CRUST AL DEFORMATION AS THE CAUSE OF CLIMATIC CHANGES. 257 



periods when the climate was so cold as to cause glaciation or a close approach thereto, 

 there have been various other periods of sudden cooling similar in character to glacial 

 periods but less marked. All of these are indicated in the chart, figure 91, which accom- 

 panies Professor Schuchert's paper, and part of which is here reproduced in figure 86. 

 The curves scarcelj^ need explanation. The lower one indicates the probable course of 

 variations of temperature during geological times. The high portions indicate a tropical 

 climate and the low portions a frigid climate. It should be noted that the curve is the 

 reverse of those used in the previous portions of this volume, in which high places, not 

 low, indicate an approach toward the conditions which induce glaciation. The upper 

 curve of figure 86 indicates the degree of aridity, the high portions indicating great dryness 

 and the low portions relative humidity. It will be seen at a glance that in a large number 

 of cases the evidences of great aridity and of glaciation appear at about the same time. 

 This may perhaps indicate that glaciation and aridity are due to the same cause. It is 

 equally possible, however, that the approximate coincidence of the two phenomena merely 

 indicates that we have to deal with periods of great climatic instabihty, during which epochs 

 of glaciation alternate with interglacial epochs of aridity. One of the Proterozoic glacial 

 periods and also the Permic and Pleistocene periods, as we have seen, point to this con- 

 clusion, and the others neither support nor oppose it, for our knowledge of them is still so 

 fragmentary that no conclusion is possible. 



Turning now from the question of the succession of glacial periods, let us see what 

 Professor Schuchert has to say as to their relation to movements of the earth's crust : 



"Of the four more or less well-determined glacial periods at least three, earliest Proterozoic, 

 Permic, and Pleistocene, occurred during or directly after times of intensive mountain-making, 

 while the fourth, late Proterozoic, apparently also followed a period of elevation. * * * On the 

 other hand, the very marked and world-wide mountain-making period * * * during late Mesozoic and 

 earliest Eocene times was not accompanied by a glacial climate, but only by a cooled one. The 

 cooled period of the Liassic also followed a mountain-making period, that of late Triassic times." 



An inspection of Professor Schuchert's diagram, as reproduced in figure 86, shows that 

 the agreement of mountain-making epochs and periods of climatic change is even closer 

 than he has indicated. In his diagram Professor Schuchert shows 22 periods of mountain- 

 making. Among these 22, Nos. 1, 4, 5, 10, and 15 accompany or immediately precede 

 great changes of climate. Nos. 19, 20, and possibly 21 are associated with distinct, but less 

 important changes, and No. 22 is associated with the great Pleistocene glacial period. 

 Small changes of chmate accompany or follow the mountain-making epochs Nos. 2, 6, 8, 

 9, 11, 13, and 17. The only mountain-making epochs not accompanied by a climatic 

 change of some sort, as indicated by Professor Schuchert's lines of temperature and aridity, 

 are Nos. 3, 7, 12, and 18 — only 4 out of 22. It is possible that these mountain-making 

 periods were also accompanied or followed by changes of climate, and that this does not 

 appear simply because the changes, like the mountain-making, were of relatively slight 

 magnitude and hence have escaped detection. This would scarcely be surprising, since it 

 is only about 30 years since the possibility of Permian glaciation began to be seriously 

 discussed, and practically all our knowledge of coolings of the earth's climate aside from the 

 Pleistocene and Permian glaciations has been obtained during the present century. A 

 basis of 18 out of 22 possible cases seems, then, to be good ground for Professor Schuchert's 

 statement that "cooled and cold cHmates, as a rule, occur during or following periods of 

 marked mountain-making." Yet the agreement between periods of mountain-making 

 and of cool climates is by no means perfect; for, as Professor Schuchert indicates, the degree 

 of coohng is not proportional to the intensity of mountain-making. This appears to be 

 especially noticeable in late Mesozoic and early Eocene times, and to a less extent in upper 

 Mississippian and late Ohgocene. In all these cases the mountain-making is proportionally 

 much more intense than the accompanying climatic change. Moreover, it must be 



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