Under these circumstances we conceive the upcasting and excavating of a normal lunar crater to have been primarily caused by a local manifestation of the force of expansion upon solidification of the subsurface matter of the moon, resulting in the creation of a mere “star” or crack in and through the outermost and solid crust. As we shall have to rely upon diagrams to explain the more complicated features, we give one of this elementary stage also as a commencement of the series; and Fig. 20 therefore represents a probable section of the lunar surface at a point which was subsequently the location of a crater. From the vent thus formed we conceive the pent-up matter to have found its escape, not necessarily at a single outburst, but in all probability in a paroxysmal manner, as volcanic action manifests itself on our globe. The first outflow of molten material would probably produce no more than a mere hill or tumescence as shewn sectionally in Fig. 21; and if the ejective force were small this might increase to the magnitude of a mountain by an exudative process to be alluded to hereafter. But if the ejective force were violent, either at the moment of the first outburst or at any subsequent paroxysm, an action represented in Fig. 22 would result: the unsupported edges or lips of the vent-hole would be blown and ground or fluxed away, and a funnel-formed cavity would be produced, the ejected matter (so much of it as in falling was not caught by the funnel) being deposited around the hollow and forming an embryo circular mountain. The continuance of this action would be accompanied by an enlargement of the conical cavity or crater, not only by the outward rush of the violently discharged material, but also by the “sweating” or grinding action of such of it as in descending fell within the hollow. And at the same time that the crater enlarged the rampart would extend its circumference, for it would be formed of such material as did not fall back again into the crater. Upon this view of the crater-forming process we base the sketch, Fig. 23, of the probable section of a lunar crater at one period of its development.

So long as each succeeding paroxysm was greater than its predecessor, this excavating of the hollow and widening of its mouth and mound would be extended. But when a weaker outburst came, or when the energy of the last eruption died away, a process of slow piling up of matter close around the vent would ensue. It is obvious that when the ejective force could no longer exert itself to a great distance it must merely have lifted its burden to the relieving vent and dropped it in the immediate neighbourhood. Even if the force were considerable, the effect, so long as it was insufficient to throw the ejecta beyond the rim of the crater, would be to pile material in the lowermost part of the cavity; for what was not cast over the edge would roll or flow down the inner slope and accumulate at the bottom. And as the eruption died away, it would add little by little to the heap, each expiring effort leaving the out-given matter nearer the orifice, and thus building up the central cone that is so conspicuous a feature in terrestrial volcanoes, and which is also a marked one in a very large proportion of the craters of the moon. This formation of the cone is pictorially described by Fig. 24.

In the volcanoes of the earth we observe another action either concurrent with or immediately subsequent to the erection or formation of the cone: this is the outflow or the welling forth of fluid lava, which in cooling forms the well-known plateau. We have this feature copiously represented upon the moon and it is presumable that it has in general been produced in a manner analogous to its counterparts upon the earth. We may conceive that the fluid matter was either spirted forth with the solid or semisolid constituents of the cone, in which case it would drain down and fill the bottom of the crater; or we may suppose that it issued from the summit of the cone and ran down its sides, or that, as we see upon the earth, it found its escape before reaching the apex, by forcing its way through the basal parts. These actions are indicated hypothetically for the moon in Fig. 25; and the parallel phenomena for the earth are shewn by the actual case (represented in Fig. 26 and on Plate I.) of Vesuvius as it was seen by one of the authors in 1865, when the principal cone was vomiting forth ashes, stones, and red-hot lava, while a vent at the side emitted very fluid lava which was settling down and forming the plateau.

