l8 NEW YORK STATE MUSEUM 



of this species of feldspar, but this phenomenon is neither so strik- 

 ing nor so common in the Marcy anorthosite as in labradorite from 

 certain other regions. Twinning striations are usually evident on 

 the shiny cleavage faces of the labradorite. 



Accessory minerals visible to the naked eye are large individuals 

 of pyroxene and hornblende, and small individuals of biotite, 

 ilmenite, pyrite, garnet, and more rarely chalcopyrite and pyr- 

 rhotite. Due to decomposition of the dark minerals, the weathered 

 anorthosite is usually light brown, but such rock is not common. 

 Locally the amount of dark-colored minerals rises to 15 to 25 per 

 cent when the rock should really be called anorthosite-gabbro. In 

 many places such anorthosite-gabbro and typical anorthosite exhibit 

 perfect gradations from one into the other, often within a few rods. 

 The gabbroic facies is, however, decidedly subordinate in amount, 

 and it has not seemed feasible to represent it separately on the 

 geologic map. 



An important facies of the Marcy anorthosite is one in which 

 dark bluish gray labradorite individuals, from a few millimeters to 

 an inch or more across, stand out conspicuously in a distinctly 

 granulated groundmass of feldspar. In the fresh rock the granu- 

 lated material varies from light gray to pale greenish gray. The 

 granules usually vary in size from microscopic to i or 2 millimeters 

 across. Even a glance at a hand specimen of such a rock makes 

 it clear 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. In this type of rock, 

 therefore, the labradorites stand out like phenocrysts, thus giving 

 the rock a distinctly porphyritic appearance, though of course 

 crystal boundaries are seldom if ever present. All degrees of 

 granulation are shown, from rocks' in which there is little or no 

 evidence of crushing, to others in which relatively large, dark lab- 

 radorites are scattered through a granulated groundmass, to 

 extreme cases where the whole rock has been so thoroughly granu- 

 lated that few, if any, labradorite cores remain. In spite of exces- 

 sive granulation, the fresh rock is very firm and hard. The 

 extremely granulated types, especially where somewhat weathered 

 to light brown, bear a close resemblance to normal weathered 

 syenite (see below) and may be readily mistaken for such in small 

 outcrops in the woods. Careful examination of a number of speci- 

 mens from an outcrop will, however, almost invariably yield at 

 least a few small uncrushed cores of labradorite to furnish the 

 clew to the nature of the rock. In connection with the granulation 



f 



