GEOLOGY OF THE LAKE PLACID QUADRANGLE 43 



These dikes consist almost wholly of microperthite and horn- 

 blende in large masses. It seems very clear that the pegmatites of 

 this sort were intruded very soon after the intrusion of the anortho- 

 site and that they are, therefore, distinctly older than the ordinary 

 granite-pegmatites above described. 



Keene Gneiss 



General statements. One of the most interesting rocks of the 

 quadrangle is locally developed as belts or irregular bodies along 

 portions of the borders between the anorthosite and syenite-granite. 

 Both the Marcy and the Whiteface types of anorthosite show such 

 border rocks. There is very strong evidence, based upon field work 

 and a study of thin sections, that this is really a transition rock 

 between the anorthosite and the syenite or granite due to actual 

 digestion or assimilation of anorthosite along its border by the 

 invading syenite-granite magma. It is here proposed that this rock 

 be called the Keene gneiss because a fine exposure of the typical 

 fresh rock occurs by the road at the northern edge of the village 

 of Keene. In view of the fact that many geologists maintain that 

 there are no definitely proved cases of magmatic assimilation on 

 considerable scales at least, the evidence furnished by these rocks 

 of the Lake Placid quadrangle has been very carefully considered 

 by the writer who is convinced that actual assimilation has taken 

 place. In other words, the Keene gneiss seems to be quite cer- 

 tainly a good example of a " hybrid " rock, to use the term sug- 

 gested by Harker, the English geologist, who has maintained that 

 such rocks have been produced either by the mixing of two distinct 

 magmas, or by the assimilation of solid rock by magmas. Fifteen 

 areas of Keene gneiss are represented on the geologic map which 

 accompanies this bulletin. Others probably exist but were not 

 located owing to scarcity of outcrops or roughness of country in 

 some places. 



The typical Keene gneiss presents a different appearance from 

 any of the other rocks of the region, and it is usually readily recog- 

 nizable. The main body of the typical rock is medium grained, 

 gneissoid, notably granulated, and looks much like a rather basic 

 phase of syenite, but it contains scattering phenocrysts of bluish 

 gray labradorite up to an inch in length. These phenocrysts, which 

 are rounded and usually elongated parallel to the foliation of the 

 rock, doubtless represent cores of crystals which survived the 

 process of granulation. Locally the phenocrysts of labradorite are 



