220 N. L. BOW EN 



of plagioclase, while other exposures, perhaps equally general and 

 widespread, would average nearly 10 per cent bisilicates. This 

 latter type seems to prevail even in the heart of the area being rep- 

 resented in most of the exposures of the Keene Valley, while the 

 bare ledges of the summit of Mt. Marcy average probably more 

 than 5 per cent bisilicates. Toward its borders, too, the anortho- 

 site commonly passes into anorthosite-gabbro and gabbro. Never- 

 theless, the mass as a whole is aptly described as consisting of 

 "little else than feldspar which is generally a blue labradorite." 1 

 If this great mass, whose volume is to be measured in thousands of 

 cubic miles, was ever molten as such, it is remarkable that none of 

 the many investigators who have studied the area have found a 

 single dike consisting of nearly pure plagioclase in the surrounding 

 rocks. The evidence of the intrusive nature of anorthosite in its 

 more typical development depends on the occurrence of Grenville 

 inclusions in it. To' be sure, the anorthosite is nearly always 

 immediately surrounded by a younger rock, the syenite, but in 

 several localities the invading power of certain phases of the anor- 

 thosite mass is well shown. Anorthosite-gabbro invades the 

 Grenville and associated older gneisses in places, and occurs as 

 outlying masses upward of 20 miles distant from the main anortho- 

 site mass. As soon, then, as the bisilicates mount to 20 or 25 per 

 cent there is no lack of evidence of the power of the mixture to 

 penetrate into openings in the surrounding rocks. The great mass 

 of the anorthosite itself contains much fewer bisilicates, yet in spite 

 of the overwhelming volume of such material it is entirely unrepre- 

 sented as dikes and small intrusions. It seems to be a reasonable 

 conclusion that this material was incapable of being injected into 

 the older rocks. 



Intimate relation of syenite and anorthosite. — The anorthosite core 

 of the Adirondack igneous mass is surrounded practically every- 

 where by the syenite-granite series with which are associated 

 numerous areas of Grenville sediments and perhaps older granite 

 gneiss. There has been a considerable tendency to consider the 

 syenite-granite as an igneous unit and anorthosite as a separate 

 unit. This tendency has been emphasized perhaps by the fact 



1 D. H. Newland, N.Y. State Museum Bull, ug, 1908, p. 17. 



