The descent from the mountains upon the series of rocks that occupy the plains, is one not merely of space, but likewise of time. A geological epoch has vanished, and a new order of things has been called into existence. This implies a change in the animal as well as in the mineral kingdom. The change may not have been sudden, but it has been thorough and pervading, accompanied by circumstances that show a general shift in the sea-bottom, and causes that have been nearly uniform in their operation over the surface of the globe. The shift in the sea-bottom is detected in the elevation of the silurian group of rocks, which have been lifted from a horizontal into a highly-inclined position: in some instances they are nearly vertical; and in most cases where the igneous rocks occur, they are bent and twisted, greatly altered and disrupted, by the process of upheaval to which they have been subjected. Geology notes in this an epoch or age of organic existence. The superjacent series of rocks are seen lying unconformably upon the silurians, that is, the older series had been consolidated and upheaved, and a period of intervening time had elapsed before the deposition of the newer. The fossils imbedded are likewise distinct and peculiar—one and the same over the superficial area of the globe—and thus we learn to mark the great and interesting cosmical changes which had already begun to be effected. We are now among the Old Red Sandstone, or Devonian system of rocks, so denominated from their great development in that district of the sister kingdom. As contrasted with the former system, the rocks of this period indicate considerable disturbance in the waters of the ocean, currents and agitations widely prevailing, and perhaps also deeper seas. The crust of the earth was still rising, and the mountains becoming higher, and these effects would necessarily follow. A superior order of animals were introduced. The fishes, which begin to appear in the upper beds of the silurian group, are now increased both in numbers and in variety of structures. The invertebrata were the prevailing types of the former age. The old red sandstone is pre-eminently characterized by the vertebrata, when, completely adapted to the element to be inhabited, mailed and plated over with thick horny scales, huge bony heads, fins and tails of corresponding strength and size; the Sauroid family appear upon the stage, capable all of buffeting the waves and fulfilling their destiny amid the greatest commotions. The fish of this early period are generally well preserved, even better than those of the tertiary age, in consequence of their osseous scales being harder than the bones, and which, from their interlocked arrangement, have contributed to preserve the general form of the body when the inner skeleton has disappeared and every other part and organ have been destroyed.