Although we cannot, obviously, see upon the moon evidence of a cone actually overtopped by the rising lake of lava, yet it is not unreasonable to suppose that such a condition of things actually occurred in many of those instances in which we observe craters without central cones, but with plateaux so smooth as to indicate previous fluidity or viscosity. From the state of things exhibited in Fig. 25 the transition to that shewn in Fig. 27 is easily, and to our view reasonably, conceivable. We are in a manner led up to this idea by a review of the various heights of central cones above their surrounding plateaux. For instance, in such examples as Tycho or Theophilus, we have cones high above the lava floor; in Copernicus, Arzachael and Alphonsus they are comparatively lower; the lava in these and some other craters does not appear to have risen so high; while in Aristotle and Eudoxus among others, we have only traces of cones, and it is supposable that in these cases the lava rose so high as nearly to overtop the central cones. Why should it not have risen so far as to overtop and therefore conceal some cones entirely? We offer this as at least a feasible explanation of some coneless craters: it is not necessary to suppose that it applies to all such, however: there may have been many craters, the formation of which ceased so abruptly that no cone was produced, though the welling forth of lava occurred from the vent, which may have been left fully open, as in Fig. 28, or so far choked as to stay the egress of solid ejecta and yet allow the fluid material to ooze upwards through it, and so form a lake of molten lava which on consolidation became the plateau. As most of the examples of coneless craters exhibit on careful examination minute craters on the surface of the otherwise smooth plateaux, we may suppose that such minute craters are evidences of the upflow of lava which resulted in the plateaux.

We have strong evidence in support of this up-flow of lava offered by the case of the crater Wargentin, (No. 26, 57·5—140·2) situated near the south-east border of the disc, and of which we give a special plate. (Plate XVII.) It appears to be really a crater in which the lava has risen almost to the point of overflowing, for the plateau is nearly level with the edge of the rampart. This edge appears to have been higher on one side than the other, for on the portion nearest the centre of the visible disc we may, under favourable circumstances, detect a segment of the basin’s brim rising above the smooth plateau as indicated in our illustration. Upon the opposite side there is no such feature visible, the plateau forms a sharp table-like edge. It is just possible that an actual overflow of lava took place at this part of the crater, but from the unfavourable situation of this remarkable object it is impossible to decide the point by observation. There is no other crater upon the visible hemisphere of the moon that exhibits this filled-up condition; but, unique as it is, it is sufficient to justify our conclusion that the plateau-forming action upon the moon has been a flowing-up of fluid matter from below subsequent to the formation of the crater-rampart, and not, as a casual glance at the great smooth-bottom craters might lead us to suspect, a result of some sort of diluvial deposit which has filled hollows and cavities and so brought up an even surface. The elevated basin of Wargentin could not have been filled thus while the surrounding craters with ramparts equally or less high remained empty: its contained matter must have been supplied from within, we must conjecture by the upflow of lava from the orifice which gave forth the material to form the crateral rampart in the first instance. We are free to conjecture that at some period of this table-mountain’s formation it was a crater with a central cone, and that the rising lava over-topped this last feature in the manner shewn by the above figure (Fig. 29).

The question occurs whether other craters may not have been similarly filled and have emptied themselves by the bursting of the wall under the pressure of the accumulated lake of lava within. We know that this breaching is a common phenomenon in the volcanoes of our globe; the district of Auvergne furnishing us with many examples; and there are some suspicious instances upon the moon. Copernicus exhibits signs of such disruption, as also does the smaller crater intruding upon the great circle of Gassendi. (See Frontispiece.) But the existence of such discharging breaches implies the outpouring of a body of fluid or semi-fluid material, comparable in cubical content to the capacity of the crater, and of this we ought to see traces or evidence in the form of a bulky or extensive lava stream issuing from the breach. But although there are faint indications of once viscous material lying in the direction that escaping fluid would take, we do not find anything of the extent that we should expect from the mass of matter that would constitute a craterfull. It is true that if the escaping fluid had been very limpid it might have spread over a large area and have formed a stratum too thin to be detected. Such a degree of limpidity as would be required to fulfil this condition we are hardly, however, justified in assuming.

To return to the subject of central cones. Although there are cases in which the simple condition of a single cone exists, yet in the majority we see that the cone-forming process has been divided or interrupted, the consequence being the production of a group of conical hills instead of a single one. Copernicus offers an example of this character, six, some observers say seven, separate points of light, indicating as many peaks tipped with sunshine, having been seen when the greater part of the crater has been buried in shadow. Erastothenes, Bulialdus, Maurolicus, Petavius, Langreen, and Gassendi, are a few among many instances of craters possessing more than a central single cone. This multiplication of peaks upon the moon doubtless arose from similar causes to those which produce the same feature in terrestrial volcanoes. Our sketch of Vesuvius in 1865 (Fig. 26) shows the double cone and the probable source of the secondary one in the diverted channel of the out-coming material. A very slight interruption in the first instance would suffice to divert the stream and form another centre of action, or a choking of the original vent would compel the issuing matter to find a less resisting thoroughfare into open space, and the process of cone-building would be continued from the new orifice, perhaps to be again interrupted after a time and again driven in another direction. In this manner, by repeated arrests and diversions of the ejecta, cone has grown upon the side of cone, till, ere the force has entirely spent itself, a cluster of peaks has been produced. It may have been that this action has taken place after the formation of the plateau, in the manner indicated by Fig. 30; a spasmodic outburst of comparatively slight violence having sought relief from the original vent, and the flowing matter, finding the one orifice not sufficiently open to let it pass, having forced other exit through the plateau.

