Genth.] 



392 



[Sep. 19, 



the Unionville locality it is rarely met with, but sometimes it is found in 

 irregular and imperfect hexagonal plates and crystals of a brownish- 

 yellow color. It exfoliates like that from West Chester. 



At the Culsagee Mine it is of more frequent occurrence, and the chlo- 

 rite shows very often an incipient alteration by a discoloration of the 

 laminae, which turns brown or brownish-yellow. It is generally found, 

 when completely altered, in plated masses of a yellowish-brown color, 

 which cannot be distinguished by their physical characters and their 

 manner of exfoliation, from that of the original locality. There is 

 another variety at the Culsagee Mine, which is associated with a consid- 

 erable quantity of corundum. It occurs in fine greenish-brownish-yellow 

 scales, not over one-eighth of an inch in diameter. It exfoliates, but not 

 as remarkably as either of the other varieties. 



The following were analyzed : 



a. The broadly foliated, from the Culsagee Mine by Dr. Koenig (a 1) 

 and Mr. Thos. M. Chatard (a 2). 



b. The greenish-brownish-yellow fine scaly, by Mr. Thos. M. Chatard 

 (b.) 



c. and for comparison the broadly foliated from West Chester, by Dr. 

 Koenig (c 1) and Mr. Thos. M. Chatard (c 2), to which is added (c 3) the 

 analysis by Prof. Brush. 



The discrepancies in these analyses hardly admit of constructing a 

 formula. The analyses a\ and 2 give nearly 6RO -(- 2R 2 3 + 5Si0 2 -f 

 10H,O, and b, cl and 2 nearly 5RO -f 2R 2 :! + 5Si0 2 + 10 H 2 0. 



The greatest, and to me unexplainable difference, exists in the results of Prof. Brush and 

 Dr. Koenig and Mr. Chatard in the amount of silicic acid and water in the West Chester Jefferi- 

 site. Dr. Koenig made his analysis with finely powdered material, which has been exposed 

 to the dry aii- of the room ; Mr. Chatard took his material from another specimen, and after 

 powdering, dried it tor four days- ovtr sulphuric acid iii a vacuum; in the same manner the 

 Culsagee mineral was treated. 



*Mr. Chatard determined only the ferric oxide, and took for the ferrous oxide Dr. Koenig's 

 result. 



