1907.] 
Proceedings. 
69 
These chemical differences corresi)ond to certain physical and 
mineralogical differences which are likewise strongly marked, whence 
the two classes may be termed alkaline and magnesian, or again 
granitic and pyroxenic ; or again igneous and aqueo-igneous. The 
first class occurs in granitic rocks ; the second in pyroxene, especially 
in connection with calcite and apatite. Micas of the first class are 
comparatively hard, brittle and anhydrous; those of the second class 
are soft, flexible and contain slighth' more water, which is more easily 
driven oft' by heat. ISIoreover, the alkaline micas are more often clear 
and transparent, but the magnesian are invariably colored, whence it 
arises that the latter are worthless for glazing purposes, which has 
been until recently the principal commercial use of mica. It was 
not, therefore, until the development of the electrical industry that 
this magnesian mica came to have a commercial value. 
It is remarkable, however, that while the granitic micas are of 
common occurrence in all parts of the world, the magnesian micas are 
comj:!aratively rare, and there is only one district where they are at 
present mined, this being the Laurentian of Canada. 
The circumstance of formation and the difference of the deposits 
point clearly to an absolutely different mode of orig'in of the alkaline 
from the magnesian micas. While different theories have been put 
forward to account for the former, the weight of evidence seems to 
show an origin by metamorphism and heat alone, probably accom- 
panied by great pressure in the cases where large crystals are formed. 
The orig-in of the magnesian micas, on the other hand, is far from 
clear, but it is certain that it was absolutely different from that 
of the alkaline micas ; and the most likely theory seems to be that 
they were formed near the surface from or in connection with a 
magnesian magma with the aid of hot water, steam and gases. 
The speaker alluded to the existence of numerous facts in support 
of the theories and conclusions given, which, however, could not be 
stated in details in a short discussion. 
After some discussion on the part of several members, Mr. Charles 
H. Doerfiinger spoke on some recent archeological discoveries in 
Southern France. 
The following papers were ordered printed by the Board of 
Directors : 
