EELATION OF SYENITE-GRANITE SERIES 427 



considered later than the anorthosite solely on the basis of his (Cush- 

 ing's) findings in the Long Lake quadrangle.'^ Now, Cnshing and I both 

 believe the syenite-granite series to be distinctly later, and I have found 

 abundant evidence in support of this view in both the Lake Placid and 

 Schroon Lake quadrangles. I feel confident that if Bowen had knov^n 

 of these important additional field facts, and he had seen my detailed 

 geologic maps, he would not have been so much inclined to minimize the 

 evidence for the distinctly later intrusion of the syenite-granite series. 

 This evidence will now be rather fully considered. 



DIKES OF SYENITE AND GRANITE IN ANORTHOSITE 



Some years ago, in his report on the geology of the Long Lake quad- 

 rangle, Cushing^^ showed that the syenite there is distinctly younger than 

 the anorthosite. In one small area several narrow, well defined dikes of 

 syenite cut the typical Marcy anorthosite, one of these dikes being several 

 miles within the border of the great anorthosite body. He presents the 

 evidence in detail. Having seen some of these dikes, I, fully agree with 

 Gushing that the evidence is decisive and it is unnecessary to repeat de- 

 tails here. In the same report Gushing states that one of the small out- 

 lying masses of anorthosite is "definitely cut by syenite which sends dikes 

 into it.'' 



As a result of the surveys of the Lake Placid and Schroon Lake quad- 

 rangles, various excellent examples of narrow tongues or dikes of syenite 

 and granite cutting anorthosite have come to light. Since these consti- 

 tute important evidence regarding the relation of the syenite-granite 

 series to the anorthosite, they will be described somewhat in detail. 



One mile south of Morgan Pond (Lake Placid quadrangle), on a prom- 

 inent spur of Wilmington Mountain, two dikes of quartz syenite clearly 

 exposed for 50 or 60 feet cut through a big bare ledge of typical White- 

 face anorthosite (see figure 1). The dikes are from 10 to 20 feet wide, 

 and they are quite certainly offshoots from the considerable body of quartz 

 syenite which grades into granite to the west. As would be expected in 

 such narrow dikes^ the syenite is finer grained than usual, but otherwise 

 it is quite normal. The dike rock is slightly gneissoid and its contacts 

 against the anorthosite are not perfectly sharp, there having been, appar- 

 ently, a little assimilation along the borders. 



Along the brook, three-quarters of a mile west of The Flume (Lake 

 Placid quadrangle), a dike of quartz syenite 20 feet wide cuts typical 

 Marcy anorthosite. This dike is an offshoot from the considerable body 

 of similar syenite extending south westward. 



3*H. p. Gushing: N. Y. State Mus. Bull. 115, 1907, pp. 480-484. 



