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.

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.”

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]

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 (p. 29).

CHAPTER XI.
CRACKS AND RADIATING STREAKS.

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.

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.

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.

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.

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.

CHAPTER XII.
COLOUR AND BRIGHTNESS OF LUNAR DETAILS: CHRONOLOGY OF FORMATIONS, AND FINALITY OF EXISTING FEATURES.

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.