46 PROCEEDINGS OF THE AMERICAN ACADEMY 



ning is the cause of this most remarkable variation in the optical angle, 

 as will be explained at length in connection with our description of 

 Culsageeite. On this last mineral the same phenomena are more 

 marked, owing chiefly to the greater transparency of the plates. 



In order to illustrate the chemical relations of the mineral to the 

 Biotite micas, we give below : 1st. The results of the analysis of 

 JeiFerisite by Professor Brush. 2d. The same results, calculated for 

 the anhydrous mineral. 3d. The results of an analysis of a Biotite 

 mica, from Pargas, Finland, by Svanberg. In each case I have added 

 the amounts of oxygen in the several oxides, to show the atomic 

 ratios : — 



Si Al Fe Fe Mg Ca E H 



(1) 37.10 17.57 10.54 1.26 19.65 0.56 0.43 13.76 = 100.87 

 19.78 8.18 3.16 .28 7.86 .16 .07 12.23 



(2) 



(3) 



The general symbol of JeiFerisite deduced from (1) would be, — 



A comparison of the results given in (2) and (3) will show that the 

 anhydrous JefFerisite corresponds very closely in its chemical constitu- 

 tion with the Biotite mica from Pargas. The chief difference is to be 

 found in the fact that the mica contains potassium and basic hydrogen, 

 in place of more than one-half of the magnesium of the JefFerisite. It 

 should, however, be remembered in this connection that the Biotites 

 present a very wide variation in the ratio between the amounts of the 



