l8 NEW YORK STATE MUSEUM 



Much of the typical Marcy anorthosite is devoid of foHation, 

 though in some local zones of almost perfectly pure plagioclase 

 rock there is a notable tendency for the feldspars to show a crude 

 parallelism (plate 6). The more gabbroid facies of the rock, how- 

 ever, often exhibit a fair to well-defined foliation accentuated by 

 the crudely parallel arrangement of the dark minerals. 



In thin section, with a low power of the microscope, the larger 

 labradorites are usually seen to be filled more or less with myriads 

 of very dark dustlike particles, probably ilmenite. The minerals 

 contained in several thin sections of the Marcy anorthosite are 

 shown in table i below. 



Chilled border facies of the anorthorsite (Whiteface anortho- 

 site). Around the borders of the great body of Adirondack 

 anorthosite, and in some places a number of miles within it, there 

 is quite generally a notable development of white or very light-gray 

 labradorite and an increase in the femic minerals causing such rocks 

 to be anorthosite-gabbro or even gabbro. Such rocks, well devel- 

 oped in the Schroon Lake quadrangle, are almost invariably finer 

 grained and lighter colored than the typical Marcy anorthosite, 

 though in some localities a few large, scattering labradorite indi- 

 viduals occur. A foliated structure is generally evident. 



Although they are more or less variable in general appearance 

 and composition, the writer has proposed that these border phases 

 of the anorthosite be classed as Whiteface anorthosite, a name 

 given by Professor Kemp to the type which occurs abundantly on 

 Mount Whiteface near Lake Placid. At the summit of Mount 

 Whiteface the rock is medium grained and consists of white plagio- 

 clase (chiefly labradorite) with lO to 15 per cent of dark minerals 

 scattered through the mass parallel to a crude foliated structure. 

 Such rock, which is quite typical of the Schroon Lake quadrangle 

 Whiteface anorthosite, 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 so well foli- 

 ated. More exceptionally the Whiteface anorthosite 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. 

 Table i gives a good idea of the mineral content of the Whiteface 

 anorthosite of the quadrangle. 



