GEOLOGY OF THE LAKE PLACID QUADRANGLE . 23 



Megascopic features. A glance at 35 or 40 specimens of White- 

 face anorthosite shows the usual rock to be medium grained and 

 white, light gray, pale greenish gray, or, more rarely, pinkish gray, 

 depending upon the color of the plagioclase feldspar. In most 

 cases the greenish tint seems to be due to stains of chlorite or 

 serpentine which have resulted from the decomposition of the dark 

 minerals. A few specimens contain no large uncrushed cores of lab- 

 radorite, but the outcrops from which such specimens come show 

 these large labradorites to be sporadically present. Nearly all the 

 specimens contain from i to 10 or 12 per cent of dark minerals, 

 these being principally pyroxene and hornblende, with garnet and 

 biotite less common, and tiny grains of oxides and sulphides of 

 iron in most specimens. 



A gneissoid structure is generally well enough developed to be 

 readily noticeable in the hand specimens, this being particularly 

 true of the rocks relatively richer in dark minerals. Many of the 

 rocks show more or less evidence of granulation, sometimes to an 

 excessive degree, but many others appear not to have been notice- 

 ably granulated. 



The most typical Whiteface anorthosite, so well exposed at the 

 top of Mt Whiteface, is medium grained, and consists of white 

 plagioclase (all or nearly all labradorite) with 5 to 12 per cent of 

 dark minerals scattered through the mass parallel to a crude foli- 

 ated structure. Such a rock stands out in marked contrast against 

 the most typical Marcy anorthosite which has nearly the same com- 

 position, but which is very coarse grained, light to dark bluish gray, 

 and rarely foliated. Since both types are differentiates of the same 

 cooling magma, they are not sharply separated, and it is often diffi- 

 cult in the field to draw other than arbitrary lines between them. 

 This matter is more fully discussed below. 



Local variations of the more typical Whiteface anorthosite are 

 richer in dark minerals, which may make up from 15 to 30 per 

 cent of the rock. In short, they are nearly always clearly foliated 

 gabbroic anorthosites with white or light-gray feldspar. Such 

 rocks are not abundant and they do not exist as rather definite 

 borders about the anorthosite like the gabbroic border phase of 

 anorthosite in the Long Lake quadrangle, as described by Gushing. 

 Rather, these gabbroic phases occur very locally as zones or belts 

 here and there throughout the areas of Whiteface anorthosite. 

 So far as could be made out, they are not different from the 

 typical Whiteface rock except for the higher content of dark 



