-inch reflector. This experience
has made us not unnaturally in some slight degree sceptical concerning
the changes alleged to have been detected by others. Those asserted by
Schroeter and Gruithuisen were long ago rejected by Beer and Maedler,
who explained them, where the accuracy of the observer was not
questioned, by variations of illumination, a cause of illusory change
which is not always sufficiently taken into account. A notable instance
of this deception occurred a few years ago in the case of the minute
bright crater _Linné_, which was for a considerable period declared,
upon the strength of observations of very promiscuous character, to be
varying in form and dimensions almost daily, but the alleged constant
changes of which have since been tacitly regarded as due to varying
circumstances of illumination induced by combinations of libratory
effects with the ordinary changes depending upon the direction of the
sun’s rays as due to the age of the moon. This explanation does not,
however, dispose of the question whether the crater under notice
suffered any actual change before the hue and cry was raised concerning
it. Attention was first directed to it by Schmidt, of Athens, whose
powers of observation are known to be remarkable, and whose labours upon
the moon are of such extent and minuteness as to claim for his
assertions the most respectful consideration.[14] He affirmed in 1866
that the crater at that date presented an appearance decidedly different
from that which it had had since 1841: that whereas it had been from the
earlier epoch always easily seen as a very deep crater, in October 1866
and thenceforward it presented only a white spot, with at most but a
very shallow aperture, very difficult to be detected. Schmidt is one of
the very few observers whose long familiarity with the moon entitles him
to speak with confidence upon such a question as that before us upon the
sole strength of his own experience; and this case is but an isolated
one, at least it is the only one he has brought forward. He is, however,
still firmly convinced that it is an instance of actual change, and not
an illusion resulting from some peculiar condition of illumination of
the object. It should be added also on this side of the discussion that
an English observer, the Rev. T. W. Webb, while apparently indisposed to
concede the supposition of any notable changes in the lunar features,
has yet found from his own observations that, after all due allowance
for differences of light and shade upon objects at different times,
there is still a “residuum of minute variations not thus disposed of”
which seem to indicate that eruptive action in the moon has not yet
entirely died out, though its manifestation at present is very limited
in extent. It appears to us that, if evidence of continuing volcanic
action is to be sought on the moon, the place to look for it is around
the circumference of the disc, where eruption from any marginal orifice
would manifest itself in the form of a protruding haziness, somewhat as
illustrated to an exaggerated extent in the annexed cut.

[Illustration: Fig. 46.]

The theoretical view of the question, which we have now to consider, has
led us, however, to the strong belief that no vestige of its former
volcanic activity lingers in the moon—that it assumed its final
condition an inconceivable number of ages ago, and that the high
interest which would attach to the close scrutiny of our satellite if it
_were_ still the theatre of volcanic reactions cannot be hoped for. If
it be just and allowable to assume that the earth and the moon were
condensed into planetary form at nearly the same epoch (and the only
rational scheme of cosmogony justifies the assumption) then we may
institute a comparison between the condition of the two bodies as
respects their volcanic age, using the one as a basis for inference
concerning the state of the other. We have reason to believe that the
earth’s crust has nearly assumed its final state so far as volcanic
reactions of its interior upon its exterior are concerned: we may affirm
that within the historical period no igneous convulsions of any
considerable magnitude have occurred; and we may consider that the
volcanoes now active over the surface of the globe represent the last
expiring efforts of its eruptive force. Now in the earth we perceive
several conditions wherefrom we may infer that it parted with its
cosmical heat (and therefore with its prime source of volcanic agency)
at a rate which will appear relatively very slow when we come to compare
the like conditions in the moon. We may, we think, take for granted that
the surface of a planetary body generally determines its _heat
dispersing_ power, while its volume determines its _heat retaining_
power. Given two spherical bodies of similar material but of unequal
magnitude and originally possessing the same degree of heat, the smaller
body will cool more rapidly than the larger, by reason of the greater
proportion which the surface of the smaller sphere bears to its volume
than that of the larger sphere to its volume—this proportion depending
upon the geometrical ratio which the surfaces of spheres bear to their
volumes, the contents of spheres being as the _cubes_ and the surfaces
as the _squares_ of their diameters. The volume of the earth is 49 times
as great as that of the moon, but its surface is only 13 times as great;
there is consequently in the earth a power of retaining its cosmical
heat nearly four times as great as in the case of the moon; in other
words, the moon and earth being supposed at one time to have had an
equally high temperature, the moon would cool down to a given low
temperature in about one fourth the time that the earth would require to
cool to the same temperature. But the earth’s cosmical heat has without
doubt been considerably conserved by its vaporous atmosphere, and still
more by the ocean in its antecedent vaporous form. Yet notwithstanding
all this, the earth’s surface has nearly assumed its final condition so
far as volcanic agencies are concerned: it has so far cooled as to be
subject to no considerable distortions or disruptions of its surface.
What then must be the state of the moon, which, from its small volume
and large proportionate area, parted with its heat at the above
comparatively rapid rate? The matter of the moon is, too, less dense
than the earth, and hence doubtless from this cause disposed to more
rapid cooling; and it has no atmosphere or vaporous envelope to retard
its radiating heat. We are driven thus to the conclusion that the moon’s
loss of cosmical heat must have been so rapid as to have allowed its
surface to assume its final conformation ages on ages ago, and hence
that it is unreasonable and hopeless to look for evidence of change of
any volcanic character still going on.

