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, 1 3/4 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, newly 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 debris, 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 every where 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

A A. Fissures gaping downwards and injected by intumescent lava beneath. B B B. Fissures gaping upwards and allowing wedges of rock to drop below the level of the intervening masses. C C. Wedges forced upwards by horizontal compression. E F. Neutral plane or pivot axis, above and below which the directions of the tearing strain and horizontal compression are severally indicated by the smaller arrows; the larger arrows beneath represent the direction of the primary expansive force.

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 ringformed 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 am 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."[1]

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.