GENERAL DESCRIPTION 405 



type of the anorthosite which occurs abundantly on Mount Whiteface, 

 near Lake Placid. At the summit of Mount Whiteface the rock is me- 

 dium grained and consists of white plagioclase (chiefly labradorite) with 

 10 to 15 per cent of dark minerals scattered through the mass parallel to 

 a crude foliated structure. This rock stands out in marked contrast 

 against the typical Marcy anorthosite, which is not so gabbroid, very 

 coarse grained, light to dark bluish gray, and generally not well foliated. 

 More exceptionally the Whiteface type is nearly pure white, being quite 

 free from femic minerals. Much of the rock, however, is locally richer 

 in dark minerals, which may constitute 15 to 30 per cent of the whole. 

 The minerals other than the feldspar are practically the same as in the 

 Marcy anorthosite. 



Whiteface anorthosite is unusually extensively developed within the 

 Lake Placid quadrangle. It there has a very irregular distribution, but 

 in a broad way it is quite certainly to be regarded as a border phase of 

 the Marcy anorthosite. 



My very recent survey of the Schroon Lake quadrangle shows that the 

 Whiteface rock there exists as a distinct border facies of the Marcy anor- 

 thosite, though it has been notably cut to pieces and more or less assimi- 

 lated by the syenite-granite series, as explained beyond. 



I have also noted Whiteface anorthosite as a border facies in the New- 

 comb quadrangle and in portions of the Ausable quadrangle. 



Kemp describes a considerable development of Whiteface anorthosite 

 as a border or rim facies of the more typical anorthosite in the Elizabeth- 

 town and Port Henry quadrangles. He maps it with "basic anorthosite 

 and related types," so that its actual extent is not shown. 



Gushing has described a commonly occurring gabbroid border facies of 

 the western border of the anorthosite, particularly in the Long Lake quad- 

 rangle. Eegarding the latter he says : 



"This border gabbro is a rather uniform grained rock, of sufficient coarse- 

 ness, so that the white of the feldspar, the red of the garnet, and the blaclv of 

 the pyroxene, hornblende, and magnetite are all prominent. In the less ex- 

 treme phases of the rock occasional small unerushed cores remain." ^ 



Judging by Cushing's description elsewhere,^ this western border facies 

 of the anorthosite differs from the typical Marcy anorthosite in being 

 usually less gneissoid and generally not so characterized by white feldspar. 



Enough detailed work has therefore been done to make it evident that 

 the great body of Adirondack anorthosite has a pretty well defined border 



H. p. Gushing: N. Y. State Miis. Bull. 115, 1007, p. 474, 

 H. P. Gushing: N. Y. State Mus. Bull. 95, 1905, pp. 310-311. 



