CHAPTER VIII.

ON LUNAR CRATERS.

As we stated in our brief general description of the visible hemisphere of the moon, and as a cursory glance at our map and plates will have shown, the predominant features of the lunar surface are the circular or amphitheatrical formations that, by their number, and from their almost unnatural uniformity of design, induced the belief among early observers that they must have been of artificial origin. In proceeding now to examine the details of our subject with more minuteness than before, these annular formations claim the first share of our attention.

By general acceptation the term “crater” has been used to represent nearly all the circular hollows that we observe upon the moon; and without doubt the word in its literal sense, as indicating a _cup_ or circular cavity, is so far aptly applied. But among geologists it has been employed in a more special sense to define the hollowing out that is found at the summit of some extinct, and the majority of active, volcanoes. In this special sense it may be used by the student of the lunar surface, though in some, and indeed in the majority of cases, the lunar crater differs materially in its form with respect to its surroundings from those on the earth; for while, as we have said, the terrestrial crater is generally a hollow on a mountain top with its flat bottom high above the level of the surrounding country, those upon the moon have their lowest points depressed more or less deeply below the general surface of the moon, the external height being frequently only a half or one-third of the internal depth. Yet are the lunar craters truly volcanic; as Sir John Herschel has said, they offer the true volcanic character _in its highest perfection_. We have upon the earth some few instances in which the geological conditions which have determined the surface-formation have been identical with those that have obtained upon the moon; and as a result we have some terrestrial volcanic districts that, could we view them under the same circumstances, would be identical in character with what we see by telescopic aid upon our satellite. The most remarkable case of this similarity is offered by a certain tract of the volcanic area about Naples, known from classic times as the _Campi Phlegræi_, or burning fields, a name given to them in early days, either because they showed traces of ancient earth-fire, or because there were attached to the localities traditions concerning hot-springs and sulphurous exhalations, if not of actual fiery eruptions. The resemblance of which we are speaking is here so close that Professor Phillips, in his work on Vesuvius, which by the way contains a historical description of the district in question, calls the moon a grand Phlegreian field. How closely the ancient craters of this famous spot resemble the generality of those upon the moon may be judged from Plate VI., in which representations of two areas, terrestrial and lunar, of the same extent, are exhibited side by side, the terrestrial region being the volcanic neighbourhood of Naples, and the lunar a portion of the surface about the crater Theophilus.

In comparing these volcanic circles together, we are however brought face to face with a striking difference that exists between the lunar and terrestrial craters. This is the difference of magnitude. None of those Plutonian amphitheatres included in the terrestrial area depicted exceed a mile in diameter, and few larger volcanic vents than these are known upon the earth. Yet when we turn to the moon, and measure some of the larger craters there, we are astonished to find them ranging from an almost invisible minuteness to 74 miles in diameter. The same disproportion exists between the depths of the two classes of craters. To give an idea of relative dimensions, we would refer to our illustration of Copernicus[8] and its hundreds of comparatively minute surrounding craters. Our terrestrial Vesuvius would be represented by one of these last, which upon the plate measures about the twentieth of an inch in diameter! And this disproportion strikes us the more forcibly when we consider that the lunar globe has an area only one-thirteenth of that of the earth. In view of this great apparent discrepancy it is not surprising that many should have been incredulous as to the true volcanic character of the lunar mountains, and have preferred to designate them by some “non-committal” term, as an American geologist (Professor Dana) has expressed it. But there is a feature in the majority of the ring-mountains that, as we conceive, demonstrates completely the fact of volcanic force having been in full action, and that seems to stamp the volcanic character upon the crater-forms. This special feature is the central cone, so well known as a characteristic of terrestrial volcanoes, accepted as the result of the last expiring effort of the eruptive force, and formed by the deposit, immediately around the volcanic orifice, of matter which there was not force enough to project to a greater distance. Upon the moon we have the central cone in small craters comparable to those on the earth, and we have it in progressively larger examples, upon all scales, up to craters of 74 miles in diameter, as we have shown in Plate VII. Where, then, can we draw the line? Where can we say the parallel action to that which placed Vesuvius in or near the centre of the arc of Somma, or the cone figured in our sectional drawing of Vesuvius (Fig. 3) in the middle of its present crater—where can we say that the action in question ceased to manifest itself on the moon, seeing that there is no break in the continuity of the crater-and-cone system upon the moon anywhere between craters of 1¾ miles and 74 miles in diameter? We have, it is true, many examples of coneless craters, but these are of all sizes, down to the smallest, and up to a largeness that _would_ almost seem to render untenable the ejective explanation: of these we shall specially speak in turn, but for the present we will confine ourselves to the normal class of lunar craters, those that have central cones, and that are in all reasonable probability truly volcanic.

