GEOLOGY OF THE SCHROON LAKE QUADRANGLE 6l 



the large body of anorthosite. In certain other quadrangles, like 

 the Blue Mountain, the writer has found af least two sets of 

 pegmatite dikes notably different in age, one occurring in the form 

 of narrow masses essentially parallel to the foliation of, and not 

 very sharply separated from, the inclosing syenite or granite, and 

 the other, generally coarser grained, cutting across the foliation 

 of, and in sharp contact with, syenite or granite. The second set 

 is quite certainly the younger, and probably all the observed peg- 

 matites in the granites of the Schroon Lake quadrangle belong with 

 the younger set.^ 



Diabase Dikes 



Diabase dikes were observed in nine localities within the quad- 

 rangle, but doubtless others exist. They are represented on the 

 geologic map. Most of these are like the usual diabase dikes of 

 the Adirondacks, more particularly like those of the North Creek 

 quadrangle described by the writer. Unlike the gabbro, aplite, and 

 pegmatite, several of the diabase dikes cut the anorthosite. The 

 diabase probably represents the latest Precambrian intrusion in the 

 Adirondack region as shown by the fine-grained texture and usually 

 very distinct chilled borders, and also by the fact that a diabase 

 dike actually cuts one of the late pegmatite dikes in the adjoining 

 North Creek quadrangle. A diabasic texture is not always evident 

 to the naked eye, but in thin section it is generally recognizable. 

 None are porphyritic, but several show very distinct magmatic 

 flow-structure foliation. All have sharp contacts against the coun- 

 try rocks. 



About one-fifth of a mile north of the old graphite mine at the 

 western base of Catamount hill, a small typical diabase dike with 

 strike N 40° E cuts Grenville gneisses. 



A diabase dike 2 feet wide with strike S 20° W is well exposed 

 in the syenite of the quarry one-half of a mile east-northeast of 

 South Schroon. 



In a ledge of gneissoid granite by the road i mile west of Schroon 

 Lake village, there are several small dikes varying in width from 

 2 to 8 inches. One is faulted 6 or 8 inches at two places. 



A very typical diabase dike with a maximum width of 40 feet 

 and strike N 20° E is clearly traceable for fully one-half of a mile 

 in the Whiteface anorthosite about 1V2 miles north-northwest of 



^ Certain Adirondack pegmatites are discussed by the writer in a recent 

 paper (Jour. Geol., vol. 27, no. i, 1919) where it is shown that some pegma- 

 tites developed as satellites of the cooling magmas of the late gabbros. 