In frequent instances we observe the state of things represented in Fig. 31, in which the plateau is studded with few or many small craters. This is the case with Plato, with Arzachael, Hipparchus, Clavius (which contains about 15 small internal craters), and many others. It is probable that these subsidiary craters were produced by an after-action like that which has produced duplicated cones, but in which the secondary eruption has been of somewhat violent character, for it may almost be regarded as an axiom that violent eruptions excavate craters and weak ones pile up cones. In the cases referred to it seems reasonable to suppose that the main vent has been the channel for an up-cast of material, but that at some depth below the surface this material met with some obstruction or cause of diversion, and that it took a course which brought it out far away from the cone upon the floor of the plateau. It might even be carried so far as to be upon the rampart, and it is no uncommon thing to see small craters in such a situation, though when they appear at such a distance from the primary vent, it seems more reasonable to suppose that they do not belong to it but have arisen from a subsequent and an independent action.

We find scarcely an instance of a small crater occurring just in the centre of a large one, or taking the place of the cone. This is a curious circumstance. Whenever we have any central feature in a great crater, that feature is a cone. The tendency of this fact is to prove that cones were produced by very weak efforts of this expiring force, for had there been any strength in the last paroxysm it is presumable that it would have blown out and left a crater. No very violent eruptions have therefore taken place from the vents that were connected with the great craters of the moon, nothing more powerful than could produce a cone of exudation or a cinder-heap. And with regard to cones, it is noteworthy that whether they be single or multiple, they never rise so high as the circular ramparts of their respective craters. This supports the inferred connexion between the crater origin and the cone origin, for supposing the two to have been independent, a supposition untenable in view of the universality of the central position of the cone, it is scarcely conceivable that the mountains should have always been located within ramparts higher than themselves. The less height argues less power in the upcasting agency, and the diminished force may well be considered as that which would almost of necessity precede the expiration of the eruption.

Occasionally a crater is met with that has a double rampart, and the concentricity suggests that there have been two eruptions from the same vent: one powerful, which formed the exterior circle, and a second rather less powerful which has formed the interior circle. It is not, however evident that this duplication of the ring has always been due to a double eruption. In many cases there is duplication of only a portion: a terrace exhibits itself around a part of the circular range, sometimes upon the outside and sometimes upon the inside. These terraces are not likely to have been formed by any freak of the eruption, and we are led to ascribe them in general to landslip phenomena. When, in the course of a volcano’s formation, the piling-up of material about the vent has continued till the lower portions have been unable to support the upper, or when from any cause, the material composing the pile has lost its cohesiveness, the natural consequence has been a breaking away of a portion of the structure and its precipitation down the inclined sides of the crater. Vast segments of many of the lunar mountain-rings appear to have been thus dislodged from their original sites and cast down the flanks to form crescent ranges of volcanic rocks either within or without the circle. Nearly every one of our plates contains craters exhibiting this feature in more or less extensive degree. Sometimes the separated portion has been very small in proportion to the circumference of the crater: Plato is an instance in which a comparatively small mass has been detached. In other cases very large segments have slid down and lie in segmental masses on the plateaux or form terraces around the rampart. Aristarchus, Treisnecker and Copernicus exhibit this larger extent of dislocation.

