Genth.] dbU [Sep. ly, 



amongst other localities from the vicinity of Chavagnac and Ourouze, in 

 France, where it is associated with mica, cyanite, red and blue corundum. 

 One of the most interesting localities is at the Falls of the Yantic, near 

 Norwich,* Connecticut, where the small crystals of sapphire are com- 

 pletely surrounded by fibrolite. This is evidently owing to an alteration 

 of corundum into fibrolite. 



At Mineral Hill, Delaware County, Pennsylvania, as I have already 

 mentioned above, corundum, more or less completely altered into fibro- 

 lite, occurs in a feldspathic rock. The corundum crystals are generally 

 rough, six-sided prisms, tapering at both ends, from one-half to one and 

 a-half inches in length and from one-eighth to one inch in thickness. 

 They are of various shades between white and brown, often of a rich 

 glove-brown color, many exhibiting a white or colorless star of six rays. 

 This star when magnified shows already a fibrous structure. Where the 

 alteration of corundum has just commenced, a thin coating of a grayish- 

 white color, sometimes not thicker than a varnish, can be observed on 

 the brown corundum. Under a strong glass it is seen to consist of a 

 vitreous mineral, which is fibrous and radiating from the corundum. 

 Many of the crystals have still a core of unaltered corundum, in others 

 every trace of it has disappeared, and fibrolite pseudomorphous after 

 corundum is left, the interior of which is made up of congeries of acicular 

 crystals. 



Only with great difficulty I was able to select a sufficient quantity of 

 pure material for analyses. 



The following are the results : 

 Spec. Grav. = 3.28G. 



a. b. c. Oxygen. 



the formula A1 2 ;! , SiO.. 



At several localities in the immediate neighborhood and frequently 

 within the serpentine region, in the State of Delaware, and Delaware 

 Couuty, Pennsylvania, fibrolite is found, which partly at least seems to 

 have been derived from the alteration of corundum. I wish to call par- 



*C. TJ. Shepard,Sill. Journ. [3] IV, ISO. 

 tBy difference. 



