REPORT OF THE DIRECTOR AND STATE GEOLOGIST 1*53 



it necessary that they be considered essential constituents of the 

 rock rather than mere accessories. 



The Adirondack anorthosites were originally very coarse 

 grained rocks, the individual crystals of labradorite often attain- 

 ing a length of three inches or more. The deformation which 

 they suffered while deeply buried, more or less broke up these 

 large crystals into a mass of granular fragments, with the result 

 that the rocks as now exposed consist partly of the unbroken 

 portions of the original crystals and partly of the granular ma- 

 terial, sometimes the one and sometimes the other predominat- 

 ing, though usually the latter. They were also somewhat 

 foliated, specially at the borders of the mass, where -there is a 

 frequent passage into a well foliated gneiss in which little or no 

 trace of the original structure remains. 



The Rand hill rock is much more thoroughly granulated than 

 the usual anorthosite and was, most probably, originally a finer 

 grained rock. In the southerly exposures the granulation is prac- 

 tically complete, and no labradorite aiujcn (a convenient term for 

 the uncrushed parts of crystals) remain. It is only in the north- 

 erly exposures that such are found, and even there the bulk of 

 the rock is granulated. Many augen however remain, running 

 up to an inch in size, though seldom exceeding that figure. As 

 in general the border portions of these intrusions are the most 

 crushed, it is not probable that we are here dealing with the 

 limit of the mass in this direction, it in all likelihood continuing 

 to the northward under cover of the Potsdam sandstone. 



Since the Rand hill rock contains a larger amount of minerals 

 other than feldspar, than does the usual anorthosite, it was more 

 definitely foliated by deformation, and much of the rock exposed 

 is thoroughly gneissoid. As a whole however it strongly resem- 

 bles some of the border phases of the larger intrusions. 



The most important difference between this rock and anortho- 

 site consists in the constant presence in considerable amount of 

 quartz; so that strictly it should be called a quartz-anorthosite- 

 gabbro. The rock is of somewhat unusual type, has not been here- 

 tofore described from the region, and is therefore deserving of 



