Sandstone, in which these impressions are principally discovered, is a rock composed chiefly of siliceous and micaceous particles cemented together by calcareous or argillaceous paste, containing salt, and colored with various shades of the oxide of iron, particularly the red, gray, brown. It has been remarked by Prof. H. D. Rogers, that the perfection of the surface containing fossil footmarks is often attributable to a micaceous deposit. The layers of sandstone have been formed by deposits from sea-water, dried in succession; such layers are also seen in the roofing slate. These deposits on the shores of the ocean, having in a soft condition received the impressions of the feet of birds, other animals, vegetables, and also of rain-drops, under favorable circumstances dried, hardened, and formed a rock of greater or less solidity. Our colleague, Dr. Gould, has exhibited to us a specimen of dried clay from the shores of the Bay of Fundy, containing beautiful impressions, recently made, of the footsteps of birds. The particles brought by the waves, and deposited in the manner described, were derived from the destruction of other rocks previously existing, particularly granite and flint, or silex, the shining atoms of which compose no small part of the sandstone rock. It is easy to conceive, that, while these deposits were taking place in the soft condition, portions of vegetable matters might become intermixed; and that these, with the impressions of the feet and other parts of animals and unorganized substances, might be preserved by the process of desiccation. The agency of internal heat may have also been employed in some cases in baking and hardening these crusty layers. The sandstone rock, though in some places actually in a state of formation at the present time, lies in such a manner in the earth's crust as to indicate an immense antiquity. The age of these beds varies in different situations. The sandstone rocks which contain the greater part of the impressions are called _new red sandstone_, to distinguish them from the _old red_, which is of a greater age. The deposits on Connecticut River may not be attributed to the action of this river, but are of higher antiquity, probably, than the river itself, and proceeded from the waves of an ancient sea, existing in a state of the surface of the globe very different from that of the present day. In 1834, tracks were discovered near Hildberghausen in Saxony, to which Prof. Kaup, of Darmstadt, gave the name of Chirotherium, from the resemblance to the impressions of the human hand. On a subsequent examination, Prof. Owen preferred the name of Labyrinthodon, from the resemblance of the folds in the teeth to the convolutions of the brain.