GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YOKK 57 



unmetamorphosed except locally in certain shear zones. One of the 

 simplest and least modified of this type is no. 115 (plate 20), a 

 basalt. 



The rock weathers to a buff color. The fresh fracture shows 

 a dense rather dark greenish gray surface, f elsitic to finely crystalline 

 with occasional fine-grained, roundish pyrite aggregates. In thin 

 section small feldspar and olivine pseudomorphs can be seen. The 

 olivine is completely ahered to serpentine. The rock is cut by many 

 small intersecting veinlets of zoisite and serpentine. 



Many dikes also cut the Storm King granite in Breakneck ridge 

 and the adjacent gneisses of Crows Nest and neighboring ground. 

 The rock here is coarser and it has more nearly the habit of a 

 diorite than of any other type. Some may have been diabasic. As 

 a matter of fact, they show considerable variety. They cut the 

 Storm King granite but do not cut the Cambrian quartzite in any 

 case yet observed. 



Peridotite or dunite. A curious rock which may be related to the 

 diabases is the dunite exposed near Tompkins hill. It forms a small 

 hillock which is cut by the road and is surrounded by gneisses or 

 granite. The contact is drift-covered. It is a fine-grained rock, 

 weathering buff colored. No phenocrysts can be distinguished. In 

 thin section it is seen to be an almost pure olivine rock, with a few 

 magnetite grains. There is slight serpentinization which gives a 

 faint silky luster to the fractured surface. This rock (no. 57) is 

 illustrated by a photomicrograph (plate 21). 



Mixed t5^es. Under this head are listed a few typical represent- 

 atives of a very large group of rocks which are less constant in their 

 petrography than those just described. In most cases they are 

 judged to be either mixtures such as syntectic products, or they are 

 impregnation effects or injection on a microscopic scale or they are 

 extreme facies of the types just described. Perhaps some belong 

 to units of still a different connection not prominent enough to be 

 mapped independently. Only one in this list, however, is considered 

 of enough independent prominence to be indicated on the map 

 and this is the hornblende gneiss which, if there is a representative 

 in this district at all, must be the equivalent of the Pochuck forma- 

 tion as described in New Jersey. All the others are included in map- 

 ping with the major units in which they are involved. 



a Hornhlende-plagioclase Gneiss {Pochuck). Hornblende-plagio- 

 clase or pyroxene-plagioclase representatives are common in small 

 and scattered development or so involved with other kinds of 



