Page:Popular Science Monthly Volume 13.djvu/664

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The trough between the New York axis and the Blue Ridge was occupied by water, and in this trough the Triassic shales and sandstones were deposited. A similar trough east of New York, where now is the valley of the Connecticut, was also a lagoon or estuary in which similar sediments accumulated, but not so quietly as the strata composing the older formations in the same region were laid down. It is evident that Nature's forces were in great activity during the period under consideration, for we find the greatest diversity in the product of these forces. The Triassic beds consist of shales, sandstones, and conglomerates. Of these the shales accumulated in comparatively clear and quiet water; and at various levels we find them filled with the remains of fishes that inhabited the lagoons where they were deposited. These fishes occur in thousands, confined to layers a few inches thick, mostly complete and mature individuals, showing that they were killed suddenly by some poisoning of the water in which they lived, its complete withdrawal, or a substitution of fresh for salt, or vice versa. These fish-bearing shales alternate with conglomerates that are sometimes beds of large bowlders—the result of violent water-action alone: a shore—or with strata of ripple-marked, sun-cracked sandstone, pitted with the impressions of rain-drops, and bearing the footprints of thousands of animals, great and small, which made these mud-banks their feeding-grounds. Here and there we find twigs of coniferous trees of the Araucarian family, or fragments of the fronds of cycads and ferns; much more frequently casts of the trunks and branches of trees mingled pell-mell, and evidently collections of drift-wood.

The footprints referred to above are generally three-toed, and resemble the tracks of birds. In dimension they vary from one to twenty inches long, and are supposed to have been made by a peculiar group of biped, birdlike reptiles, which possessed the world in Mesozoic times, and inhabited the shores of North America in great numbers during the Triassic age.

The alternations of coarse and fine strata, with their characteristic fishes and footprints, are repeated in the Trias on the west side of the Hudson until they form a series which has a thickness of several thousand feet. As the ripple-marks, sun-cracks, and other evidences of exposure to the air, occur at several levels, they prove the gradual subsidence of the trough where those sediments accumulated, with which the filling from the wash of the land kept pace, affording a succession of fresh surfaces where the winds and waves as well as living creatures left their autographs. Although as yet but partially examined and imperfectly read, these records, like the Assyrian tablets, have told us many interesting things, and they constitute a treasury of ancient lore which is destined for ages to supply new material for the geological history of this region.

From what we have already learned of the circumstances in which the Triassic rocks of our neighborhood were formed, we may conclude