GEOLOGY OF THE SCHROON LAKE QUADRANGLE 45 



road just north of the village of Keene in the Lake Placid quad- 

 rangle. 



Fifteen areas of mostly Keene gneiss are represented on the 

 writer's Lake Placid geologic map and the rocks are described in 

 the accompanying report. Gushing has described rocks which 

 probably belong in the same category, from two localities on the 

 western side of the great anorthosite area. Cushing suggests that 

 these rocks, particularly in the Long Lake quadrangle, are mag- 

 matic assimilation products. Kemp has described certain peculiar 

 types of gabbro, called the Woolen Mill and Split Rock Falls types, 

 as occurring in the Elizabethtown quadrangle. Kemp says 

 nothing regarding the origin of these types, but, in the writer's 

 judgment, they are to be classed as Keene gneiss. These seem to 

 be the only rocks of the sort in the Adirondack region regarding 

 which even brief published statements by other workers have been 

 made. The whole problem of the Keene gneiss is rather fully dis- 

 cussed in the writer's recent paper 1 on "Adirondack Anorthosite." 

 "~ Megascopic characters. The typical Keene gneiss presents a 

 different appearance from any other Adirondack rock. In the 

 Lake Placid and Schroon Lake quadrangles, the typical rock is 

 medium grained, gneissoid, notably granulated, and looks much 

 like some facies of the syenite-granite series except for scattering 

 phenocrysts of bluish gray labradorites up to an inch long. 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 pheno- 

 crysts are absent or only sparingly present, and such facies of the 

 Keene gneiss are often difficult to distinguish in the field from 

 certain phases of the syenite-granite series. Under the micro- 

 scope, however, the distinction may generally be made. A gneissoid 

 structure is nearly always present but it varies notably, in some 

 leases being practically absent. The fresh rock is usually greenish 

 gray, and it weathers brown. 



Microscopic characters. The mineral contents of thin sections 

 of selected samples of various phases of the rock from the Schroon 

 Lake quadrangle are shown in table 3. It is quite clear from this 

 table that the Keene gneiss is mostly distinctly intermediate in 

 composition between the syenite-granite and the anorthosite. 



1 Geol. Soc. Amer. Bui. v. 29, no. 4. 1918. 



