372 DR T. THOMSON on Aslestus, Chlorite, and Talc. 



A little lead was found in the scales, which was obviously a 

 foreign substance. It was separated with much care ; though 

 it is not impossible that it may have affected the weight of some 

 of the constituents a little. From the care taken to separate 

 the lead, and from the repetition of the analysis, I believe that 

 the error resulting from it, if any, must be very trifling. 



It is obvious at first sight, that the constitution of this mine- 

 ral is quite different from that of chlorite. I believe it to be a 

 variety of rhomboidal mica. From the analysis of mica by KLA- 

 PROTH and ROSE, there cannot be a doubt, that at least three dif- 

 ferent species of mineral are at present confounded together 

 under the name of mica. One of the most common of these 

 species crystallizes in oblique rhomboidal prisms, with the fol- 

 lowing angles : 



P on M' 98 40', 

 P on M 81 20', 

 W on M 60. 



I analyzed a very pure crystallized mica of this kind from the 

 United States, and found its composition as follows : 



Silica, 49.380 



Alumina, 23.668 



Protoxide of iron, . . 7.312 



Lime, . . . . . . 6.134 



Potash, '..'.'. . . 15.292 



Lithia, ..'.". . . . 0.060 



101.846 



