DB T. THOMSON on Asbestus, Chlorite, and Talc. 367 

 This is equivalent to 



30 atoms silica, 

 4.5 atoms lime, 

 5 atoms magnesia, 

 4.5 atoms protoxide of iron, 

 1 atom alumina. 



The atoms of the bases amount to 15, and those of silica to 

 30. Hence it is obvious that the mineral is composed of bisili- 

 cates. If we admit a little of the lime and of the protoxide of 

 iron to be replaced by alumina, then rock-cork will be a com- 

 pound of 



1 atom bisilicate of lime, 



1 atom bisilicate of magnesia, 



1 atom bisilicate of iron. 



Its symbol will be CS 2 + MS 2 -f /S 2 . 



It is therefore most probably a variety of pyroxene. 



If any confidence can be placed in the preceding discussion, 

 it follows that the minerals hitherto arranged as subspecies of 

 asbestus, constitute in reality four distinct species. 



1. Amianthus and common asbestus are varieties of am- 

 phibole. 



2. Rock-wood is 4 (|M + ^A/) S 2 -f/S 2 + 1 Aq. 



3. Mountain-leather 6 ( y C + yA/) S 3 + ( y M + y /) S 4 

 -i- 17 Aq. 



4. Mountain-cork is a variety of pyroxene. 



