42 PEOCEEDINGS OP THE AMERICAN ACADEMY 



There appear to be, therefore, these two distinct types of hydrous 

 micas related to the species Muscovite, all rich in alumina and alkali, 

 destitute, or only containing very small amounts, of magnesia, and 

 having a vpide optical angle. 



The anhydrous Muscovites have not been investigated nearly as 

 fully as the hydrous varieties ; and I can find no analyses of any of the 

 beautiful specimens from our American localities. I cite here, for the 

 sake of comparison, an analysis of the Fuchsite, from the Zillerthal, 

 by Schafhautl (No. 1), and one of a mica, from Fahlun, by H. Rose 

 (No. 2): — 



Si Al. Fe er Mg Ca Na E F 

 (1) 47.95 34.45 1.80 3.95 0.71 0.59 0.37 10.75 0.35 = 100.92 

 25.57 16.05 .54-1.24 .28 0.17 0.09 1.83 



25.57 17.83 2.37 



Si Al Fe Mg. E H F 



(2) 46.22 34.52 6.04 2.11 8.22 0.98 1.03 

 24.65 16.08 1.81 0.84 1.39 0.87 



24.65 17.89 3.10 



If now we should add to (1) 6.93 per cent water, and to (2) 3.42 per 

 cent, we should obtain the ratios : — 



(1) 25.57 : 17.83 : 8.52, or approx. 3:2:1, 



as in the hydrous micas analyzed by Haughton. 



(2) 24.65 : 17.89 : 6.16, or approx. 4:3:1, 



as in Damourite and Sterlingite. 



There would seem, then, to be two definite varieties of hydrous micas 

 of the Muscovite family, distinguished by the atomic ratios 3:2:1 and 

 4:3:1. Corresponding to these, it is probable that there are two 

 varieties of anhydrous mica, containing an excess of SiO.,, which, by the 

 addition of sufficient water to saturate the excess of the acid radical, 

 are reduced to one or the other of the two normal types. In a pre- 

 vious paper (Am. Jour. Sci,, II. xii. 217, 1867) the author suggested the 

 idea that the excess of silica in this class of micas might result from 

 a mixture of two isomorphous species corresponding to the two 

 hydrates : — 



H=-0:=Si and II,= OfSiOy 



