404 W. J. MILLER ADIRONDACK AXORTHOSITE 



the play of colors geuerallv so characteristic of this species of feldspar. 

 Twinuing striations are often evident on the cleavage faces. 



Accessory minerals visil)le to the naked eye are large individuals of 

 pyroxene and liornblende and small individuals of biotite. ilmenite. 

 pyrite, garnet, and more rarely chalcopyrite or pyrrhotite. These acces- 

 sor}' minerals ordinarily constitute 5 to lU per cent of the typical coarse 

 anorthosite. but there are local developments of the rock which are made 

 up almost entirely of plagioclase. and still others, rather abundantly de- 

 veloped, which contain from 10 to 25 per cent of dark minerals, these 

 last named tyj^es really being anorthosite-gabl)ros. 



An important facies of the anorthosite is one in which the dark labra- 

 dorite individuals, from a few millimeters to an inch or more across, 

 stand out conspicuously in a distinctly granulated ground-mass of feld- 

 spar. The granulated material varies from light gray to pale greenish 

 gray. It is very evident that the large labradorites are roughly rounded 

 uncrushed cores of what were considerably larger individuals before the 

 rock was subjected to the process of granulation. All degrees of granu- 

 lation are exhil^ited to extreme cases where the rock has been so thor- 

 oughly granulated that few. if any, labradorite cores remain. 



Much of the typical Marcy anorthosite is practically devoid of folia- 

 tion, though in some local zones of almost perfectly pure plagioclase rock 

 there is a notal)le tendency for the feldspars to show a crude parallelism. 

 The more gabbroid facies of the rock, however, often exhibit a fair to 

 well defined foliation accentuated by the parallel arrangement of the 

 femic minerals. 



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

 dorites are usually seen to be more or less filled with myriads of very dark 

 dustlike particles, probably ilmenite. 



CHILLED BORDER FACIES OF THE AXORTHOSITE (WHITEFACE 

 ASORTHOSITE) 



Around the borders of the great body of anorthosite, and in some places 

 a number of miles within it, there is quite generally a notable increase in 

 femic minerals, causing the rocks to be anorthosite-gabbro or even gabbro. 

 Such rocks are almost invariably medium grained and therefore notably 

 finer grained than the typical Marcy anorthosite, though in some locali- 

 ties a few large, scattering labradorite individuals occur. A foliated 

 structure is generally well developed. 



Although they are more or less variable in general appearance and com- 

 position, I here propose that these border phases of the anorthosite be 

 classed as ^Miiteface anorthosite. a name given by Professor Kemp to a 



