Thus far we can conjecture, with great security, taught by the manifestly wild and fierce convulsions which once threw up the mountains, since in them distortion is the rule and regularity of structure the exception, and also by the equally manifest quiet and sustained process of upheaval, when the plateaus were formed; their strata being in a state of regularity and unbroken repose. When the great vents produced by the outward pressure of internal volcanic forces occurred beneath the sea, they were filled up in the same manner as on the dry land, excepting that the summits of the mountains emerge above the surface in the form of isolated islands, or when there was a chain or group of mountains upheaved, as an archipelago. When there was no rifting of the surface, and no forcing up of whole chains of peaks through a thinned crust, the fierce action of the internal heat appears to have necessitated the upheaval of solitary volcanoes here and there, in some cases even rows of them, to give vent to the pent-up steam and gases, and to convey away the molten tide within. When such volcanic series rose in parallel ranges, they lifted, or may have lifted up the whole district between them, as if upon their shoulders, and so formed the American type of plateaus, of less breadth and greater length than the Asiatic, and in height corresponding with the volcanic peaks which form their rim, and to which they are probably indebted for the form of their structure. It needs hardly to be added to what has been said above, that the general direction of existing mountain chains depends upon the direction of the primitive seams made in the earth’s surface by internal forces. The Ural Mountains, the Scandinavian chain, the Alleghanies, the Ghauts, run on meridian lines; others more or less transversely. The various kinds of rock which have been thrown up in mountains enlighten us as to the process and results of the internal heat of the earth; the successive formations display not only the various eruptions of molten matter, and its discharge in new layers above what had been thrown out before, but reveal the relative age of the various formations. We have in a single chain sometimes a whole volume of history, marking off the epochs of upheaval with the most perfect legibility and exactness. Many crystallised rocks result evidently from the gradual process of cooling after the ancient exposure to the intense heat of the inner portions of the earth—granite, porphyry, gneiss, slates, and the so-called metamorphosed rocks. These used to be considered the oldest formations, but the upheaval theory treats them as the latest formed.