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, Per- 

 mian, 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. 



