250 Prof. J. P. Cooke on the Vermiculites. 



from the Zillerthal, by Shafhautl (No. 1), and one of a mica, 

 from Fahlun, by H. Rose (No. 2):— 



Si. Al. Fe. Gr. Mg. Ca. Ka. K. F. 



(1) 47-95 34-45 1-80 3*95 0*71 059 037 10*75 0-35 = 100-92 

 25-57 1605 -54 1*24 -28 0-17 0-09 1-83 



25-57 17-83 2-37 



Si. =41. Fe. Mg. K. H. F. 



(2) . 46-22 34-52 6-04 2'11 8'22 098 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 hy- 

 drous micas of the Muscovite family, distinguished by the atomic 

 ratios 3:2:1 and 4:3:1. Corresponding to these, it is pro- 

 bable that there are two varieties of anhydrous mica, containing 

 an excess of SiO 2 , 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 previous paper (Amer. 

 Journ. Sci. (II.) vol. xii. p. 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 4 §0 4 =Si and H 2 =0 2 =SiO; 



and he there described a mica whose atomic ratio was very 

 closely that of the second type, and which he called Cryophyl- 

 lite. Now the simplest theory of the relation of the hydrous to 

 the anhydrous Muscovites would seem to be that, while in the 

 molecules of the anhydrous micas a portion of the silicon atoms 

 (the number varying in the different varieties) are in the condi- 

 tion of the first anhydride (H 2 =0 2 =SiO), the hydrous micas con- 

 tain sufficient basic hydrogen to bring all the silicon atoms into 

 the condition of the normal hydrate (H 4 §0 4 lSi). The two 

 graphic formulae which follow indicate more clearly than any 

 other language can, the relations we have attempted to describe. 

 In these formulae E stands for the double atom in the radical of 

 the sesquioxides, having the quantivalence of six : — 



