GEOLOGY OF THE SCHROON LAKE QUADRANGLE 17 



quadrangle where, as a result of its careful study, important light 

 has been thrown upon the age, relations, structure, and origin of 

 the great anorthosite body. Most of the northeastern half of the 

 quadrangle, or an area of over 80 square miles, is occupied by 

 anorthosite to the exclusion of all other formations except Pleis- 

 tocene deposits and a few small basic dikes. 



Marcy type of the anorthosite. By far the most abundant 

 general facies of the rock may be called Marcy anorthosite because 

 of its great exposures on Mount Marcy in the quadrangle next 

 to the north. The most typical portions of the Marcy anorthosite 

 are coarse grained, light to dark bluish gray, and consist largely of 

 basic plagioclase feldspar, mainly labradorite. The dark bluish 

 gray labradorite crystals usually vary in length from a fraction of 

 an inch to several inches, crystals about an inch long being very 

 common. Among other places, labradorites from 5 inches to I 

 foot long were observed on the western of the three Peaked hills, 

 and on the ridge i mile north-northwest of Blue Ridge village. 

 Only occasionally do these labradorites exhibit the play of colors 

 so characteristic of this species of feldspar. Twinning striations 

 are often evident to the naked eye on the cleavage faces. 



Accessory minerals visible to the naked eye are large individuals 

 of pyroxene and hornblende, and small individuals of biotite, 

 ilmenite, pyrite, garnet, and more rarely chalcopyrite or pyrrhotite. 

 These accessory minerals ordinarily constitute 5 to 10 per cent of 

 the typical coarse anorthosite, but there are local developments of 

 the rock which are made up almost entirely of plagioclase, and still 

 others, rather abundantly developed as zones, bands and irregular 

 masses, which contain from 10 to 25 or more per cent dark minerals, 

 these last named types being really anorthosite-gabbros. Such 

 gabbroid facies are more fully described below. 



An important facies of the anorthosite is one in which the dark 

 labradorites, from a few millimeters to an inch or more across, 

 stand out conspicuously in a distinctly granulated groundmass of 

 feldspar. The granulated material varies from light gray to pale 

 greenish gray. It is very evident that the large labradorites are 

 roughly rounded uncrushed cores of what were considerably larger 

 individuals before the rock was subjected to the process of granu- 

 lation. All degrees of granulation are exhibited to extreme cases 

 where the rock has been so thoroughly granulated that few, if 

 any, labradorite cores remain. 



