Genth.] 



384 



[August 18, 



locality Tve find occurring in small quantities, remnants of crystals of gray 

 corundum, generally surrounded by a little silvery mica in fine scales, in 

 brownish-wliite or light brown cleavable feldspar, in masses sometimes 

 from 25 to 30™" in diameter. The feldspar shows distinct triclinic stria- 

 tion. The corundum, where it is in contact with the feldspar or the mi- 

 caceous coating, has a corroded appearance. The analysis gave : 



= 3.6 



2.80 



99.93 



This feldspar, after its fine powder had been dried over sulphuric acid 

 for several days, gave on ignition 2.45 % of water and, in another sample, 

 2.55 %. As there is not a sufficient amount of bases R^O and RO present 

 for oligoclase, may not a portion of this water be basic water ? 



b. Another interesting occurrence of the alteration of corundum into 

 a feldspar, is that at the "Black Horse" tavern, near Media, in Dela- 

 ware county. The corundum, of a dark gray color in rough crystals, 

 generally coated with a film of fine scaly mica, is imbedded in a finely 

 granular brownish-white feldspar, which has probably resulted from its 

 alteration. It has a specific gravity of 2.611, and the mean of two closely 

 agreeing analyses, is as follows : 



} = 10.87 



.45 



100.06 



The constituents of R20(R0) : RjOj : SiOj are in the ratio of 3.45 : 

 10.87 : 31.16, ivhich is = 1 ! 3.1 : 9, or almost exactly that of oligo- 

 clase. 



It is an interesting fact that a part of the calcium oxide is replaced by 

 barium oxide. 



