232 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



Of the other theories only two may be examined with any hope of a 

 rational application to the late Paleozoic: the effect of volcanic dust in the 

 upper atmosphere in reducing the amount of solar radiation which reached 

 the earth, and the effect of deformation. 



The first would have world-wide effect if the calculations of Abbot and 

 Fowle^ and Humphreys^ are correct and the continuance of the conditions 

 cited by them and quoted in Clements, Plant Succession, pages 322-324, 

 would be sufficient to bring on a glacial period. But, as noted by Clements, 

 "the only evidence of such continuance in geological time would have to 

 be sought in the coincidence, or immediate sequence, of cold or cooled 

 climates with periods of great eruptive activity." 



Schuchert^ has opposed the sufficiency of this cause, citing the fact that 

 the period of violent volcanic activity at the close of the Mesozoic was not 

 followed by a glacial but only by a "slightly cooled climate." The great 

 Paleozoic and Cenozoic glaciations were not in coincidence with the moun- 

 tain-making disturbances of those eras. Smaller disturbances localized 

 within periods of the eras produced a slight drop in temperature, but not 

 sufficient to produce glacial conditions. 



On the whole Schuchert is inclined to attribute the climatic changes to 

 deformational causes, perhaps accentuated by volcanic dust. He concludes :^ 



"We may therefore conclude that volcanic dust in the isothermal region of 

 the earth does not appear to be a primary factor in bringing on glacial climates. 

 On the other hand, it can not be denied that such periodically formed blankets 

 against the sun's radiation may have assisted in cooling the climates during some 

 of the periods when the continents were highly emergent." 



With this idea Huntington is apparently in agreement. The deforma- 

 tional theory is by far the most obvious and easily applied to the explanation 

 of most of the climatic changes. Evidence has been cited to show that the 

 eastern border of the continent of North America was being elevated in the 

 late Paleozoic, and this movement was but a part of the much greater move- 

 ment which elevated the Armorican-Variscan chains in Europe and the 

 Appalachian Mountains in North America. As the European portion of 

 this movement was accompanied by vigorous volcanic activity, it is very 

 possible that some late eruptions of great vigor supplied an adequate amount 

 of ash and that, as Clements suggests, the glacial conditions in North 

 America were induced by a coincidence of causes, perhaps in the order of 

 their importance, deformation, volcanic dust, and deficiency of CO2 in the 

 atmosphere. 



1 Abbott, C. G., and F. E. Fowle, Volcanoes and Climate, Smiths. Misc. Coll., vol. 60, No. 



29, 1913. 

 ^ Humphreys, W. J., Volcanic Dust and Other Factors in the Production of Climatic Changes, 



and Their Possible Relation to Ice Ages, Mount Weather Observatory Bull., vol. 6, 



No. I, 1913. 

 ^ Loc. cit., p. 287. 



