230 THE GEOLOGIST. 



Mica group. — The most diverse compounds may occur in the form 

 of mica. The following table (from Bischof ) of maximum and mini- 

 mum contents shows how great this diversity may be, combined with 

 great uniformity of physical character. 



Maximum. Minimum. 



Silica 71 ..... 36 



Alumina 38 6 



Protoxide Iron ) n 



Peroxide Iron \ • ' " 



Magnesia .... 29 .... . 0 



Potash 14 2 



Lithia 5.7 0 



Fluorine 10.4 0 



The three regular mica species are Potash-mica, Magnesia-mica, and 

 Lithia-mica. They contain essentially Al, and K, and (in the two 

 first-named varieties) Fe, and usually some water. Chlorite and Ripi- 

 dolite are physically allied to the micas, but they differ chemically in 

 not containing an alcaline base. 



Hornhlendic and Talc groups, — These extend from Wollastoiiite, a 

 simple silicate of Ca, to Talc and Olivine, essentially silicates of Mg 

 with a small proportion of Fe. The minerals of these groups occupy 

 the opposite end of the series to the felspars, and in geological 

 importance are next to that group. 



XXVII. Instability of Mineral Species. — Within the narrow limits in 

 which these papers are necessarily confined, it is impossible even to 

 indicate many of the important physical and chemical relations of 

 minerals, and the different theories of genetic origin deduced from 

 them. I shall, however, give a few extracts illustrating Bischof's 

 theory of the instability, and consequently slow cyclical changes, of 

 mineral species, which of late years has attracted so much attention 

 and discussion in Germany ; and which, if established, will throw so 

 much light upon the genesis and metamorphism of rocks. The ex- 

 tracts are from the Cavendish Society's translation of his " Chemical 

 and Physical Geology." 



" Strictly speaking, we do not know with regard to any single mineral whether 

 it is still ni its original condition, or has been more or less altered. . . . The 

 alteration of a mineral is an extremely slow process. The material changes go 

 on so gradually that they are not chemically recognisable until after long periods 

 of time. In the analysis of a mineral in which such changes have already com- 

 menml, especially by the addition of new constituents, although in very minute 

 <iuantities, it is not unlikely to happen that they may be considered as accidental. 



