FETTKE, MANHATTAN SCHIST OF NEW YORK 197 



usually unsymmetrical and frequently are overturned toward the west. 

 The axes of the folds have a general northeast and southwest trend and 

 usually have a gentle pitch toward the southwest. As a result of later 

 planation through erosion, the formations are now exposed in a series of 

 fairly parallel belts running nearly northeast and southwest. The lime- 

 stone belts on account of their easier erosion are usually carved out by the 

 valleys. Of the other two formations, the Fordham gneiss is the most 

 resistant one, but usually the outcrops of both these formations are marked 

 by ridges. Faulting in two directions, parallel with the folds and across 

 them, has occurred. This has complicated their exposures. The lime- 

 stone which normally should appear between the' schist and gneiss has at 

 times been cut out entirely or else its apparent thickness has been con- 

 siderably reduced. 



Historical Review 



Since the region underlain by the Manhattan schist was explored and 

 settled long before the science of geology had begun to attract any atten- 

 tion in this country, we find references made to the local formations as 

 soon as men began the study of geology in North America. 



One of the earliest references to the Manhattan Schist appeared in 

 P. Cleveland's "Elementary Treatise on Mineralogy and Geology," which 

 was published in 1816. In it H. H. Hay den called attention to a granite 

 ridge which crossed Manhattan Island, appeared again at Hurlgate on 

 Long Island and then extended into Connecticut. This ridge is now 

 known to have been merely a protruding portion of the Manhattan schist 

 which underlies the greater part of the island. William Maclure's first 

 geological map of the United States appeared in the same volume. He 

 placed the rocks underlying Manhattan Island in his primitive formation. 



Among other of the earlier discussions on the geology of southeastern 

 New York is that of Samuel Akerly,'* who described the formations under- 

 lying Manhattan Island and Westchester County in 1820. Akerly recog- 

 nized granites, gneisses, schists and limestones, all of which he placed in 

 the Primitive formation on account of their crystalline character and 

 absence of fossils. 



L. D. Gale*^ in 1839, in his account of the geology of New York County, 

 described the rocks as consisting chiefly of gneisses and associated serpen- 

 tine, hornblende, primary limestones and anthophyllite rock. 



* An essay on the geology of the Hudson River, and the adjacent regions, Illustrated 

 by a geological section of the county, from the neighborhood of Sandy Hook, in New Jer- 

 sey, northward, through the Highlands in New York, towards the Catsklll Mountains. 

 New York, 1820. 



<^ "Report on the Geology of New York County." Third ann. rept. Geol. Surv. New 

 York, pp. 177-199. 1839. 



