[Illustration]

[Illustration]

Although the Vertebrate division of the Animal Kingdom still waited for its higher classes, yet it had received one important addition since the Silurian and Devonian periods.  The Carboniferous marshes were not without their reptilian inhabitants; but they were Reptiles of the lowest class, the so-called Amphibians, those which are hatched from the egg in an immature condition, undergoing metamorphosis after birth.  They have no hard scales, and lay a large number of eggs.  I am unable to present any figure of one of these ancient Reptiles, as they are found in so imperfect a state of preservation that no plates have been made from them.  I would add in connection with this subject that I believe a large number of animals found in the Carboniferous deposits, and referred to the class of Reptiles, to be Fishes allied to Saurians.

Before leaving the Carboniferous period, let us see what territory the United States has conquered from the Ocean during that time.  All its central portion, from Canada to Alabama, and from Western Iowa, Missouri, and Arkansas to Eastern Virginia, was raised above the water.  But as yet the Alleghanies and the Rocky Mountains did not exist; a great gulf ran up to the mouth of the Ohio, for the Mississippi had not yet accumulated the soil for the fertile valley through which it was to take its southern course; the Coral-Builders had still their work to do in constructing the peninsula of Florida; and, indeed, all the borders of the continent of North America, as well as a large part of its Western territory, were still to be added.  But although its central portion held its ground and was never submerged again, yet the continent was slowly subsiding during the middle geological periods, so that, instead of enlarging gradually by the increase of deposits, its limits remained much the same.

This accounts for the very scanty traces to be found in America of the secondary deposits; for the Permian, Triassic, and Jurassic beds, instead of being raised to form successive shores, along which their deposits could be accumulated in regular sequence, as had been the case with the Azoic, Silurian, and Devonian deposits in the northern part of the United States, were constantly sinking, so that the Triassic settled above the Permian, the Jurassic above the Triassic, and so on, each set of strata thus covering over and concealing the preceding one.  Though we find the stratified rocks of these periods cropping out here and there, where some violent disturbance or the abrading action of water has torn asunder or worn away the overlying strata, yet we never find them consecutively over any extensive region; and it is not till the Cretaceous and earlier Tertiary periods that we find again a regular succession of deposits around the shores of the continent, marking its present outlines.  It is, then, in Europe, where the sequence of their beds is most complete, that we must seek to decipher the history of the middle geological ages; and therefore, when I meet my readers again, it will be in the Old World of civilization, though more recent in its physical features than the one we leave.