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.