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 ) „^ „ 
Peroxide Iron j ' ' " 
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. Tiiey contain essentially Al, and K, and (in the two 
first-named varieties) Fe, and usually some water. Chlorite arid Ripi- 
doUte are physically allied to the micas, but they differ chemically in 
not containing an alcaline base. 
Hornhlendic and Talc groups. — These extend from Wollastonite, 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. InstahiUty of Mineral Species. — Withia the nai'row 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 \ni\\ regard to any single mineral wliether 
it is stiU in its original condition, or has been more or less altered. . . . The 
altei-ation of a mineral is an extremely slow process. The material changes go 
on so gradually that they are not chemically recognisable until after long jieriods 
of time. In the analysis of a mineral in which such changes have already com- 
menced, es))ecially by the addition of new constituents, although in very minute 
quantities, it is not unlikely to happen that they may be considered as accidental. 