And in the first place let us take a passing glance at the probable formative process of a terrestrial volcano. Rejecting the hypothesis of Von Buch, which geologists have on the whole found to be untenable, and which ascribes the formation of all mountains to the elevation of the earth’s crust by some thrusting power beneath, we are led to regard a volcano as a pyramid of ejected matter, thrown out of and around an orifice in the external solid shell of the earth by commotions engendered in its molten nucleus. What is the precise nature and source of the ejective force geologists have not perfectly agreed upon, but we may conceive that highly expanded vapour, in all probability steam, is its primary cause. The escaping aperture may have been a weak place since the foundations of the earth were laid, or it may have been formed by a local expansion of the nucleus in the act of cooling, upon the principle enunciated in our Third Chapter; or, again, the expansile vapour may have forced its own way through that point of the confining shell that offered it the least resistance. The vent once formed, the building of the volcanic mountain commenced by the out-belching of the lava, ashes, and scoria, and the dispersion of these around the vent at distances depending upon the energy with which they were projected. As the action continued, the ejected matter would accumulate in the form of a mound, through the centre of which communication would be maintained with the source of the ejected materials and the seat of the explosive agency. The height to which the pile would rise must depend upon several conditions: upon the steady sustenance of the matter, and upon the form and weight of the component masses, which will determine the slope of the mountain’s sides. Supposing the action to subside gradually, the tapering form will be continued upwards by the comparatively gentle deposition of material around the orifice, and a perfect cone will result of some such form as that represented below, which is the outline ascribed by Professor Phillips to Vesuvius in pre-historic, or even pre-traditional times, and which may be seen in its full integrity in the cases of Etna, Teneriffe, Fussi-Yamma, the great volcanic mountain of Japan, and many others. The earliest recorded form of Vesuvius is that of a truncated cone represented in Fig. 17, which shows its condition, according to Strabo, in the century preceding the Christian Era.

Now this form may have been assumed under two conditions. If, as Phillips has surmised, the mountain originally had a peaked summit with but a small crater-orifice at the point, then we must ascribe its decapitation to a subsequent eruption which in its violence carried away the upper portion, either suddenly, or through a comparatively slow process of grinding away or widening out of the sides of the orifice by the chafing or fluxing action of the out-going materials. But it is probable that the mountain never had the perfect summit indicated in our first outline. The violent outburst that caused the great crater-opening of our second figure may have been but one paroxysmal phase of the eruption that built the mountain: a sudden cessation of the eruptive force when at its greatest intensity, and when the orifice was at its widest, would leave matters in an opposite condition to that suggested as the result of a slow dying out of the action: instead of the peak we should have a wide crater-mouth. It is of small consequence for our present purpose whether the crater was contemporaneous with the primitive formation of the mountain, or whether it was formed centuries afterwards by the blowing away of the mountain’s head; for upon the vast scale of geological time, intervals such as those between successive paroxysms of the same eruption, and those between successive eruptions, are scarcely to be discriminated, even though the first be days and the second centuries. We may remark that the widening of a crater by a subsequent and probably more powerful eruption than that which originally produced it is well established. We have only to glance at the sketch, Fig. 18, of the outline of Vesuvius as it appeared between the years A.D. 79 and 1631 to see how the old crater was enlarged by the terrible Pompeian eruption of the first-mentioned year. Here we have a crater ground and blown away till its original diameter of a mile and three-quarters has been increased to nearly three miles. Scrope had no hesitation in expressing his conviction that the external rings, such as those of Santorin, St. Jago, St Helena, the Cirque of Teneriffe, the Curral of Madeira, the cliff range that surrounds the island of Bourbon, and others of similar form and structure, however wide the area they enclose, are truly the “basal wrecks” of volcanic mountains that have been blown into the air each by some eruption of peculiar paroxysmal violence and persistence; and that the circular or elliptical basins which they wholly or in part surround are in all cases true craters of eruption.

When the violent outburst that produces a great crater in a volcanic mountain-top more or less completely subsides, the funnel or escaping orifice becomes choked with débris. Still the vent strives to keep itself open, and now and then gives out a small delivery of cindery matter, which, being piled around the vent, after the manner of its great prototype, forms the inner cone. This last may in its turn bear an open crater upon its summit, and a still smaller cone may form within _it_. As the action further dies away, the molten lava, no longer seething and boiling, and spirting forth with the rest of the ejected matter, wells upwards slowly, and cooling rapidly as it comes in contact with the atmosphere, solidifies and forms a flat bottom or floor to the crater.