It is possible that these landslips occurred long after the formation of the craters that have been subject to them. They are probably attributable to recent disintegration of the lunar rocks, and we have a powerful cause for this in the alternations of temperature to which the lunar crust is exposed. We shall have occasion to revert to this subject by-and-bye; at present it must suffice to point out that the extremes of cold and heat, between which the lunar soil varies, are, with reasonable probability, assumed to be on the one hand the temperature of space (which is supposed to be about 200° below zero), and, on the other hand, a degree of heat equal to about twice that of boiling water. A range of at least 500° must work great changes in such heterogeneous materials as we may conjecture those of the lunar crust to be, by the alternate contractions and expansions which it must engender, and which must tend to enlarge existing fissures and create new ones, to grind contiguous surfaces and to dislodge unstable masses. This cause of change, it is to be remarked, is one which is still exerting itself.

In a few cases we have an entirely opposite interruption of the uniformity of a crater’s contour. Instead of the breaking away of the ring in segments we see the entire circuit marked with deep ruts that run down the flanks in a radial direction, giving us evidence of a downward streaming of semi-fluid matter, instead of a disruption of solid masses. We cannot doubt that these ruts have been formed by lava currents, and they indicate a condition of ejected material different from that which existed in the cases where the landslip character is found. In these last the ejecta appears to have been in the form of masses of solidified or rapidly solidifying matter, which remained where deposited for a time and then gave way from overloading or loss of cohesiveness, whereas the substances thrown out in the case of the rutted banks were probably mixed solid and fluid, the former remaining upon the flanks while the latter trickled away. Nothing so well represents, upon a small scale, this radial channelling as a heap of wetted sand left for a while for the water to drain off from it. The solid grains in such a heap sustain its general mass-form, but the liquid in passing away cuts the surface into fissures running from the summit to the base, and forms it into a model of a volcanic mountain with every feature of peak, crag, and chasm reproduced, This similarity of effect leads us to suspect a parallelism of cause, and thus to the inference that the material which originally formed such a crater-mountain as Aristillus (which is a most prominent example of this rutted character, and appears in Plate IX., side by side with a crater that has its banks segmentally broken), must have been of the compound nature indicated; and that an action analogous to that which ruts a damp sand-heap, rutted also the banks of the lunar crater.

Before passing from the subject of craters it behoves us to say a few words upon the curious manner in which these formations are complicated by intermingling and superposition. Yet, upon this point, we may be brief, for in the way of description our plates speak more forcibly than is possible by words. In particular we would refer to Plate XII., which represents the conspicuous group of craters of which the three largest members have been respectively named Theophilus, Cyrillus, and Catherina. But the area included in this plate is by no means an extraordinary one; there are regions about Tycho wherein the craters so crowd and elbow each other that, in their intricate combinations, they almost defy accurate depiction. Our map and Plate XVI. will serve to give some idea of them. This intermingling of craters obviously shows that all the lunar volcanoes were not simultaneously produced, but that after one had been formed, an eruption occurred in its immediate neighbourhood and blew a portion of it away; or it may have been that the same deep-seated vent at different times gave forth discharges of material the courses of which were more or less diverted on their way to the surface.

We have before alluded to the frequent occurrence of lines of craters upon the moon. In these lines the overlapping is frequently visible; it is seen in Plate XII. before referred to, where the ring mountains are linked into a chain slightly curved, and upon the map, Plate IV., the nearly central craters Ptolemy and Alphonsus, the latter of which overlaps the former, are seen to form part of a line of craters marking a connection of primary disturbance. An extensive crack suggests itself as a favourable cause for the production of this overlaying of craters, for it would serve as a sort of “line of fire” from various points at which eruptions would burst forth, sometimes weak or far apart, when the result would be lines of isolated craters, and sometimes near together, or powerful, when the consequence would be the intrusion of one upon the other, and the perfect production of the latest formed at the expense or to the detriment of those that had been formed previously. The linear grouping of volcanoes upon the earth long ago struck observant minds. The fable of the Typhon lying under Sicily and the Phlegreian fields and disturbing the earth by its writhings, is a mythological attempt to explain the particular case in that region.

