48 PEOCEEDINGS OP THE AMERICAN ACADEMY 



discuss what are the relations of the water thus combined to the mo- 

 lecular structure of the mineral. This question is still in suspense, and 

 we are persuaded that our science is not yet in a condition to solve the 

 problem. All that we can at present do is to classify the phenomena 

 presented by the exfoliation of the vermiculite minerals with the efflo- 

 rescence of Glauber's salt or the intumescence of alum. 



If, then, I am justified in the inferences I have drawn, Jefferisite 

 differs chemically from Biotite mica chiefly in containing water of 

 crystallization, very much in the same way that gypsum differs from 

 anhydrite, or common salt from the crystals with two molecules of 

 water, which form in brine at a low temperatm^e. In these bodies, 

 however, the forms of the crystals of the hydrous are wholly different 

 from those of the anhydrous salts, while the crystallographic relations 

 of Jefferisite are very similar, if not identical, with those of the mag- 

 nesian micas. It is, however, also true that the crystalline structure of 

 the micas seems to admit of a very wide variation of chemical com- 

 position. In cryophyllite, which has all the crystallographic charac- 

 ters of the micas strongly developed, is the atomic ratio 3:4:14; 

 while in the Biotite, whose analysis was cited above, it is 2:3:5. 

 The first is an acid, and the last a neutral silicate. The other varieties 

 of mica have a composition intermediate between these extremes ; 

 and it might be said, if such a variation of composition is compatible 

 with the crystalline structure exhibited by tliis group of minerals, it 

 would not be surprising if the structure were sufficiently elastic to admit 

 of the insertion of the water of crystallization without great alteration 

 of external form. 



It may be further urged that the normal mica crystal (a rhombic 

 prism with angles of 60° and 120°) is a form which appears to be 

 related to a mode of molecular structure common to a great many sub- 

 stances, and the phenomena which will be described in this paper seem 

 to sustain this opinion. Still, we do not care to theoi'ize about the 

 subject. Our object is simply to make prominent the two points : 

 1st. That the crystallographic structure of Jefferisite is identical with 

 that of the magnesian micas. 2d. That the chemical constitution of 

 the anhydrous mineral is closely allied to that of Biotite. 



Culsageeite or the Vermiculite of the Jenks Mine, North Carolina. — 

 Through the kindness of Colonel C W. Jenks, the proprietor of the 

 corundum mines on the Culsagee River in JMacon County, N. C, I 

 have had an opportunity of examining a variety of vermiculite whicli at 

 that locality forms an important part of the matrix of the corundum. The 

 associations of the mineral in North Carolina are very similar to those 