It may happen that a subsequent eruption from the original vent will be comparable in violence to the original one, and then the inner cone assumes a magnitude that renders it the principal feature of the mountain, and reduces the old crater to a secondary object. This has been the case with Vesuvius. During the eruption of 1631 the great cone which we now call Vesuvius was thrown up, and the ancient crater now distinguished as Monte Somma became a subsidiary portion of the whole mountain. Then the appearance was that shown in Fig. 19, and which does not differ greatly from that presented in the present day. The summit of the Vesuvian cone, however, has been variously altered; it has been blown away, leaving a large crateral hollow, and it has rebuilt itself nearly upon its former model.

When we transfer our attention to the volcanoes of the moon, we find ourselves not quite so well favoured with means for studying the process of their formation; for the sight of the building up of a volcanic mountain such as man has been permitted to behold upon the earth has not been allowed to an observer of the moon. The volcanic activity, enfeebled though it now be, of which we are witnesses from time to time on the earth, has altogether ceased upon our satellite, and left us only its effects as a clue to the means by which they were produced. If we in our time could have seen the actual throwing up of a lunar crater, our task of description would have been simple; as it is we are compelled to infer the constructive action from scrutiny of the finished structure.

We can scarcely doubt that where a lunar crater bears general resemblance to a terrestrial crater, the process of formation has been nearly the same in the one case as in the other. Where variations present themselves they may reasonably be ascribed to the difference of conditions pertaining to the two spheres. The greatest dissimilarity is in the point of dimensions; the projection of materials to 20 or more miles distance from a volcanic vent appears almost incredible, until we realize the full effect of the conditions which upon the moon are so favourable to the dispersive action of an eruptive force. In the first place, the force of gravity upon our satellite is only one-sixth of that to which bodies are subject upon the earth. Secondly, by reason of the small magnitude of the moon and its proportionally much larger surface in ratio to its magnitude, the rate at which it parted with its cosmical heat must have been much more rapid than in the case of the earth, especially when enhanced by the absence of the heat-conserving power of an atmosphere of air or water vapour; and the disruptive and eruptive action and energy may be assumed to be greater in proportion to the more rapid rate of cooling; operating, too, as eruptive action would on matter so much reduced in weight as it is on the surface of the moon, we thus find in combination conditions most favourable to the display of volcanic action in the highest degree of violence. Moreover, as the ejected material in its passage from the centre of discharge had not to encounter any atmospheric resistance, it was left free to continue the primary impulse of its ejection without other than gravitative diminution, and thus to deposit itself at distances from its source vastly greater than those of which we have examples on the earth.

We can of course only conjecture the source or nature of the moon’s volcanic force. If geologists have had difficulty in assigning an origin to the power that threw up our earthly volcanoes, into whose craters they can penetrate, whose processes they can watch, and whose material they can analyze, how vastly more difficult must be the inquiry into the primary source of the power that has been at work upon the moon, which cannot be virtually approached by the eye within a distance of six or eight hundred miles, and the material of which we cannot handle to see if it be compacted by heat, or distended by vapours. Steam is the agent to which geologists have been accustomed to look for explanation of terrestrial volcanoes; the contact of water with the molten nucleus of our globe is accepted as a probable means whereby volcanic commotions are set up and ejective action is generated. But we are debarred from referring to steam as an element of lunar geology, by reason of the absence of water from the lunar globe. We might suppose that a small proportion of water once existed; but a small proportion would not account for the immense display of volcanic action which the whole surface exhibits. If we admitted a Neptunian origin to the disturbances of the moon’s crust, we should be compelled to suppose that water had existed nearly in as great quantity, area for area, there as upon our globe; but this we cannot reasonably do.

Aqueous vapour being denied us, we must look in other directions for an ejective force. Of the nature of the lunar materials we can know nothing, and we might therefore assume anything; some have had recourse to the supposition of expansive vapours given off by some volatile component of the said material while in a state of fusion, or generated by chemical combinations. Professor Dana refers to sulphur as probably an important element in the moon’s geology, suggesting this substance because of the part which it appears to play in the volcanic or igneous operations of our globe, and on account of its presence in cosmical meteors that have come within range of our analysis. Any matter sublimated by heat in the substrata of the moon would be condensed upon reaching the cold surrounding space, and would be deposited in a state of fine powder, or otherwise in a solid form. Maedler has attributed the highly reflective portions of some parts of the surface, such as the bright streams that radiate from some of the craters, Copernicus and Tycho for instance, to the vitrification of the surface matter by gaseous currents. But in suppositions like these we must remember that the probability of truth diminishes as the free ground for speculation widens. It does not appear clear how expansive vapours could have lain dormant till the moon assumed a solid crust, as all such would doubtless make their escape before any shell was formed, and at an epoch when there was ample facility for their expansion.