The capricious manner in which these intrusions occur is very curious. Very commonly a small crater appears upon the very rampart of a greater one, and a more diminutive one still will appear upon the rampart of the parasite. Stoeffler presents us with one example of this character, Hipparchus with another, Maurolycus with a third, and these are but a few cases of many. Here and there we observe several craters ranged in a line with their rims in one direction all perfect, and the whole appearing like a row of coins that have fallen from a heap. There is an example near to Tycho which we reproduce in Plate XX. In this case one is led to conjecture that the ejective agency, after exerting itself in one spot, travelled onward and renewed itself for a time; that it ceased after forming crater number two, and again journeyed forward in the same line, recommencing action some miles further, and again subsiding; yet again pushing forward and repeating its outburst, till it produced the fourth crater, when its power became expended. In each of these successive eruptions the centre of discharge has been just outside the crater last formed; and the close connexion of the members of the group, together with the fact of their nearly similar size, appears to indicate a community of origin. For it seems feasible that as a general rule the size of a crater may be taken as a measure of the depth of force that gave rise to the eruption producing it. This may not be true for particular cases, but it will hold where a great number are collectively considered; for if we assume the existence of an average disturbing force, it is apparently clear that it will manifest itself in disturbing greater or less surface-areas in proportion as it acts from greater or less depths. Or, mutatis mutandis, if we assume an uniform depth for the source of action, the greater or less surface disturbance will be a measure of greater or less eruptive intensity.

Perhaps the most remarkable case of a vast number of craters, which, from their uniform dimensions, suggest the idea of community of source-power or source-depth, is that offered by the region surrounding Copernicus, which, as will be seen by our plate of that object, is a vast Phlegreian field of diminutive craters. So countless are the minute craters that a high magnifying power brings into view when atmospheric circumstances are favourable, and so closely are they crowded together, that the resulting appearance suggests the idea of froth, and we should be disposed to christen this the “frothy region” of the moon, did not a danger exist in the tendency to connect a name with a cause. The craters that are here so abundant are doubtless the remains of true volcanoes analogous to the parasitical cones that are to be found on several terrestrial mountains, and not such accidental formations as the Hornitos described by Humboldt as abounding in the neighbourhood of the Mexican volcano, Jurillo, but which the traveller did not consider to be true cones of eruption.[9] Although upon our plate, and in comparison with the great crater that is its chief feature, these countless hollows appear so small as at first sight to appear insignificant, we must remember that the minutest of them must be grand objects, each probably equal in dimensions to Vesuvius. For since, as we have shown in an early chapter, the smallest discernible telescopic object must subtend an angle to our eye of about a second, and since this angle extended to the moon represents a mile of its surface, it follows that these tiny specks of shadow that besprinkle our picture, are in the reality craters of a mile diameter. This comparison may help the conception of the stupendous magnitude of the moon’s volcanic features; for it is a conception most difficult to realize. It is hard to bring the mind to grasp the fact that that hollow of Copernicus is fifty miles in diameter. We read of an army having encamped in the once peaceful crater of Vesuvius, and of one of the extinct volcanoes of the Campi Phlegræi being used as a hunting preserve by an Italian king. These facts give an idea of vastness to those who have not the good fortune to see the actual dimensions of a volcanic orifice themselves. But it is almost impossible to conjure up a vision of what that fifty-mile crater would look like upon the moon itself; and for want of a terrestrial object as a standard of comparison, our picture, and even the telescopic view of the moon itself, fails to render the imagination any help. We may try to realize the vastness by considering that one of our average English counties could be contained within its ramparts, or by conceiving a mountainous amphitheatre whose opposite sides are as far apart as the cathedrals of London and Canterbury, but even these comparisons leave us unimpressed with the true majesty which the object would present to a spectator upon the surface of our satellite.

CHAPTER IX.
ON THE GREAT RING-FORMATIONS NOT MANIFESTLY VOLCANIC.

