322 PAST CLIMATES AND CLIMAXES. 



3, 7, 12, and 18. Numbers 1, 4, 5, 10, 15, and 22 are associated with great 

 changes of climate; 19, 20, and 21 are connected with distinct changes, and 

 2, 6, 8, 9, 11, 13, and 17 with minor changes of climate. Huntington states 

 that — 



"A basis of 18 out of 22 possible cases seems, then, to be good ground for 

 Professor Schuchert's statement that 'cooled and cold climates, as a rule, 

 occur during or following periods of marked mountain-making.' Yet the 

 agreement between periods of mountain-making is by no means perfect; for, 

 as Professor Schuchert indicates, the degree of cooUng is not proportional to 

 the intensity of mountain-making. This appears to be especailly noticeable 

 in late Mesozoic and early Eocene times, and to a less extent in upper Missis- 

 sippian and late Oligocene. In all these cases the mountain-making is pro- 

 portionally much more intense than the accompanying climatic change. 

 With reference to the causal relation of deformation to glacial periods, Schu- 

 chert (1914 : 286) says : '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 duriiig 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.'" 



As is shown later, the effect of a crustal deformation must have been modi- 

 fled repeatedly or regularly by the various cycles of solar radiation. The 

 coincidence of a maximum sun-spot cycle, such as that of 1780, and of volcanic 

 eruptions, like those of 1783-1785, but of much greater magnitude and extent, 

 such as happened in nearly every geological period, with a major deformation 

 would necessarily have produced a maximum glaciation. Conversely, a 

 mininniim sun-spot cycle coincident with a major deformation would have 

 tended to reduce the coohng effect of the latter, particularly if the coincidence 

 recurred at more critical times. The presence or absence of coincidence of 

 solar cycles seems to afford a plausible explanation of the varying effects of 

 mountain-making as shown above, particularly if we accept the importance 

 ascribed to solar cycles and volcanic eruptions by Huntington, Humphreys, 

 and others. The coincidence of solar and deformational cycles seems also to 

 furnish a possible solution of the difficulty felt by Huntington (1914:261), 

 namely, that "in the absence of any assignable cause, there seems to be some 

 ground for the hypothesis that throughout the course of geological history 

 disturbances of the earth and of the sun have occurred at about the same 

 time." 



The volcanic hypothe_sis.— Abbot and Fowle (1913 : 24), and Humphreys 

 (1913 : 1) appear to have shown beyond a doubt that the dust of great volcanic 

 eruptions exerts a measurable and important lowering upon atmospheric 

 temperatures. Abbot and Fowle have given the following sununary of their 

 investigations into the effect of the eruption of Mount Katmai upon the 

 direct radiation of the Sim: 



"The transparency of the atmosphere was much reduced in the summer of 

 1912 by dust from the volcanic eruption of Mount Katmai, June 6 and 7. 



