ELIZABETHTOWN AND PORT HENRY QUADRANGLES 35 



the anorthosite. They themselves are chiefly a mixture of quartz 

 and diopside. Each of these minerals makes up about half of the 

 slide, the only additional component being a small grain of apatite. 

 The crystals range up to 2 millimeters in diameter. The diopside 

 is of irregular outline, of a pale green color, with slight pleo- 

 chroism to yellow. The quartz is strained around the edges, and 

 in one slide is cloudy at the center of the crystals because of 

 innumerable, minute acicular inclusions, probably rutile, and ori- 

 ented in every direction. Presumably the original rock was a 

 quartzose and somewhat calcareous clastic, which on recrystalliza- 

 tion has afforded the minerals described. The edges of the inclu- 

 sion, being melted into the more calcareous anorthosite yielded the 

 garnet rims. 



Rocks of this composition are well recognized members of the 

 Grenville, and have been described by H. P. Gushing under 

 the name of quartz-diopside rock in Museum Bulletin 115, 

 pages 504-8. To the unaided eye they resemble coarsely crystal- 

 line quartzites and as such were collected in the field. The best 

 explanation of these curious masses of rock is the one which refers 

 them to original Grenville strata pierced by the intrusive anortho- 

 site, which tore off and included fragments and did not entirely 

 absorb them. 



Border facias of anorthosite. Around the borders of the main 

 great intrusion, the anorthosites in this as in neighboring areas take 

 on more dark silicates and lose also the distinctive bluish or green- 

 ish color of the feldspar, which is quite characteristic of the cen- 

 tral portions. Professor Gushing has observed the same feature in 

 the Long Lake area,^ and has given it a special color on his map. 



Several years ago while in the field upon the Lake Placid quad- 

 rangle, this feature was noted in the rocks of Whiteface mountain 

 and in the notes the rock was called the Whiteface type. A sample 

 from the summit of this mountain was analyzed by George Steiger 

 in the laboratory of the United States Geological Survey with the 

 results given below. The feldspar of the rock is white and shows 

 evidence of crushing and granulation. The dark silicates are much 

 more abundant than in the typical anorthosite and besides the 

 pyroxenic minerals, diopside and hypersthene, hornblende is fre- 

 quent. 



This same type of rock has been noted northeast and east of 

 New Russia and it forms a small prong of Oak hill. 



The Whiteface type has a medium percentage of silica as the 

 anorthositic rocks run. There are other varieties with somewhat 



IN. Y. State Mus. Bui. 115, p. 473. 



