414 W. J. MILLER ADIRONDACK ANORTHOSITE 



eral differentiation in situ shown by the anorthosite is into anorthosite- 

 gabbro and gabbro, and not into syenite." I am in hearty accord with 

 this thesis. In all my work I have never found anything like positive 

 proof for either syenite or granite as a differentiate of anorthosite. 



Daly-* says: "Most petrographers agree that anorthosite is a direct 

 derivative of gabbroid magma/' and that transitional forms and intimate 

 relationships between anorthosite and gabbro are common phenomena. 

 He gives illustrations from various regions ; bnt he does not refer to any 

 such relationship between syenite or granite and anorthosite. There are, 

 however, certain rocks in the xldirondacks intermediate between S3^enite 

 or granite and anorthosite-gabbro or anorthosite; but there is very strong 

 evidence that such rocks were formed by assimilation of anorthosite by 

 the later syenite or granite magma and not by differentiation in si- v. 

 The nature and origin of these rocks are rather fully considered beyond 

 in this paper. 



The chilled Gabbroid Boeder Facies axd its Significance 



general obsertatioxs 



As already pointed out, the great body of Adirondack anorthosite has 

 a more or less well developed gabbroid border facies. Not only is tliis 

 border facies usually richer in femic minerals and notably finer grained 

 than the typical Marcy anorthosite, but it is also usually more gneiss(nd 

 with white instead of dark bluish gray feldspar. The Whiteface anor- 

 thosite above described is rather typical of this sort of rock. 



DISTRIBUTION AXD RELATION TO OTHER ROCKS 



Gushing was the first to definitely recognize the existence of the gab- 

 broid border facies on the western side of the anorthosite area. He re- 

 gards it as a chilled gabbroid border of the anorthosite. Within the Long 

 Lake quadrangle he maps a belt of this rock usually from 1 to 2 miles 

 wide, and he states that there is a perfect transition from anorthosite 

 through anorthosite-gabbro to gabbro. It is very important to note that 

 he finds no evidence whatever for either the present or former existence 

 of syenite intermediate between the true anorthosite and the gabbroid 

 border facies. His map does, however, show one broad tongue of syenite 

 to have locally cut out most of the border phase, this syenite containing 

 several mappable inclusions of the gabbroid border rock. 



My recent work in the Schroon Lake quadrangle very clearly shows the 

 Whiteface anorthosite there to be a gabbroid border facies of the slyiot- 



2* R. a. Daly : Igneous Rocks and Their Origin, 1914, p. 324. 



