THE SOLAR HYPOTHESIS. 235 



The four theories which have just been mentioned, with their appropriate modifications, 

 are the only ones which have hitherto had any permanent standing as attempts to explain 

 glaciation or the other climatic vicissitudes of geological times. The precession theory 

 seems to have been thoroughly tested and found wanting. It demands a rigid periodicity 

 which ought to cause the recurrence of the same phenomena repeatedly at precisely the 

 same intervals. Possibly precession may account for slight chmatic variations, but the 

 larger changes fail entirely to meet its requirements. The three other theories — that is, 

 those of carbonic acid, elevation of the lands, and solar changes — may be considered as 

 standing at the present time upon an equal footing. It is incumbent upon us now to test 

 them in the Ught of the new knowledge which seems to have come from the study of the 

 climate of historic times. 



The outstanding fact to which our investigations seem to lead is that at the present 

 time the chmate of the world is highly unstable. Contrary to the old idea of uniformity, 

 we find, apparently, that cycles, large and small, are continually in progress. So far as 

 our present knowledge goes, it is impossible to differentiate between the larger and the 

 smaller except in the matter of size. If this is so, it seems essential that an acceptable 

 theory should explain not only the great changes but the small ones. In other words, if 

 we omit for the moment the great phenomena of the redistribution of the climatic zones of 

 the earth as a whole, and consider only glacial epochs and smaller phenomena, the causes 

 of chmatic variations must apparently be capable not only of large, slow changes, but 

 also of those which are small and rapid. A cause must be found to explain long cycles, 

 such as the glacial and interglacial epochs of the Pleistocene period, and that same cause 

 or some other must also be of such a nature that in the space of half a century it can produce 

 a change as great as that which apparently occurred between 1300 and 1350 a. d. At this 

 point both the theory of elevation and that of carbon dioxide seem to break down. It seems 

 as if neither the altitude of the mountains and continents of the world, nor yet the amount 

 of carbon dioxide in the air can, almost in our own day, have changed so quickly and 

 markedly as to cause an epoch such as that which seems to have culminated in the four- 

 teenth century. 



Moreover, thi'oughout geological time there is good reason to think that minor changes 

 of chmate are of much more frequent occurrence than is commonly supposed. I have 

 discussed this matter in an article upon "Characteristics of the Glacial Period in Non- 

 glaciated Regions,"* and shall not dwell on it here. The work of Gilbert upon hmestones, 

 and the fuller work of Barrell upon sedimentation, seem to indicate the constant succession 

 of minor climatic fluctuations during a large portion of geological time. These, too, must 

 be explained by any complete chmatic theory. 



Finally, we may add that so far as any absolute measurements have been made, we have 

 no definite evidence of variations in chmatic conditions corresponding to any observed 

 change in the altitude of the earth's surface or in the amount of carbon dioxide in the 

 air. Therefore we may say that so far as the small climatic changes are concerned, it is 

 hard to conceive of their having arisen in accordance with either of these two theories. 

 This by no means must be taken as implying that changes both in the composition of the 

 air, and far more in the altitude and position of the land, have not had most pronounced 

 effects. It merely means that they appear to be of importance chiefly in reference to 

 long-continued, slow changes, but fail when an attempt is made to use them in explanation 

 of anything except major phenomena of long endurance. 



At this point we must raise the query whether purely meteorological causes now in 

 operation may not suffice to account for minor climatic changes. The varying conditions 

 of the seasons, the accidental accumulation of masses of clouds, the fortuitous convergence 

 of an unusual number of storms, the heating of a portion of the earth's surface by lava, 



* Bulletin of the Geological Society of America, vol. 18, 1907, pp. 351-388. 



