226 J, P. Cooke, Jr.. on Cryophyllite. 
the mica of the Rockport granite is an isomorphous mixture of 
these two distinct species. This conclusion, moreover, is favored 
by the fact that lepidomelane—the species to which this mica 
undoubtedly belongs—contains in other localities neither lithia 
nor fluorine; while on the other hand these same ingredients 
are strikingly characteristic of the lepidolite micas, to whic 
eryophyllite is allied. Assuming then that the lithia and fluo- 
rine in the analyses belong to the cryophyllite and not to the 
lepidomelane, we have endeavored in the following table to 
eliminate this disturbing element from our results, Thus, in 
column 1 we have repeated the mean result of our analyses. 
In column 2 we have the amounts of the several ingredients of 
cryophyllite corresponding to 0°59 per cent of lithia, deduced 
from the analyses of this mineral given above. Subtracting 
the same to a percentage composition, and these numbers, ac- 
cording to our theory, represent the true composition of the 
Rockport lepidomelane. In column 5 we have placed for com- 
parison the analysis of lepidomelane by Soltmann. 
Reduction of analyses of Lepidomelane. 
1 2 3. 4. Oxygen. 5. Oxygen. 
Silica, 39:55 7-48 32:07 37°39 ....==1994/39740  =1994 
Fluorid of silica, O62: (60 jeoes Soest Mets 
Alumina, 16-73 2-44 1429 16°66 7-78 11°60 5-42 
Sesquiox. of manganese, 0°60 0:05 0 5 F 
Sesquiox, of iron, 1207 0-29 11-78 13°74 4:12 27°66 829 
Protoxyd of iron, 17-48 1:16 16:32 19°03 4:23 12°43 276 
Magnesia, 062 O11 O51 Q'59 0-24 0'26 0-10 
Potassa, 10°66 1-91 875 10°20 1-73 9°20 1°61 
Lithia, O60 059 dene eee Se 
Water, 150.... 150 1°75 156=19°85! 0°60 0:53=18°71 
, 100'00 99°15 
Oxygen ratio, 19°85 : 19°94 = 1:1, 
General formula 2RO, Si0,. Probable rational formula, 2(¢R%, $4)3Si- 
the alumina and the oxyds of iron and manganese are compa 
together in the two analyses, and the difference in the distribu- 
tion of the iron between the two oxyds is unimportant if we 
