GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK IQ 



of the Pochuck gneiss. I'he igneous is represented by three gneisses, 

 the Pochuck, Byram and Losee, which are the dark-colored, medium, 

 and Hght-colored types respectively. The Pochuck gneiss is horn- 

 blendic and probably corresponds to the hornblendic gneiss of the 

 West Point sheet. It is believed to be partly igneous and partly 

 sedimentary, but so much metamorphosed that the original character 

 is indeterminable. Its relation to the other gneisses is unknown 

 but probably it is older. The metamorphism may have been pro- 

 duced during the invasion of the granitoid gneisses. Those, known 

 as the Losee and Byram, are more distinctly granitic and are inter- 

 layered with the Pochuck and with each other. Granite and peg- 

 matite cut all the gneisses. 



The following quotations suggest the chief structural concep- 

 tions : " The limestone and dark gneiss together seem to constitute 

 a matrix holding the intrusive granitoid rocks in the form of rela- 

 tively thin but extended plates." The gneisses are " so intricately 

 mingled that detailed representation of their distribution is quite 

 impracticable." " The varieties of gneiss are seldom found in large 

 masses free from intermixture with other sorts, but the different 

 facies or varieties occur in tabular masses which are interlayered 

 both on a large and on a small scale." " That large amounts of pre- 

 existing rock material have been more or less completely dissolved 

 or assimilated by the invading magmas is suspected, but can not be 

 ascertained." " Throughout New Jersey evidence of crushing in 

 the minerals of the gneisses is almost entirely wanting, and appear- 

 ances strongly favor the belief that the gneissic foliation is original 

 in the invading rocks of the Precambrian complex." 



On the opposite side of this field in close enough proximity to 

 demand equally careful consideration is the work of the Connecticut 

 State Survey. The new state map includes representations of some 

 of the same formations. The Connecticut geologists have rendered 

 a good service in the discrimination of differences and in marking 

 bounds and limits, but little attempt is made in the matter of cor- 

 relation or genesis of the more obscure types. 



In 1905 the senior author of this bulletin began work on the 

 crystallines of southeastern New York for the New York Survey, 

 starting in the Tarrytown quadrangle. The New York City area 

 im.mediately south had been mapped and the description of its geol- 

 ogy had been issued as a geologic folio'^ so that this seemed to be the 

 most logical place to begin. 



'Geologic Folio No. 83, U. S. Geo!. Survey (1902). New York City and 

 Vicinity. F. J. H. Merrills, N. H. Darton, A. Hollick, R. D. Salisbury, 

 R. E. Dodge, Bailey Willis, H. A. Pressey. 



