[Illustration: THE FORMATION OF THE CENTRAL CONE. FINAL ACTION OF A LUNAR VOLCANO.] [Illustration: 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. [Illustration: Fig. 32.]