T he task of distributing the palæozoic sedimentary rocks of New York into natural divisions became easy after the publication of the ‘Silurian System’ of Sir Roderick Murchison. This State is peculiarly favourable to such studies, from the directions, lofty sides, and other features of its principal lakes and rivers. Many of the latter, such as the Genesee, Black, and Onondaga Rivers, rise in the south, and, running north, cross and lay bare successive strata in the most satisfactory manner. Other streams, like the Mohawk, in the eastern part of New York, rise in the west, and also traverse nearly the whole series. Glacial and other natural erosions, as well a s some prolonged artificial excavations, disclose the outcrops of the strata in a succession of exposures unparalleled in number and extent. In the counties of Herkimer and Otsego the cliffs often attain an elevation of a thousand feet, followed closely by others surmounting them. No country in the world, says De Verneuil, presents so complete and uninterrupted a development of the Silurian and Devonian systems, if we except Sweden, where it is on a small scale. In these regions, therefore, our investigations are far simpler and more decislve than amid the disturbances of manv parts of Europe. We are in the position of the anatomist who examines a perfect animal instead of a mass of dismembered fragments. As types of comparison, the palaeozoic rocks of this State are to be preferred to any other, as a whole. In the following