While we are not insensible of the value of an expansive vapour explanation, if it could be based on anything beyond mere conjecture, we are disposed to attach greater weight to that afforded by the principle sketched in our third chapter, viz., of expansion upon solidification. We gave, as we think, ample proof that molten matter of volcanic nature, when about passing to the solid state, increases its bulk to a considerable degree, and we suggested that the lunar globe at one period of its history must have been, what our earth is now, a solid shell encompassing a molten nucleus; and further, that this last, in approaching its solid condition, expanded and burst open or rent its confining crust. At first sight it may seem that we are ascribing too great a degree of energy to the expansive force which molten substances exhibit in passing to the solid condition, seeing that in general such forces are slow and gradual in their action; but this anomaly disappears when we consider the vast bulk of the so expanding matter, and the comparatively small amount that in its expansion it had to displace. It is true that there are individual mountains on the moon covering many square miles of surface, that as much as a thousand cubic miles of material may have been thrown up at a single eruption; but what is this compared to the entire bulk of the moon itself? A grain of mustard-seed upon a globe three feet in diameter represents the scale of the loftiest of terrestrial mountains; a similar grain upon a globe one foot in diameter, would indicate the proportion of the largest upon the moon. A model of our satellite with the elevations to scale would show nothing more than a little roughness, or superficial blistering. Turn for a moment to our map (Plate IV.), upon which the shadows give information as to the heights of the various irregularities, and suppose it to represent the actual size of some sphere whose surface has been broken up by reactions of some kind of the interior upon the exterior—suppose it to have been a globe of fragile material filled with some viscous substance, and that this has expanded, cracked its shell, oozed out in the process of solidification, and solidified: the irregularity of surface which the small sphere, roughened by the out-leaking matter, would present, would not be less than that exhibited in the map under notice. When we say that a lunar crater has a diameter of 30 miles, we raise astonishment that such a structure could result from an eruption by the expansive force of solidifying matter; but when we reflect that this diameter is less than the two-hundredth part of the circumference of the moon, we need have no difficulty in regarding the upheaval as the result of a force slight in comparison to the bulk of the material giving rise to it. We have upon the moon evidence of volcanic eruptions being the final result of most extensive dislocations of surface, such as could only be produced by some widely diffused uplifting force. We allude to the frequent occurrence of chains of craters lying in a nearly straight line, and of craters situated at the converging point of visible lines of surface disturbance. Our map will exhibit many examples of both cases. An examination of the upper portion (the southern hemisphere of the moon) will reveal abundant instances of the linear arrangement, three, four, five or even more crateral circles will be found to lie with their centres upon the same great-circle track, proving almost undoubtedly a connexion between them so far as the original disturbing force which produced them is concerned. Again, in the craters Tycho (30), Copernicus (147), Kepler (146), and Proclus (162), we see instances of the situation of a volcanic outburst at an obvious focus of disturbance. These manifest an up-thrusting force covering a large sub-surface area, and escaping at the point of least resistance. Such an extent of action almost precludes the gaseous explanation, but it is compatible with the expansion on consolidation theory, since it is reasonable to suppose that in the process of consolidation the viscous nucleus would manifest its increase of bulk over considerable areas, disturbing the superimposed crust either in one long crack, out of the wider opening parts of which the expanded material would find its escape, or “starring” it with numerous cracks, from the converging point of which the confined matter would be ejected in greatest abundance and, if ejected there with great energy and violence, would result in the formation of a volcanic crater.

The actual process by which a lunar crater would be formed would differ from that pertaining to a terrestrial crater only to the extent of the different conditions of the two globes. We can scarcely accept Scrope’s term “basal wrecks” (of volcanic mountains that have had the summits blown away) as applicable to the craters of the moon, for the reason that the lunar globe does not offer us any instance of a mountain comparable in extent to the great craters and whose summit has _not_ been blown away. Scrope’s definition implies a double, or divided process of formation: first the building up of a vast conical hill and then the decapitation and “evisceration” of it at some later period. There are grounds for this inferred double action among the terrestrial volcanoes, since both the perfect cone and its summitless counterpart are numerously exemplified. But upon the moon we have no perfect cone of great size, we have no exception whereby the rule can be proved. It is against probability, supposing every lunar crater to have once been a mountain, that in every case the mountain’s summit should have been blown away; and we are therefore compelled to consider that the moon’s volcanic craters were formed by one continuous outburst, and that their “evisceration” was a part of the original formative process. We do not, however, include the central cone in this consideration: that may be reasonably ascribed to a secondary action or perhaps, better, to a weaker or modified phase of the original and only eruption.

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.