192 
AMERICAN GEOLOGY. 
the ingenious explanations which were offered to explain the 
easterly dipping of the quartz, limestone and slate rocks of 
Berkshire, Mass., beneath the gneiss of the Hoosick mountain. 
But this does not exist immediately adjacent to the range; there 
is a synclinal axis, the dip being westerly and from the range. 
The error in the dynamics arose from having determined the 
statics only in part. 
Among the points in statical geology which always require 
considerate attention, is the derivation of the materials compos¬ 
ing a formation or mass, when mica and talc enter into their 
composition. In cases when those minerals compose a rock, we 
might infer that it was metamorphic; especially when they 
exist in large proportions. But such a conclusion becomes 
doubtful if other facts in the statics of those rocks are wanting 3 
for mica and talc from the form of their particles, and the 
nature of their constitution, reproduce the same lithological 
masses as those from which they were derived. Hence it is 
certainly safe to be cautious in pronouncing a rock metamor- 
phic when composed of mica and talc. 
There are three kinds of forces which are recognizable in 
the earth’s crust; physical, chemical and biological. The first 
leaves its imprint upon its masses, and from those imprints its 
nature and its degree are inferential. 
The second furnishes results generally complete, and activity 
is rather inferential, and our conlusions must be based upon our 
knowledge of chemical principles, and the known mutual action 
of bodies upon each other. Chemical forces must frequently 
give origin to the physical, and they no doubt become the 
source of some of the most energetic of this class of operations. 
The third is entirely passive, leaves no mechanical impress, 
but they leave memorials of the highest moment. Biological 
forces are entirely passive when put in conflict with the dyna¬ 
mical and chemical, and always yield to their movements. 
It is in the construction of the globe that these forces are 
to be studied, and it is here that the conditions are revealed 
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