REPORT OF THE DIRECTOR I917 41 



anorthosite is usually very coarse grained, bluish gray, and highly 

 feldspathic, though locally developed gabbroid facies are common. 

 It makes up the greater bulk of the anorthosite, which occupies 

 most of the northeastern half of the quadrangle. The other facies, 

 known as Whit ef ace anorthosite, is usually medium grained, light 

 gray, and more or less gabbroid. This is a broader development of 

 the Marcy anorthosite. Many small and large inclusions of anor- 

 thosite, chiefly the Whiteface type, occur in the syenite-granite 

 series as far out as 7 or 8 miles from the main body of the anorthosite. 



Next in order of age is the syenite-granite series, which is clearly 

 intrusive into both the Grenville and the anorthosite. It is wholly 

 confined to the southwestern half of the quadrangle where the 

 granite facies is the most prominent of all the rocks. 



A rock type of particular interest, called the Keene gneiss, is well 

 developed, and observations within the quadrangle have thrown 

 much light upon its origin. The two largest bodies mapped each 

 occupy several square miles. This Keene gneiss is considered to 

 have resulted from the actual digestion or assimilation of anortho- 

 site by the syenite-granite magma. 



Gabbro stocks of the usual Adirondack kind are prominently devel- 

 oped in the southwestern half of the quadrangle, but none occur 

 within the great body of anorthosite. Each of the two stocks covers 

 several square miles. 



Pegmatite and diabase dikes, both later than the gabbro, are 

 moderately represented. Also a few dikes of aplite occur, some of 

 them younger than the gabbro and some probably older. 



A small area of dolomitic limestone has long been known to occur 

 in Schroon Lake village. It is probably of Little Falls (Upper 

 Cambrian) age and about 135 feet thick. 



In 19 16 an outlier of Potsdam sandstone was discovered from 

 1 J to 2 miles southwest of Schroon Lake village. It is about one- 

 half of a mile long. 



In 191 7 an area of about 2 acres of sandstone and dolomite, 

 belonging to the Potsdam-Little Falls series, was discovered in the 

 vSchroon valley 7 miles north of Schroon Lake village and one-fourth 

 of a mile east of the border of the Schroon Lake quadrangle. 



Fifteen faults and zones of excessive jointing have been located, 

 and these have notably influenced the topographic development. 

 A number of conspicuous ridges and valleys with north-northeast 

 strike have been determined by these fault zones of weakness in 

 the rocks. 



