GEOLOGY OF THE LAKE PLACID QUADRANGLE 4I 



the large area of granite porphyry. Hornblende appears to be a 

 constant constituent, while pyroxene often fails. No. 49 of table 2 

 is from the small area of mixed rock in the gorge one-half of a mile 

 south of Keene. 



Like the other members of the syenite-granite series, inclusions 

 of Grenville gneiss sometimes occur in this coarse granite as, for 

 instance, on Catamount mountain, and on top of the hill i mile 

 east of the south end of Silver lake. 



It seems very clear, as a result of both field and laboratory 

 studies, that the granite and granite porphyry are both only facies 

 of a single great cooling magma. The perfect gradations from 

 syenite through granitic syenite to granite and granite porphyry 

 strongly support the view that the members of the syenite-granite 

 series (except possibly the basic phase of the syenite) are differ- 

 entiation products of a single great cooling magma. Possibly some 

 of the granite may be distinctly older or younger than the syenite 

 but, except for the small granitic dikes below described, no such 

 rock could be proved to be present. 



Granite, Aplite, and Pegmatite Dikes 



The various acidic dikes which have been definitely located are 

 represented on the geologic map (each dike by a number) there 

 being twenty-one in all. There must be many others concealed 

 under glacial drift, or hidden in the woods, or not discovered where 

 actually outcropping. In some cases the age relations are not 

 altogether clear. 



Granite dikes. At dike localities i and 2 there are several pink 

 granite dikes cutting anorthosite. No. 3 is a dike of pink granite 

 75 feet long cutting Whiteface anorthosite, but not with very 

 sharply defined contacts. No. 4 is a dike 4 feet wide clearly cut- 

 ting Grenville gneiss. Dike no. 11 is pink granite cutting Marcy 

 anorthosite. At locality no. 12 several dikelike masses of granite 

 are involved with syenite and Whiteface anorthosite. 



Most of these dikes may be merely off-shoots from the great 

 syenite-granite intrusive body, but some may possibly be consider- 

 ably later in age. 



Aplite dikes. Interesting dikes of aplite were observed. At 

 no. 9, in the bed of the river east of Copperas pond, a dike of pink 

 aplite, with several small tongues, clearly cuts a gabbroic, very 

 gneissoid facies of the Whiteface anorthosite. A thin section of 

 this aplite shows 20 per cent orthoclase, 20 per cent microperthite, 



