422 W. J. MILLER ADIRONDACK ANORTHOSITE 



masses and bunches of dark monoclinic pyroxene crystals, these masses 

 ranging' in size from mere specks to an inch or two long and roughly 

 parallel, causing the rock to exhibit a crude foliated structure. Closely 

 associated with this rock is a true injection gneiss which is gray, medium 

 grained, and clearly foliated. A thin section reveals the following min- 

 eral joercentages : Andesine to labradorite, 82 ; hornblende, 9 ; both green 

 and colorless pyroxenes, 7I/2 ; biotite, 1, and ilmenite, one-half. Still an- 

 other phase of the rock from the same ledge is very similar, but it is finer 

 grained and contains several per cent of pale red garnets in small scatter- 

 ing grains. It is very evident that these several facies together represent 

 a border phase of the considerable body of adjacent Grenville which has 

 been penetrated more or less intimately by Whiteface anorthosite magma. 



It might be argued that the above described intimate associations are 

 not very significant because it is Whiteface, rather than Marcy, anortho- 

 site involved with the Grenville. ^ow, although some inclusions of Gren- 

 ville in Marcy anorthosite are knov^n (see below), it does, nevertheless, 

 appear to be true, as Bowen maintains, that Grenville and Marcy anor- 

 thosite are never very intimately associated. But this is readily ex- 

 plained, first, because the outer and upper (border) portion, rather than 

 the inner portion, of the great intrusive came into contact with, and-more 

 or less cut into, the Grenville, and, second, because, as pointed out else- 

 where in this paper, there is good reason to think that the inner or Marcy 

 anorthosite portion of the intrusive was probably too viscous to h^ve had 

 much power of penetration. Furthermore, opportunity for observing the 

 effect of Marcy anorthosite on Grenville is very restricted, because the 

 Grenville has been almost, if not completely, removed from the interior 

 of the great anorthosite area. 



The occurrences above described are very significant, since most of the 

 Whiteface rock which has so intimately penetrated the Grenville is just 

 as truly anorthosite as the typical Marcy anorthosite. Much of it is, in 

 fact, made up almost entirely of pure white plagioclase. In most of these 

 mixed gneiss localities in the Lake Placid quadrangle the field evidence 

 strongly points to the occurrence of the nearly pure plagioclase Whiteface 

 anorthosite far under its surface or, in other words, not far from the out- 

 side or upper limit of the Marcy anorthosite. In such positions the 

 border facies would scarcely be any more gabbroid than the typical Miarcy 

 anorthosite, and locally it might even carry fewer femic minerals, as is 

 the case locally within the Marcy anorthosite. The very occurrence of 

 the nearly pure plagioclase anorthosite as numerous bands or dikes in or 

 as injections into the Grenville proves the rock to have once been in a 

 true magmatic condition, either wholly, or at least largely, molten as such. 