In our previous chapter we have given a reason for regarding as true volcanic craters all those circular formations, of whatever size, that exhibit that distinctive feature the central cone. Between the smallest crater with a cone that we can detect under the best telescopic conditions, namely, the companion to Hell, 1¾ mile diameter, and the great one called Petavius, 78 miles in diameter, we find no break in the continuity of the crater-cum-cone system that would justify us in saying that on the one side the volcanic or eruptive cause ceased, and on the other side some other causative action began. But there are numerous circular formations that surpass the magnitude of Petavius and its peers, but that have no central cone, and are, therefore, not so manifestly volcanic as those which possess this feature. Our map will show many striking examples of this class at a glance. We may in particular refer inter alia to Ptolemy near the centre of the moon, to Grimaldi (No. 125), Shickard (No. 28), Schiller (No. 24), and Clavius (No. 13), all of which exceed 100 miles in diameter. Even the great Mare Crisium, nearly 300 miles in diameter, appears to be a formation not distinct from those which we have just named. These present little of the generic crater character in their appearance; and they have been distinguished therefrom by the name of Walled or Ramparted Plains. Their actual origin is beyond our explanation, and in attempting to account for them we must perforce allow considerable freedom to conjecture. They certainly, as Hooke suggested, present a “broken bubble”-like aspect; but one cannot reasonably imagine the existence of any form of mineral matter that would sustain itself in bubble form over areas of many hundreds of square miles. And if it were reasonable to suppose the great rings to be the foundations of such vast volcanic domes, we must conclude these to have broken when they could no longer sustain themselves, and in that case the surface beneath should be strewed with débris, of which, however, we can find no trace. Moreover, we might fairly expect that some of the smaller domes would have remained standing: we need hardly say that nothing of the kind exists.

The true circularity of these objects appears at first view a remarkable feature. But it ceases to be so if we suppose them to have been produced by some very concentrated sublunar force of an upheaving nature, and if only we admit the homogeneity of the moon’s crust. For if the crust be homogeneous, then any upheaving force, deeply seated beneath it, will exert itself with equal effects at equal distances from the source: the lines of equal effect will obviously be radii of a sphere with the source of the disturbance for its centre, and they will meet a surface over the source in a circle. This will be evident from Fig. 32, in which a force is supposed to act at F below the surface s s s s. The matter composing s s being homogeneous, the action of F will be equal at equal distances in all directions. The lines of equal force, F f, F f, will be of equal length, and they will form, so to speak, radii of a sphere of force. This sphere is cut by the plane at s s s s, and as the intersection necessarily takes place everywhere at the extremity of these radii, the figure of intersection is demonstrably a circle (shown in perspective as an ellipse in the figure). Thus we see that an intense but extremely confined explosion, for instance, beneath the moon’s crust must disturb a circular area of its surface, if the intervening material be homogeneous. If this be not homogeneous there would be, where it offered less than the average resistance to the disturbance, an outward distortion of the circle; and an opposite interruption to circularity if it offers more than the average resistance. This assumed homogeneity may possibly be the explanation of the general circularity of the lunar surface features, small and great.

We confess to a difficulty in accounting for such a very local generation of a deep-seated force; and, granting its occurrence, we are unprepared with a satisfactory theory to explain the resultant effect of such a force in producing a raised ring at the limit of the circular disturbance. We may indeed, suppose that a vast circular cake or conical frustra would be temporarily upraised as in Fig. 33, and that upon its subsidence a certain extrusion of subsurface matter would occur around the line or zone of rupture as in Fig. 34. This supposition, however, implies such a peculiarly cohesive condition of the matter of the uplifted cake, that it is doubtful whether it can be considered tenable. We should expect any ordinary form of rocky matter subjected to such an upheaval to be fractured and distorted, especially when the original disturbing force is greater in the centre than at the edge, as, according to the above hypothesis, it would be; and in subsiding, the rocky plateau would thus retain some traces of its disturbance; but in the circular areas upon the moon there is nothing to indicate that they have been subjected to such dislocations.

Mr. Scrope in his work on volcanoes has given a hypothetical section of a portion of the earth’s crust, which presents a bulging or tumescent surface in some measure resembling the effect which such a cause as we have been considering would produce. We give a slightly modified version of his sketch in Fig. 35, showing what would be the probable phenomena attending such an upheaval as regards the behaviour of the disturbed portion of the crust, and also that of the lava or semifluid matter beneath: and, as will be seen by the sketch, a possible phase of the phenomena is the production of an elevated ridge or rampart at the points of disruption c c; and where there is a ring of disruption, as by our hypothesis there would be, the ridge or rampart c c would be a circle. In this drawing we see the cracking and distortion to which the elevated area would be subjected, but of which, as previously remarked, the circular areas of the moon present no trace of residual appearance.

