REPORT OF THE DIRECTOR I918 2^ 



and amphibolite of the district are facies of the old gabbro. Some 

 may be younger, and some may possibly represent metamorphosed 

 portions of the Grenville series. 



Anorthosite, which is clearly younger than the Grenville, and 

 older than the syenite-granite series, occurs in several small areas. 

 It is mostly of the Whiteface type. Its age relation to the old gabbro 

 could not be definitely determined. 



Several varieties of granite, granitic syenite and quartz syenite 

 constitute the main bulk of the rocks of the quadrangle. Among 

 these the variety of chief interest is a fine to medium-grained, 

 usually pink, rock locally varying from a true granite through 

 granitic syenite to quartz syenite or even quartz diorite. A mag- 

 matic flow-structure foliation is generally more or less well developed. 

 The writer has called this rock the "Lyon Mountain granite" because 

 of its excellent exposures in and near Lyon Mountain village. The 

 rock is also extensively developed in Johnson mountain, Duncan 

 mountain, and in the valley of the Saranac river. 



There is considerable evidence to show that the Lyon Mountain 

 granite grades into, and is only a facies of, a coarse-grained, usually 

 gneissoid rock which the writer has called the "Hawkeye granite" 

 because of its excellent exposures in the vicinity of Hawkeye village. 

 This rock is extensively developed in Silver Lake mountain, in the 

 southwestern portion of the quadrangle, and in the southeastern 

 half of the great mass of Lyon mountain. 



Special mention should be made of the dikes and other masses 

 of pegmatite and silexite, and dikes of aplite, which, in age and 

 origin, are closely associated with the granites. The Lyon Mountain 

 granite contains a great profusion of pegmatite and silexite ^ masses 

 and no aplite dikes, while the Hawkeye granite contains many 

 aplite dikes and relatively few pegmatite and silexite masses. The 

 relations of the pegmatite and silexite masses to the inclosing granite 

 make it clear that they both began to develop, probably as segrega- 

 tion masses, while the granite magma still possessed a considerable 

 degree of fluidity, and that they continued to form, probably both 

 as segregation products and as dikes, until the inclosing granite 

 almost or possibly completely solidified. The silexite masses repre- 

 sent very siliceous facies of the pegmatitic development, gradations 

 from nearly pure silica to ordinary pegmatite being not uncommon. 



The term "silexite" has recently been proposed by the writer (Science, 49: 149) 

 for any body of pure or nearly pure silica of igneous or aqueo-igneous origin which 

 occurs as a dike, segregation mass, or inclusion within or without its parent body. 



