REPORT OF THE DIRECTOR AKD STATE GEOLOGIST 1903 197 



a great formation of banded gneiss, cut and injected at many 

 points by later eruptives and of which nothing more could be said 

 than that it was Precambrian. 



After extended reconnaissance and local study, in the Pre- 

 cambrian area, certain conclusions have been reached which make 

 it possible to express more definitely the character of the Pre- 

 cambrian formations. 



In the vicinity of New York city the prevailing rock is a gray 

 and black banded gneiss, the gray bands consisting of quartz, 

 orthoclase and biotite and the dark bands containing much 

 biotite with some hornblende. With local variations in the pro- 

 portions of the two materials of its banding, the general character 

 of the rock persists throughout the Precambrian areas of West- 

 chester county and it may be recognized here and there along 

 certain lines in Putnam county, chiefly on the lower slopes of the 

 mountain ranges as a rule showing a predominance of the lighter 

 colored rock which is essentially a white feldspar gneiss with a 

 small amount of biotite. The same characters prevail as one 

 passes northeastward through Dutchess county over the Pough- 

 quag area and to Dover mountain and its Connecticut extension, 

 the Kent-Cornwall area of Percival^ who used the following 

 description. 



The predominant rock of this formation is the white felspathic, 

 dark mica-seamed granitic gneiss, varying, by the different pro- 

 portions of its feldspar and mica, from a nearly white, thicker, 

 more granitic variety, to a lighter or darker gray, thinner and 

 more schistose variety. 



These variations occur in alternate beds, one or the other pre- 

 dominating in different sections of the formation. 



Passing farther northeastward into Massachusetts one en- 

 counters the Becket gneiss formation of Emerson which has in 

 places the same general characters. 



The facts intended to be shown by the map are self-explanatory. 

 Details of the geology of the Precambrian rocks are reserved 

 for a separate paper on this subject which the writer has now in 

 preparation. 



^Percival. Report on Connecticut. New Haven 1843. 