Those who have offered other explanations of these vast ring-formed mountain ranges, have been no more happy in their conjectures. M. Rozet, who communicated a paper on selenology to the French Academy in 1846, put forth the following theory. He argued that during the formation of the solid scoriaceous pelicules of the moon, circular or tourbillonic movements were set up; and these, by throwing the scoria from the centre to the circumference, caused an accumulation thereof at the limit of the circulation. He considered that this phenomenon continued during the whole process of solidification, but that the amplitude of the whirlpool diminished with the decreasing fluidity of the surface material. Further, he suggested that when many vortices were formed, and the distances of their centres, taken two and two, were less than the sums of their radii, there resulted closed spaces terminated by arcs of circles; and when for any two centres the distance was greater than the sum of the radii of action, two separate and complete rings were formed. We have only to remark on this, that we are at a loss to account for the origination of such vorticose movements, and M. Rozet is silent on the point. If the great circles are to be referred to an original sea of molten matter, it appears to us more feasible to consider that wherever we see one of them there has been, at the centre of the ring, a great outflow of lava that has flooded the surrounding surface. Then, if from any cause, and it is not difficult to assign one, the outflow became intermittent, or spasmodic, or subject to sudden impulses, concentric waves would be propagated over the pool and would throw up the scoria or the solidifying lava in a circular bank at the limit of the fluid area.

This hypothesis does not differ greatly from the ebullition theory proposed by Professor Dana, the American geologist, to explain these formations. He considered that the lunar ring-mountains were formed by an action analogous to that which is exemplified on the earth in the crater of Kilauea, in the Hawaiian islands. This crater is a large open pit exceeding three miles in its longer diameter, and nearly a thousand feet deep. It has clear bluff walls round a greater part of its circuit, with an inner ledge or plain at their base, raised 340 feet above the bottom. This bottom is a plain of solid lavas, entirely open to day, which may be traversed with safety (we are quoting Professor Dana’s own statement written in 1846, and therefore not correctly applying to the present time): over it there are pools of boiling lava in active ebullition, and one is more than a thousand feet in diameter. There are also cones at times, from a few yards to two or three thousand feet in diameter, and varying greatly in angle of inclination. The largest of these cones have a circular pit or crater at the summit. The great pit itself is oblong, owing to its situation on a fissure, but the lakes upon its bottom are round, and in them, says Professor Dana, “the circular or slightly elliptical form of the moon’s craters is exemplified to perfection.”

Now Dana refers this great pit crater and its contained lava-lakes to “the fact that the action at Kilauea is simply boiling, owing to the extreme fluidity of the lavas. The gases or vapours which produce the state of active ebullition escape freely in small bubbles, with little commotion, like jets over boiling water; while at Vesuvius and other like cones they collect in immense bubbles before they accumulate force enough to make their way through; and consequently the lavas in the latter case are ejected with so much violence that they rise to a height often of many thousand feet and fall around in cinders. This action builds up the pointed mountain, while the simple boiling of Kilauea makes no cinders and no cinder cones.”

Professor Dana continues, “If the fluidity of lavas, then, is sufficient for this active ebullition, we may have boiling going on over an area of an indefinite extent; for the size of a boiling lake can have no limits except such as may arise from a deficiency of heat. The size of the lunar craters is therefore no mystery. Neither is their circular form difficult of explanation; for a boiling pool necessarily, by its own action, extends itself circularly around its centre. The combination of many circles, and the large sea-like areas are as readily understood.”[10]

In justice to Professor Dana it should be stated that he included in this theory of formation all lunar craters, even those of small size and possessing central cones; and he put forth his views in opposition to the eruptive theory which we have set forth, and which was briefly given to the world more than twenty-five years ago. As regards the smallest craters with cones, we believe few geologists will refuse their compliance with the supposition that they were formed as our crater-bearing volcanoes were formed: and we have pointed out the logical impossibility of assigning any limit of size beyond which the eruptive action could not be said to hold good, so long as the central cone is present. But when we come to ring-mountains having no cones, and of such enormous size that we are compelled to hesitate in ascribing them to ejective action, we are obliged to face the possibility of some other causation. And, failing an explanation of our own that satisfied us, we have alluded to the few hypotheses proffered by others, and of these Professor Dana’s appears the most rational, since it is based upon a parallel found on the earth. In citing it, however, we do necessarily not endorse it.

CHAPTER X.
PEAKS AND MOUNTAIN RANGES.

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.