I.

THE GEOLOGICAL HISTORY OF NEW YORK
ISLAND AND HARBOR.

By J. S. NEWBERRY.

NEW YORK IN ANCIENT GEOLOGICAL TIMES.-The rocks which compose New York Island and underlie the adjacent country on the north and east are chiefly gneiss and mica-schist, with heavy intercalated beds of coarse-grained, dolomitic marble and thinner layers of serpentine. These are all distinctly stratified, and have once been sedimentary beds deposited horizontally-sandstones, shales, and limestones-but now, upheaved and set on edge, are by metamorphism converted into compact crystalline strata with the obliteration of all fossils—if fossils they contained. The age of these rocks has not yet been accurately determined, although they have usually been supposed to be Lower Silurian, and a continuation of those which contain the marble-beds of Western Massachusetts and Vermont. There are some reasons, however, why they should be regarded as still older. That they do not form the southern prolongation of the marble belt of Vermont is indicated by the facts that both the marble-beds and the rocks associated with them are so unlike in the two localities that they can hardly be parts of the same formation. In Vermont, the marbles occur in what is essentially a single belt, are fine-grained, usually banded and mottled, are nearly pure carbonates of lime, and the rocks immediately associated with them are gray siliceous limestones, quartzites, and slates. In Westchester County, and on New York Island, on the contrary, the marbles are very coarsely crystalline dolomites (double carbonates of lime and magnesia), which occur in a number of parallel belts, are generally of uniform white or whitish color, and have no rocks associated with them that can represent the quartzites and argillites of Vermont. On the whole, the group of strata which forms New

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York Island has so great a similarity to some portions of the Laurentian series in Canada that it is difficult to resist the conviction that they are of the same age.

The Canadian series is supposed to be not less than 50,000 feet in thickness, consisting of somewhat different elements in different parts, but mainly of gneiss and crystalline schists with numerous beds of dolomitic marble and serpentine, and containing, as most characteristic minerals, magnetic iron-ore and apatite (phosphate of lime). The beds stand at a high angle, and, although having once formed great folds and even mountains, by ages of surface-erosion they have been worn down to a merely undulating surface. On the east bank of the Hudson, at and above New York, we have almost precisely the same state of things, viz. 1. A belt of crystalline rocks forming apparently a continuous series to and beyond the Connecticut line; 2. Strata set nearly vertical, once forming high hills or mountains, now worn down by long exposure to a mere rolling surface; 3. The series composed chiefly of gneiss and crystalline schists, with heavy beds of dolomitic marble and thinner bands of serpentine; and, 4. Containing in its western portion where it joins the New Jersey iron belt-with which it is inseparably connected-important beds of magnetic iron-ore, while apatite is one of the most common disseminated minerals. From these and other reasons which might be mentioned, the New York rocks are regarded by the writer as of Laurentian age. They seem to have formed a ridge which was a part of a range of highlands that ran down on the eastern side of our continent, having the same general direction with the Alleghanies, but being very much older than the more recent folds of that chain. Indeed, judging from the character of the rocks composing it, the immense amount of surface-erosion it has suffered, and the absence of overlying strata, we must regard it as one of the oldest portions of the continent.

Staten Island is in part a continuation of the New York belt of Laurentian rocks—the eastern side being composed of granite and serpentine, the western of trap and Triassic sandstone-and owes its relief to that fact. South of this point the ridge sinks down and is covered

with more recent strata, but it apparently reappears at Trenton and Philadelphia. Thus it would seem to be a sort of spur of the Blue Ridge, the oldest chain of the Alleghany belt, diverging from it in Fulton County, New York, and following a nearly parallel course southwestward.

During the Palæozoic ages, the New York ridge seems to have been a land-surface; for the Silurian, Devonian, and Carboniferous rocks were deposited on both sides of it in New England, New York, and Pennsylvania, but no traces of them have been found upon it. In each of these ages the sea flowed in over some portion of the continent, and deposited on the inundated surfaces sediments containing more or less complete representatives of the prevalent forms of life; and these, now fossilized, afford means for identifying and classifying the strata.

In the Cambrian age the continent, composed of Laurentian and Huronian rocks, was broad and high, and the Cambrian strata (Acadian group) were deposited only along its margin.

At the beginning of the Silurian age the sea rose over its shores, covering most of the land-surface, but leaving the Canadian highlands, the Adirondacks, the Blue Ridge, with its New York spur, unsubmerged. Then during all the thousands of years in which the Trenton limestone group was accumulating by organic agencies, the slow growth and deposition after death of the hard parts of animals, and the other thousands of years in which the Hudson River and Utica shales were formed in a shallowing sea, this old land was exposed to wear from rain and wind, sun and frost.

In like manner when the Upper Silurian and Devonian seas in turn flooded more limited portions of the adjacent lands, covering them with new layers of sediment, the old ridges and highlands which have been enumerated, with large additions to their areas made in the Silurian age, were suffering constant abrasion and reduction of altitude.

In the Carboniferous age all the country for a great distance east, north, and west of New York, was above the sea, but along the coast in Rhode Island and Eastern Massachusetts were marshes where a luxuriant vegetation was forming peat-beds that were destined, in after-times, to become seams of coal; and in Pennsylvania, and thence westward in ' Ohio and Illinois, were vast tracts of swamp-half water, half land— which are now the most extensive coal-basins in the world. During all these ages the belt of highlands which separates the valley of the Hudson from that of the Connecticut was probably much higher than now, and stood as a witness of the varying phases of the unending war between land and sea, and saw the continent created and destroyed again and again; but in all these changes it took no part.

In the latter part of the Carboniferous age the Alleghanies proper were gradually elevated, the convex folds forming mountain-ridges, the depressed or synclinal arches becoming the slowly-deepening coalbasins. In the end all the country between the Atlantic and the Mississippi stood as a broad and elevated continental area. Subsequently the sea rose and fell upon its margin, leaving there the record of its oscillations in the deposits of the recent geological ages, but no considerable portion of its surface has since been submerged.

The Triassic age was a stormy one in the region about New York.

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 along 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