Genth.] OiO [Sep. 19, 



which is = 0.92 : 3 : 8.30, or very near 1:3:9 in accordance 

 with the formula of oligoclase = 2 (K 2 0, Na 2 0, CaO), 3 Si0 2 + 2 (A1 2 3 , 

 3 Si0 2 ). 



/. The yellowish-white granular feldspar of Mineral Hill, Delaware 

 County, Pennsylvania, above referred to, which contains crystals of 

 corundum altered into fibrolite and other minerals, seems also to be 

 oligoclase. 



g. The so-called indianite from Chester, Massachusetts, has been proved 

 by C- T. Jackson's* analysis to be granular oligoclase. 



h. The white granular feldspar from the locality on the road from 

 Unionville to Kennett Square has been analyzed by Brush and Weld,t 

 who found it to be albite, containing a small percentage of lime. It is 

 frequently the matrix of gray corundum associated with euphyllite (?). 

 The corundum crystals are rough and irregular and are always more or 

 less coated, enveloped or changed into euphyllite, and also a soft, appa- 

 rently amorphous, mineral, which is probably a variety of margarite, of 

 which, however, I had not enough for an analysis. 



The very interesting question presents itself : are the feldspars found 

 in connection with the corundum, the result of the alteration of the 

 latter ? I am not prepared to give a positive answer, and believe that it 

 requires a great many more observations before it can be decided. From 

 what I have seen, it appears that there are cases where feldspars have 

 been formed from corundum, and it is very probable that many have thus 

 been formed, but that at the same time a portion of the alumina re-crys- 

 tallized as corundum, which we then find imbedded in the feldspathic 

 matrix. 



Very similar occurrences must have taken place in several other in- 

 stances, to which I will here refer, viz. ; the crystals of corundum in a 

 matrix of tourmaline and the beautiful, very well defined crystals of it 

 in the chlorite of the Culsagee Mine. 



11. Tourmaline. 



Tourmaline has been found in association with corundum at most of 

 the localities already referred to ; for example, at Kassoibrod in the 

 Ural, at Naxos in Greece, at Chester, Massachusetts, at Unionville, Penn- 

 sylvania, at the Culsagee Mine, rarely at the Cullakenee Mine, and more 

 frequently at Dudleyville, Alabama. 



a. At Unionville black tourmaline is frequently found in irregular 

 masses of different sizes, from a small grain to several inches in diam- 

 eter, in the corundum as well as in the minerals resulting from its alter- 

 ation, especially in the foliated margarite or with zoisite and euphyllite. 

 It sometimes shows crystalline prismatic planes, but is mostly granular 

 and occupies the interstices between the corundum itself. 



A far more interesting occurrence is mentioned by Dr. Isaac Lea (loc. 



*C. T. Jackson's. Sill. Journ. [2] XLII.107. 

 tBrush and Weld, Sill. Journ. [23 VIII ,390. 



