ROGERS, GEOLOGY OF THE CORTLAXDT SERIES 21 



ders of the area are not, however, susceptible of close determination; a 

 heavy blanket of drift covers the district and obscures the geology. Out- 

 crops are few and are generally considerably altered, except wherj the rock 

 is artificially exposed. The land surface is on the whole rather flat for a 

 rock of this character, as compared to the hilly topography southward; 

 this may be due to the fact that it lies in the lee of the Highlands, which 

 would cause an undue accumulation of till at this point, fading away to 

 the south. The granite is surrounded in general by mica schist, but to 

 the northwest lies an area of pinkish granitic gneiss, related to the typical 

 Highland gneisses. 



As stated above, the credit for the recognition of this rock as part of 

 the Cortlandt series belongs to Dr. Berkey. He has as yet published 

 only a short note on the subject. 22 Dana does not mention this area at 

 all ; and since he apparently guided Williams over the country, the latter 

 likewise overlooked it. Closer work on the geology, however, reveals its 

 unmistakable relationship with the neighboring basic rocks. Its entire 

 lack of (megascopic) metamorphism separates it sharply from the sur- 

 rounding schists and gneisses and places its age as approximately the same 

 as that of the rocks to the south. It has undergone about the same de- 

 gree of weathering as have the basic rocks. Furthermore the latter are 

 frequently penetrated by aplite and pegmatite dikes, which are closely 

 akin to the granite ; these are especially abundant in the northern part of 

 the basic area. There are, however, apparently no areas of granite in the 

 latter district; the acidic flow seems to have been more sharply separated 

 than the various intermediate and basic facies of the magma. The pene- 

 tration of the basic rocks by the acidic dikes would seem to indicate that 

 the acid extreme of the series is the youngest; the chronological and 

 chemical relations of the various members will, however, be more fully 

 discussed at the end of the petrographical descriptions. 



In the hand specimen, the granite is practically white when fresh, 

 being made up almost entirely of quartz and feldspar, with very subor- 

 dinate amounts of muscovite and biotite. The rock is generally weath- 

 ered at the surface to a faint dirty brownish color, owing to the formation 

 of epidote. The grain is medium, and the rock is very firm. A number 

 of specimens were taken from the surface outcrops; several from the 

 quarries, and one from a diamond drill boring at the 200 ft. level. The 

 thin sections show that the rock is remarkably uniform in composition. 

 Quartz, in large angular grains, constitutes in general about one-half the 

 rock. It shows numerous rehealed cracks but few inclusions. The feld- 



- "The acid extreme of the Cortlandt Series," Science, XXVIII, 575. 1908. 



