Geology and Natural History. 153 
depolarizes like a uniaxial erystal having the principal axis per- 
pendicular to the cleavage. When the mica scales conform thus 
In position to the plane of bedding in a slate, the author holds 
that they are probably of fragmental origin; since he has found 
In other slates in which the mica was “formed in situ, that the 
crystals of mica are not stratified but lie at all possible azimuths, 
and moreover are collected about special centers.” 
As an inference from the study of slates and other rocks, he 
j of ¢ 
true mica schist,” the only essential difference being in the differ- 
ence in size of the crystals. Some fine-grained mica schists are 
land were originally slates” is worked out microscopically with 
great care, and the very probable conclusion is reached that 
taking into consideration the character of both the feldspathic 
and quartzose grains,” the material was to a considerable extent 
derived from a granite of a type very unlike that of Cornwall, but 
in some respects analogous to that of Aberdeen, though differing 
om it in being more like a quartz felsite. 
These are a few of the deductions, in the Address, arrived at 
from the study of the least promising of all rock-formations— 
sand-beds and ‘slates, 
7. Geological Survey of Pennsylvania.—Great progress has 
been made toward the completion of the geological survey of 
