l8 NEW YORK STATE MUSEUM 



term in the east necessitates holding either that the Grenville 

 series is also older than the Lower Huronian, or that the Lauren- 

 tian granite is younger than about Lake Superior, since the granite 

 is certainly younger than the Grenville. 



GRENVILLE SCHISTS 



As has been stated, Grenville schists exist in such great variety 

 and with such rapid alternations as to defy detailed mapping and 

 to render detailed description laborious and profitless. The schists 

 are everywhere intricately involved with hard, white, garnetif- 

 erous gneisses which, heretofore regarded as sediments, seem to 

 the writer to be plainly igneous rocks. They cut the schists intru- 

 sively and develop pegmatites. In the majority of exposures they 

 are merely injected along the foliation planes of the schist, form- 

 ing injection gneisses and looking extremely like interbanded 

 sediments. The more common of the schists are mica schists, and 

 the prevailing Grenville combination of the quadrangle consists 

 of the interbanded mica schist and white granite. 



These mica schists vary from very weak rocks with abundant 

 mica to much 'firmer ones in which mica is scant. Because of 

 weakness, the former variety is seldom seen in outcrop, but several 

 cuts through such schists expose them well along the Adirondack 

 Railroad a mile north of Saratoga. The firmer varieties outcrop 

 everywhere. 



These schists are feldspar-quartz-mica combinations, and nearly 

 everywhere contain in addition pink garnets. The mica is biotite 

 and the bulk of the feldspar is plagioclase, oligoclase to andesine. 

 Quartz forms in general from lo to 25 per cent of the rock. Folia- 

 tion is thorough and even. 



On the one hand these mica schists grade over into amphibolites, 

 which are heavy black gneisses composed essentially of plagioclase 

 feldspar and hornblende, with usually black mica and pyroxene in 

 addition; on the other hand they grade into hard, light colored 

 feldspar-quartz gneisses, by diminution in the mica present. 



The garnets seem to owe their origin to contact action of the 

 white granite upon the schists, as will be later shown. Graphite is 

 a frequent mineral in the schists. 



The schists are in chief part metamorphosed shales, as indicated 

 clearly by their composition and structure. They have been entirely 

 recrystallized, injected in complex fashion by granite, and vastly 

 changed in appearance and character. Originally they varied 



