GEOLOGY OF MOUNT MARCY 39 



SOME GARNET REACTION-RIMS IN ANORTHOSITE 



BY MAX ROESLER 



The hand specimens show irregular masses, of rudely lenticular 

 shape, of brownish green pyroxene with associated magnetite, or 

 else single pyroxene crystals, surrounded by rims of dark-red garnets 

 with associated quartz. These garnets are very small and no indi- 

 vidual crystal has been found that has a diameter greater than one 

 millimeter. The ground mass in which these lenses and rims lie is a 

 typical granulated anorthosite such as is common in the Adirondacks 

 and as has been described by F. D. Adams ^ from the areas in 

 Quebec. That is, it is light colored, granular, with occasional indi- 

 viduals of larger size that retain the blue-gray coloring of the fresh 

 labradorite. This description applies to all the fragments except one, 

 in which there is in the ground mass a very considerable amount of 

 quartz in scattered crystals. For reasons which will appear later, 

 and for the sake of distinguishing it from the ordinary anorthosite, 

 this specimen will be spoken of as the aplitic anorthosite. 



Thin sections were made of the various parts of several of the 

 specimens, and their examination gives the following results. 



The ground mass of the ordinary anorthosite is composed of 

 plagioclase, granular, irregular in outline, twinned as a rule with 

 the twinning lamellae often bent. In some of the grains of plagioclase 

 there are inclusions of a clear material in a poikilitic arrangement 

 and of lower index of refraction than the plagioclase. The inclu- 

 sions are probably orthoclase. The plagioclase, judged by the extinc- 

 tion angles, is very close to labradorite, more often varying toward 

 the more acid than toward the more basic varieties. There are 

 occasional grains of orthoclase in one of the sections. As a rule they 

 are limited to the inclusions in the labradorite. Plate i6 A illustrates 

 these inclusions of orthoclase in the labradorite. 



The section made of the ground mass of the " aplitic " anorthosite 

 showed the following : Quartz in large grains, badly strained, but 

 with very little evidence of granulation. Orthoclase in large grains, 

 also badly strained, but not granulated. Microcline, several grains. 

 Neither the quartz nor the feldspar shows good crystal outlines. 

 Labradorite in a few small scattered and broken grains. These 

 plagioclase grains show at times a graphic intergrowth with quartz. 

 In one case the quartz within the plagioclase showed the same 

 orientation as the quartz in interstices between the plagioclase grain 



1 Geol. Survey of Canada, Annual Rep't, v. 8, pt. J, p. 107-10. 



