Prof. B. Silliman on some American Mitierals. 453 



sembles the last, but its composition is different in important 

 particulars. It is also found associated with the corundum 

 and emery of Unionville, Chester County, Pennsylvania. 

 The specimen here analysed is marked " D," and was taken 

 by me in May last from the mineral collection of the Chester 

 County Cabinet formed by Mr. Williams. It is in broad 

 foliated masses of a yellowish-white colour, easily cleavable, 

 and apparently hexagonal in form, penetrated by hexagonal 

 crystals of corundum. Inelastic, brittle ; resembles common 

 mica, but not so strikingly as A. Hardness, 3*5. Gravity, 3. 

 B. B. gives the same characters as the last species. No reac- 

 tion could be obtained for lithia or boracic acid in any of the 

 minerals of this series. The reaction for fluorine in this one 

 was feeble. It is unaffected by strong acids even on long 

 boiling, except partially by very strong sulphuric acid. Its 

 qualitative analysis gave silica, alumina, lime, potash, soda and 

 water, with a trace of iron and fluorine. 



The following analysis was made by Mr. J. J. Crooke, on 

 r389 grm. of the substance fused with carbonate of baryta. 

 It yielded — 



Oxygen. 



Silica .... 0-496 = 35-708 p. c. 18'553=18*55= 9 



Alumina . A '. 0-738 53-131 24-872 24-87 12 



Lime . luu iui* 0-101 7*271 2-042^ 



Potash -^Tevabaoqi 0017 1-224 0-207 [- 2-36 1 



Soda 0-006 0-413 0-1 loj 



Water and fluorine 0-032 2-303 2-050 205 1 



1-390 100-068 

 This gives the ratios 

 3Si03, 4 Alb^, RO + HO= RO, SiO^ + 2(2Alb% SiO^) + HO. 



Atoms. Required. Found. 



4769-76 100-00 100-068 



This species somewhat resembles margarite, and it may be 

 shown on further examination that margarite is a hydrated 

 mineral. At present it is reported as anhydrous, and its pro- 

 portions of silica and alumina are different from the present 

 species. Its analysis, given by Hausmann on the authority of 

 the Gottingen Laboratory, is — 



