GEOLOGICAL SOCIETY. 



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FOURTH ORDER — MICA. 



Pearly lustre. 



Mono-axis. Perfect cleavage through the basis, or a 

 very flat half bevelment, (hemidoma, Br.:) thin la- 

 minae easily obtained. Neither tesseral, tetragonal? 

 or opaline. 

 H. from 1 to 6|. 



G. from 2, 5 to 3, 4. 



H. less than 2 : G. from 2, 6 to 2, 8, very 



unctuous to the touch. 

 H. from 2 to 3 : G. from 2, 7 to 3. 

 H. 5f and more : hexagonal and cleavable 



through the basis. 

 G. from 2, 5 to 2, 6, without green streak. 



G. more than 3 : H. 3 and more. 



Examples — Optic monoaxic, and optic bi- 

 axic mica, margarite, clintonite, pyro- 

 smalite. 



FIFTH ORDER— PORODINE, 



Opaline, conchoidal fracture. 



H. from 1 to 5|. 



G. from 1, 8 to 3, 1. 



H. 4f and more : G. 2, 3 and more. 

 Examples— Alumo-calcite, and kero!ite f 



halloysite, bole and sider-bole, allophane, 

 common copper green, pimelite 3 chloro- 

 pal, picrolite, magnesite, chlorophaite, 



