QUARRY MATERIALS OF NEW YORK 53 



Lower Siluric systems. Berkey would separate them into an earlier 

 Precambric and a later or Paleozoic group, of which the Precambric 

 group is made up of the Lowerre quartzite, Inwood limestone and 

 Manhattan schist — the members that belong more strictly to the 

 Highlands region. The later, or Paleozoic formations, are the 

 Poughquag quartzite, Wappinger limestone and Hudson River 

 slates ; they occur only in small down-faulted areas in the Highlands, 

 but have a very widespread distribution north of there, particularly 

 the slates which outcrop along the whole central Hudson valley. 

 The Yonkers gneiss may be mentioned in connection with the 

 Precambric, as an igneous derivative, later than the Fordham 

 gneiss with which it is in contact. It occurs in a long narrow belt 

 and in isolated bodies in southern Westchester county. According 

 to the earlier interpretation, as advanced by Merrill, its age is later 

 than the Hudson River slates. The rock has considerable local im- 

 portance as a building stone. 



The period of Precambric history, so far as it can be formulated 

 from the rocks of the New York areas, began, therefore, with the 

 accumulation of sediments composed of quartzose, argillaceous and 

 calcareous materials that are collectively known as the Grenville 

 series. They must have been derived from some preexisting rocks 

 which, if still found anywhere, represent the Archean or basal 

 complex of the Lake Superior and Canadian regions, but so far 

 no vestiges of this older surface have been identified. Subsequent 

 to their deposition, there was a long lapse of time in which the 

 forces of upheaval, metamorphism and igneous activity were mani- 

 fested at intervals on a tremendous scale. The sediments were 

 compressed, plicated and completely recrystallized. Their lower 

 parts were invaded and broken up by deep-seated intrusions, 

 representing several different periods and rock varieties. Volcanic 

 energy was also displayed and led no doubt to extensive accumula- 

 tions of lavas and other igneous materials at the surface. By these 

 agencies, the land areas must have acquired a very considerable 

 elevation, probably with a rugged mountainous topography to which 

 the surface of the present day is hardly comparable as to altitude 

 and massive features. Upon such land surfaces erosion would be 

 very active and powerful in its results. Destruction thus was in 

 progress while the upbuilding went on ; while in the latter part of 

 the Precambric time there was a long period of continued erosion 

 without compensation by uplift. The effects of this were the re- 

 moval of an immense but unknown thickness of rock from the 

 upper zone, leaving the deeper buried parts exposed much as they 