We conceive it possible, however, that minute changes of a non-volcanic
character may be proceeding in the moon, arising from the violent
alternations of temperature to which the surface is exposed during a
lunar day and night. The sun, as we know, pours down its heat
unintermittingly for a period of fully 300 hours upon the lunar surface,
and the experimental investigations of Lord Rosse, essentially confirmed
by those of the French observer, Marie Davy, show that under this
powerful insolation the surface becomes heated to a degree which is
estimated at about 500° of Fahrenheit’s scale, the fusing point of tin
or bismuth. This heat, however, is entirely radiated away during the
equally long lunar night, and, as Sir John Herschel surmised, the
surface probably cools down again to a temperature as low as that of
interstellar space: this has been assumed as representing the absolute
zero of temperature, which has been calculated from experiments to be
250° below the zero of Fahrenheit’s scale. Now such a severe range of
heat and cold can hardly be without effect upon some of the component
materials of the lunar surface.[15] If there be any such materials as
the vitreous lavas that are found about our volcanoes, such as obsidian
for instance, they are doubtless cracked and shivered by these extreme
transitions of temperature; and this comparatively rapid succession of
changes continued through long ages would, we may suppose, result in a
disintegration of some parts of the surface and at length somewhat
modify the selenographic contour. It is, however, possible that the
surface matter is mainly composed of more crystalline and porous lavas,
and these might withstand the fierce extremes like the “fire-brick” of
mundane manufacture, to which in molecular structure they may be
considered comparable. Lavas as a rule are (upon the earth) of this
unvitreous nature, and if they are of like constitution on the moon,
there will be little reason to suspect changes from the cause we are
considering. Where, however, the material, whatever its nature, is piled
in more or less detached masses, there will doubtless be a grating and
fracturing at the points of contact of one mass with another, produced
by alternate expansions and contractions of the entire masses, which in
the long run of ages must bring about dislocations or dislodgments of
matter that might considerably affect the surface features from a close
point of view, but which can hardly be of sufficient magnitude to be
detected by a terrestrial observer whose best aids to vision give him no
perception of minute configurations. And it must always be borne in mind
that changes can only be _proved_ by reference to previous observations
and delineations of unquestionable accuracy.

Speaking by our own lights, from our own experience and reasoning, we
are disposed to conclude that in all visible aspects the lunar surface
is unchangeable, that in fact it arrived at its terminal condition
_eons_ of ages ago, and that in the survey of its wonderful features,
even in the smallest details, we are presented with the sight of objects
of such transcendent antiquity as to render the oldest geological
features of the earth modern by comparison.

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