328 PAST CLIMATES AND CLIMAXES. 



directly, the researches of KuUmer and Huntington indicate almost conclu- 

 sively that the sun-spot cycle is causally connected with the shifting of the 

 storm-track and hence with efficient variations of rainfall and temperature. 

 To-day, as in the historic past, the sun-spot process of the sun seems to be in 

 control of the major and minor climatic cycles of the earth, and the effect of 

 volcanic dust is merely to modify the solar control. During geological times 

 this relation was doubtless the same, though volcanic agencies must have 

 often had a greater relative importance. The chief difference in causation 

 must have been in the assumption of the primary r61e by deformation. While 

 this point will perhaps never be susceptible of complete proof; the corre- 

 spondence between deformation and glaciation as shown in Schuchert's table 

 is very conviucing. Major geological climates seem then to have been due 

 primarily to deformation, reinforced or modified by solar activity, often in a 

 critical fashion, which was in its tiu-n somewhat affected by volcanic eruptions. 

 Coincidence of causes. — ^Hmnphreys (1913 : 32) has suggested the coinci- 

 dence of causes in the production of climatic changes: 



"It is surmised, therefore, that the greatest of our past climatic changes 

 may have been caused by the combined and roughly simultaneous variations 

 in continental level and volcanic activity; cold periods coming with increase 

 in elevation and increase in vulcanism, minor climatic oscillations with tempo- 

 rary changes in vulcanism, and warm periods when the land had gone back 

 to low levels and volcanoes had ceased greatly to veil the skies with dust." 



As Schuchert (1914 : 286) has pointed out, the coincidence of times of great 

 volcanic activity in the past with cold or cooled climates is far from convincing. 

 There does, however, seem to be much, if not general, coincidence of deforma- 

 tion and vulcanism, and it appears probable that increasing knowledge of the 

 climates of the past will connect these coincidences with cold or cooled climates. 



The absence of efficient deformation at present and during the Human period 

 confines the possibiUties of coincidence to solar cycles and volcanic eruptions. 

 Since sun-spot cycles form a continuous series, volcanic dust becomes a coin- 

 cident cause whenever it is carried into the upper atmosphere in efficient 

 quantity. Such a coincidence seems to have been amply demonstrated by 

 Humphreys, as shown in plate 57. It is especially striking in the case of 

 Mayon, Asamayama, Tomboro, Babuyan, and Krakatoa. In the case of 

 Kotlugia, Fuego, and Katmai, the effect was to diminish or destroy the normal 

 influence of the sim-spot miniTniiTn in increasing the mean temperature. 

 Coincident vulcanism may thus have a plus or a roinus effect. Its plus effect 

 seems the most important, in that it emphasizes the normal influence of a sun- 

 spot maximum, but the minus effect may actually produce a greater departure 

 relatively by destroying the influence of a minimum. When two more or less 

 complete minus effects are followed by a striking plus effect, as from 1754 to 

 1789, the combined action must be considerable, if not decisive. 



In the geological past, the question of coincidence is more complex because 

 three causes are concerned, namely, deformation, sun-spot cycles, and vul- 

 canism. At the same time it is evident that coincidences were correspondingly 

 more frequent, if indeed they may not better be regarded as continuous, at 

 least over long periods. Great body deformations must have lasted through 

 thousands of years, in which major sun-spot maxima and minima must have 



