GEOLOGY OF THE SCHROON LAKE QUADRANGLE 21 



it clear that the Adirondack anorthosite is by no means an almost 

 perfectly homogeneous mass of plagioclase. The main bulk of the 

 Marcy anorthosite contains at least 5 to 10 per cent of minerals 

 other than plagioclase. Portions with about 10 per cent are com- 

 mon, and in many places there are 10 to 20 per cent, or even more, 

 of dark minerals. It is also true that some portions of the great 

 mass contain less than 5 per cent of femic constituents. Con- 

 servatively estimated, the average Marcy anorthosite carries fully 

 10 per cent of minerals other than plagioclase. 



In the writer's work in both the Lake Placid and Schroon Lake 

 quadrangles, many observations have been made of anorthosite- 

 gabbro and more typical anorthosite exhibiting perfect gradations 

 from one into the other. Such gabbroid f acies exist locally through- 

 out the body of Marcy anorthosite, in many places as rather dis- 

 tinct zones or belts a few feet or rods wide, and in other places 

 on much larger scales. Many other gabbroid portions are much 

 more irregular in shape, and not so distinctly separated from the 

 purer Marcy anorthosite. 



The anorthosite-gabbro very commonly, and the typical Marcy 

 anorthosite less commonly, locally exhibit more or less well-devel- 

 oped foliation with exceedingly variable strikes. Marked varia- 

 tions in degree of foliation often occur in single ledges. It is also 

 important to note that granulation, so prevalent throughout the 

 anorthosite body, shows many extreme variations, often in single 

 outcrops. 



Another variation of the anorthosite from a pure plagioclase 

 rock consists in the dustlike (schillerization) inclusions of a dark 

 mineral, probably ilmenite, in the labradorite. These are so numer- 

 ous as to cause most of the labradorites to have a dark bluish gray 

 color. Thus even the plagioclase crystals are not pure lime-soda 

 feldspar. 



Finally in this connection, attention should be called to the pres- 

 ence of a very appreciable amount of potash in the typical Marcy 

 anorthosite, as shown in an analysis made for Professor Kemp. 

 Whether this potash exists in regular potash-feldspar form, or is 

 part of the labradorite proper, it is additional proof that the anor- 

 thosite is not a practically pure mass of lime-soda feldspar. 



Some exam-pies of variations of Marcy anorthosite. Near the 

 top of the hill I mile a little to the west of north of Blue Ridge 

 village, the following variations across the strike from south are 

 finely exhibited: first, there is typical Marcy anorthosite; then, a 

 band 2 feet wide of gneissoid, moderately coarse-grained, gabbroid 



