GEOLOGY OF THE LAKE PLACID QUADRANGLE 49 



The little hill in the eastern portion of the area consists of rather 

 mixed rocks, but it is mostly Keene gneiss with large labradorites. 

 At one place fairly coarse granite is intimately associated with 

 gabbroic Whiteface anorthosite with local development of what 

 appears to be an assimilation product of the two containing some 

 quartz. Small masses of Grenville gneiss are also commonly 

 involved with the rocks of this hill. 



Other areas. The narrow band of Keene gneiss at the southern 

 base of Catamount mountain contains large labradorites but it is 

 scarcely gneissoid. No. 11 of table 3 gives its mineral content. 



In the small area one-half of a mile north of Franklin Falls, a 

 medium-grained, rather gabbroic, gneissoid rock, without pheno- 

 crysts, has the composition of Keene gneiss as shown by no. 12 of 

 table 3. This rock grades into gabbroic Whiteface anorthosite, but 

 its relation to the nearby granite could not be determined. 



The area just north of Ov^/en pond shows big ledges of homo- 

 geneous, typical Keene gneiss. 



The small area i^ miles south-southeast of The Flume shows 

 Keene gneiss closely associated with much syenite and some White- 

 face anorthosite. 



Significance of the distribution of the Keene gneiss. That the 

 Keene gneiss is actually an assimilation product of the fusion and 

 digestion of anorthosite by syenite or granite magma is regarded 

 as proved by the evidence above presented. But such rock is not 

 universally present, as a transition or border rock between anor- 

 thosite and syenite or granite. For instance, the long boundaries 

 between the Whiteface anorthosite and granite of Mt Whiteface, 

 and between the Whiteface anorthosite and syenite from the 

 southern side of Mt Whiteface to west of Knapp hill, were crossed 

 at many places without noting any rock like the Keene gneiss. As 

 seen on the map, other areas also show an absence of Keene gneiss 

 as a border rock. It is possible that some masses of Keene gneiss 

 may have been overlooked in the rough, densely wooded country, 

 or that some may exist under cover of glacial and postglacial 

 deposits, but, in view of the detailed survey, it is certain that any 

 such masses of Keene gneiss are relatively small. How is this 

 difference in distribution of the Keene gneiss to be accounted for? 

 Also why do the borders between the Grenville and syenite-granite 

 series, as well as in the mixed gneiss areas of Grenville and syenite- 

 granite, show little or no evidence of magmatic assimilation? The 

 writer believes that the answer to these questions may be found in 

 the temperature relations of the rocks at the time of the intrusion 



