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.
THE FORMATION OF THE CENTRAL CONE. FINAL ACTION OF A LUNAR VOLCANO.
PLATE XIII
ARZACHAEL, PTOLEMY, and the RAILWAY.
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.
