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serpentine has evidently a l;uk ot aliiniiiia; otlunvise it closely re- 
sembles the Vermont rock. 
Origin. 
The mineralogical composition of these serpentines strongly sug- 
gests that they are the alteration product of a basic igneous rock. 
The presence of original olivine in a large number of specimens, with- 
out any trace of pyroxene, makes it seem that the parent rock was 
almost a pure dunite; for if much pyroxene had originally been 
present, it would in all probability have resisterl alteration as long as 
the olivine. In the cases where pyroxene does occur it may be inferred 
that the parent rock was less basic, probably an ordinary peridotite. 
The alteration of peridotite to serpentine is caused by hydration. 
The source of the water may be either magmatic or meteoric. If 
magmatic, some later intrusive which supplied the water should be 
found either cutting the serpentine, or in its immediate vicinity. Such 
intrusives are not found, and consequently the theory of meteoric 
origin must be relied on to account for water. In support of this, 
it may be pointed out that ground water contains carbon dioxide and 
oxygen, which acting on the iron and magnesium liberated by the 
change from peridotite to serpentine, would help to explain the exist- 
ence of so much magnetite and dolomite in the serpentine. The 
alteration must have taken place at some depth, as ordinary surface 
weathering of peridotite would merely cause decomposition and 
disintegration, the products of which would tend to be simpler com- 
pounds such as carbonates, silica, and limonite, rather than the 
complex hydrous silicate forming serpentine. 
No very strong evidence can be obtained in regard to the origin 
of the serpentine from the forms in which it occurs. The most that 
can be said is that they are concordant with the existing schistosity. 
If the schistosity corresponds to the original bedding planes, they 
were true concordant intrusions. They do not, howe\'er, resemble 
sills or laccoliths, nor is it easy to imagine how they could have been 
changed from such to their present form, even as a result of later moun- 
tain building. The size of some of the partially altered grains of 
olivine implies that the intrusions did not occur as surface flows but 
have been revealed by long continued erosion. If the schistosity 
does not correspond to the original planes, they may have been vol- 
