L.AWSON.] 



Plumasite, <in Oligoclase- Corundum Rock. 



227 



When the corundum crystals are broken off the fracture 

 surfaces show not uncommonly scaly films of a lustrous silvery 

 white, or pearly mineral of micaceous habit. The scales arc 

 brittle, and when examined on the stage of the microscope they 

 have the characters of a biaxial mineral with small optic angle, 

 rather low double refraction and negative character. They are, 

 therefore, identified as margarite. There occurs through the 

 feldspathic part of the rock a similarly appearing secondary 

 mineral in the form of small veinlets and patches. In this case, 

 however, the luster is more waxy and the color somewhat green- 

 ish. The mineral is foliated, and the scales when examined on 

 the stage of the microscope show it to be unaxial and positive 

 and to have a feeble double refraction. Its specific gravity is 

 2.74. These data are insufficient for identification with any 

 known mineral, but they agree most closely with the characters 

 of a light colored chlorite. It appears to occur in the rock along 

 minor breaks and dislocations. 



Chemical Composition of the Rock. — Representative hand speci- 

 mens of the rock composed practically of only oligoclase and 

 corundum were taken to determine the proportion in which these 

 two minerals are preseut in the rock. The specific gravity of 

 the rock was found from a series of four samples, weighing 165 

 to 442 grms., to be 2.789. Knowing this and the specific gravity 

 of both constituent minerals, as given above, the proportion was 

 found to be oligoclase 83.64 per cent., corundum 16.36 per cent. 

 Taking the round numbers 84 and 16 as the respective percent- 

 ages of the two minerals, the bulk analysis of the rock is found 

 to be as follows: 



The Bock Type Defined. — It appears from the foregoing 

 description that this interesting rock has resulted from the con- 

 solidation of a magma having approximately the composition of 

 oligoclase with an excess of alumina. This particular type of 



SiOo 



AI2O3 



CaO 



Na 2 



H,0 



51.80 

 35.39 

 -1.54 

 6.82 

 1.45 



100.00 



