PRE-LAURENTIAN HISTORY.


Furthermore, geologists, by a more critical examination of volcanic
ejections, have arrived at conclusions adverse to their origin from a
liquid interior. At the first flash of thought, the two or three hun-
dred large, active volcanoes, and the thousands of extinct ones,
scattered over the face of the earth, seem to point quite explic-
itly to a molten interior. But when, on critical examination, it is
found that they eject different kinds of matter at different stages,
that adjacent volcanoes may, at the same time, be giving forth very
different material, and especially that the liquid matter, in neigh-
boring vents, may stand at heights differing by hundreds, and even
by thousands of feet, it seems an almost necessary conclusion that
volcanoes do not have their origin in a common liquid reservoir.
Other evidences may, be deduced in support of the same con-
clusion.'
Restraining ourselves to these mere hints as to the lines of argu-
ment, we may pass the subject with the statement that the recent
tendency of scientific opinion has been toward the theory of an
essentially solid interior.
It is quite likely that the true view is not correctly repre-
sented either by the notion of a solid or of a molten. mass, as those
terms are commonly understood, but rather by the peculiar state of
a solid under great pressure. It is a well known fact that solid
bodies, under great pressure, yield, by flowing like a liquid or plastic
body. A familiar instance is found in the punching of holes in
steel plates. A part of the steel flows laterally and only a part is
punched out, the amount of steel in the core never equaling that
displaced by the punch. That the interior of the earth is yielding
and plastic, the phenomena of geology seem to quite clearly indi-
cate, but the plasticity may be of the nature of a solid under press-
ure, rather than that of a liquid due to temperature.
None of these views, however, militate against the theory of the
former molten condition of the earth, or of its igneous, and ulti-
mately, nebulous origin.
Chkareters of the Prinmitive Critst. It is a question of some theo-
retical interest and importance to determine wvhat was the nature of
the primitive crust of the earth, using that term, as is commonly
done, for convenience, to express the original outer layer, without
1 The student will find valuable discussions of this and kindred themes in
the
Geological Exploration of the Fortieth Parallel, Vol. I, by Clarence King;
in
the Geology of the High Plateaus, by C. E. Dntton; in Richthofen's Memoir
on
The Natural System of Volcanic Rocks; in Volcanoes (International Scienitific
Scries), by J. W. Judd, and in Dana's Manual, pp. 716, 808.


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