In Vermont the marbles occur in what is essentially a single belt, are fine-grained, unusually 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 niarbles 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 east bank of the Hudson, at and above New \''ork, wc have : 1st. A belt of crystalline rocks forming apparently a continuous series to and beyond the Connecticut line; 2d. Strata set nearly vertical, once forming high hills or mountains, now worn down by long exposure to a more rolling surface ; od. The series composed chiefly of gneiss and crystalline schists, with heavy beds of dolomite marble and thinner bands of serpentine; and 4th. Containing in its western portion, where it adjoins 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. For these and other reasons Mr. Newberry regards the New York rocks as belonging to the Laureutian age.

On the other hand. Prof. James D. Dana ' holds that Westchester County is comprised within t\u Green Mountain region, that it borders the southern side of the Putnam County Archa'an, as Dutchess County does the northern, and resembles in its ordei that part of the Green Mountain region which now makes Western Connecticut. The topographical features of the county owe much to the lime-stont bolts, which, by their easy erosion, have determined the courses of river valleys, and the lines of marshes along such valleys, as well as located many of tlu lakes.