CLIMATIC CYCLES. 329 



recurred at least several times, as also volcanic eruptions of major intensity. 

 Minor or regional deformations probably occurred at any time during a defor- 

 mational cycle. Remote as they are from the present, only the effects of 

 major climatic changes can have come down to us, though it is clear that these 

 changes were of different intensity. In accordance with probability, the 

 greatest climatic changes of the past should have been the combined result of 

 major deformation, maximum sun-spot cycle, and marked eruptive activity. 

 The coincidence of extraordinary sun-spot maxima can only be inferred, 

 though their recurrence seems beyond question, if we may judge from historic 

 times. In any event, Himtington has proposed an ingenious and plausible 

 explanation of glaciation upon the basis of the cyclonic effects of sun-spot 

 periods. On the other hand, while Schuchert is doubtless correct in regarding 

 deformation as a primary and volcanic action as a secondary cause, it seems 

 probable that they were regularly associated. The general assmnption that 

 vulcanism is more or less directly connected with crustal deformation lends 

 support to this view. If this be correct, then volcanic dust must have been a 

 secondary though constant concomitant of the 22 deformations indicated by 

 Schuchert, of which 18 were followed by cooled or cold climates. Moreover, 

 it seems plausible to assume that many, if not all, of these, and especially the 

 greater ones, were associated with maximum sun-spot effects. Since the out- 

 standing changes of geological climates were those of cooling, they would 

 appear to be due to the plus effect of great sun-spot maxima and intense vol- 

 canic eruptions coincident with major crustal deformations. Sun-spot cycles 

 and volcanic activity, together or separate, must have repeatedly exerted a 

 minus effect during gradational phases, but our knowledge of the record is 

 still too imperfect to reveal this. 



As has been previously suggested, the coincidence of causes with the same 

 but cumulative or plus effects seems ample to explain the great glaciations of 

 the past. This seems especially true when it is recalled that a primary or the 

 primary cause of glaciation is assumed to be deformation by Schuchert, solar 

 activity by Huntington, and volcanic action by Hiunphreys. As a conse- 

 quence, there seems no need to assume some unknown cause of coincident or 

 related deformation and solar activity. Since deformation extends over long 

 periods, it is inevitable that times of great sim-spot maxima, such as those of 

 1370 to 1385 and 1775 to 1785, must have occimred repeatedly, augmenting the 

 climatic consequences of deformation, and often being reinforced by intense 

 volcanic activity. 



CLIMATIC CYCLES. 



Relation of changes and cycles. — It is here assumed that all climatic changes 

 recur in cycles of the most various intensity and duration. In fact, this seems 

 to be established for historic times by Huntington and for geologic times by 

 the studies of glacial periods which have made possible the table compiled by 

 Schuchert (fig. 26). The cychc nature of climatic changes has been strongly 

 insisted upon by Huntington (1914 : 1; 1914^ : 529): 



"The considerations which have just been set forth have led to a third 

 hypothesis, that of pulsatory climatic changes. According to this, the earth's 

 climate is not stable, nor does it change imiformly in one direction. It appears 

 to fluctuate back and forth not only in the little waves that we see from year 



