Download Frederic Clements' Book on Plant Succession in PDF ...

CAUSES OF CLIMATIC CHANGES. 321
Humphreys (1913:3, 28), and Huntington (1914:258, 289; 1914^566).
This leaves the deformational, solar and volcanic hypotheses for consideration
upon the basis of actual observation at the present. The first task is to trace
the correlation of each of these with climatic changes. The second task is to
determine the relative importance of the changes induced by these three
causes, and the third is to relate the cycles which result.
The deformational hypothesis.— ^The assumption that major and minor
changes of climate are caused by body and superficial deformations of the
earth's crust has already been discussed in considerable detail. The relation
of crustal deformation to the climates of geologic time has recently been pre-
sented by Schuchprt in clear and convincing fashion (Huntington, 1914 : 265,
Fig. 26.—Schuchert's chart of geological climates and deformations, slightly modified.
255). He recognizes seven periods of glaciation, namely, Pleistocene, Permian,
Devonian, Cambrian, Latest, Undated, and Earliest Proterozoic, of
which the first two at least were marked by several glacial-interglacial cycles.
The evidence of sediments as to aridity and the biologic evidences of climate
are sketched in a comprehensive manner. The most significant feature of
the discussion, however, is the chart of geological climates, which is here
reproduced (fig. 26). In this the major deformations are indicated in the
hne "Times of mountain-making." The direct effects of these upon climate
are seen in the temperatm^ curves above, and the indirect action upon deposits
and vegetation is shown in the remaining curves. As Huntington points out
(1914 : 257), the chart shows 22 periods of deformation or mountain-making.
Of these, but 4 are imaccompanied by a chmatic change of some kind, viz.