ON THE CRYSTALLIZATION OF IGNEOUS ROCKS. 
BY 
Joseph Paxson Iddings. 
[Read in abstract before the Society, May 25, 1889.] 
INTRODUCTION. 
In the search for evidences of the character and composi¬ 
tion of the interior of the earth attention has naturally been 
turned to those rocks which, by their mode of occurrence on 
the surface of the globe, give ample evidence of having come 
from greater depths than those once occupied by the lowest 
upturned sediments or by the great body of crystalline schists 
and gneisses. 
Volcanic lavas that reach the surface in a highly heated, 
fluid condition, and other crystalline masses that traverse 
the earth’s crust in dikes, sheets, and laccolites, which are of 
similar origin and are classed with the former as igneous 
rocks, have evidently been erupted from deep-seated sources 
in a molten state, and appear to have been the same in the 
earliest geological ages that we have records of, as now. 
They may fairly be considered as the material evidence 
concerning the greatest depths within our planet of which 
we can hope to obtain direct knowledge. 
How far the phenomena of igneous rocks may contribute 
to the solution of the problem of the earth’s interior, or 
whether they may, in fact, belong to the superficial portion 
of the earth’s mass, is still an open question, an answer to 
which will not be found in the present paper. 
It is not the purpose of this paper to discuss all the varied 
phenomena of igneous rocks; nor is it considered that those 
here treated of are the phenomena which may contribute 
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