Page:Popular Science Monthly Volume 13.djvu/662

642 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.