1882.] ^^^ [Genth. 



ture of grains of menaccauite ; in some of the crystals there were ouljr a 

 few small ones, rarely over one millimetre in size, in others, the quantitj^ 

 disseminated through the corandum, is very large, and a great portion, 

 probahly in the act of crystallization, has been pushed to the outside of 

 the corundum crystals, and gives them a coating of menaccanite, ■which 

 sometimes reaches a thickness of S™". The menaccanite grains have no 

 distinct form, they have an iron-black color, and, on a fracture, submetallic 

 lustre, they are not magnetic, and gave the following composition : 



TiOj = 17.62 



FCjOs = 67.36 



AljO,, = 3.7a 



FeO = 11. 14' 



MgO = 0.27 



Sib, = 0.41 



100.53 



The only alteration of this corundum which I have noticed, is that into 

 mica and small quantities of tourmaline, but the specimens which have 

 been obtained from this locality are the most beautiful and of great scien- 

 tific interest. 



The brown corundum is surrounded with greenish-white subfibrous 

 mica, showing under a good magnifier a very fine scaljr structure ; this 

 mica is sometimes only a very thin coating, but frequently from 2 to 5""' 

 in thickness, surrounded by fine scaly mica, much of which has changed 

 to brown scales with submetalliclustre, which largely exfoliate when heated, 

 like jefferisite or maconite. In some specimens the subfibrous mica peels 

 off and then shows the edges of the corundum rounded, and the whole 

 surfaces acted upon, as if by a solvent ; other specimens contain a core of 

 lirown corundum with the star of six rays ; the corundum still shows the 

 rounded hexagonal form, but the subfibrous greenish-white mica forms a 

 ring around it with perfect hexagonal sides and sharp edges (analysis e), 

 the whole being imbedded in fine scaly mica schist. Where many of such, 

 partly altered corundum crystals are crowded together in the mica schist, 

 the appearance reminds one of plum pudding. 



Many of the corundum crystals are almost completely changed into 

 mica. Then they are often flattened out, and form irregular nodules in 

 the mica schist, having a whitish or greenish-white color, are fine scaly on 

 the surface (sometimes imbedding small slender crystals of black tourma- 

 line), but compact or very fine granular in the interior. On breaking, some 

 show yet minute traces of unaltered corundum ; others have not a trace of 

 it left, and have not the remotest resemblance to mica, but more the 

 appearance of a grayish white compact limestone. Analysis of the com- 

 pact mica (e2). 



The menaccanite which was in the original mineral is also present in 

 the altered. 



PROC, AMER. PHILOS. SOC. XX. 113. 2w. PRITifTED SEPT. 6, 1882. 



