ORIGIN AND DIFFERENTIATION 563 



thereby be lowered by about 7° C. But it is not certain that the effect of this 

 considerable diminution of heat was not counteracted by some change in the 

 average cloudiness, or in the nocturnal radiation of the earth to space. It is 

 conceivable that the cloud of haze prevented the escape of radiation of the 

 earth to space in the same manner that it prevented the incoming of radiation 

 from the sun to the earth, so that the decrease of heat available to warm the 

 earth may have been in part or in whole compensated by a decrease in the 

 rate of escape of heat from the earth, owing to the presence of the haze."- 



Although further and perhaps more exact observational data ma}^ be 

 required to determine accurately the several factors involved, it appears 

 to have been demonstrated with a remarkable degree of certaint}' that the 

 presence of a veil of volcanic dust in the higher atmosphere may have an 

 appreciable effect in decreasing solar radiation, and hence in reducing 

 the amount of the sun^s heat that reaches the earth. From the historical 

 record, which covers onl}^ about 150 years, it appears that practically 

 all considerable volcanic eruptions have been followed by cold or cooler 

 temperatures. It is therefore assumed that the action was similar ia 

 past geologic ages; but whether it is now or has ever been sufficient 

 to initiate glaciation may well be doubted, though it may well enougli 

 have been a minor contributing factor. 



VIEWS OF BARRELL 



It appears to be the conviction of many geologists that the world-wide 

 mild climates of earlier geologic ages, together with middle latitude gla- 

 ciation, etcetera, may have resulted from the combination of a number 

 of local factors rather than from a single compelling cause. For ex- 

 ample, Barrell writes me as follows : 



"I am inclined to think that a number of fundamental factors have been 

 involved. The earth now has a very pronounced relief with high mountain 

 ranges and lack of shallow seas. The high mountain ranges comb out the 

 moisture from the air. Less moisture means cold continental climates. Gen- 

 eral coldness reduces evaporation and the smaller water area also results in 

 less evaporation. In former ages a greater evaporation area and lesser moun- 

 tains would result in far higher normal humidity for the whole earth. Rain 

 would result from a greater degree of super saturation, chiefly in higher lati- 

 tudes, and its precipitation would tend to keep the temperature high by giving 

 up the latent heat of the water vapor. Warm waters and cloudy skies would 

 aid in giving a winter mildness of climate. We have that now prevailing in 

 western Ireland, western Patagonia, and certain other regions with latitude 

 over 50°. The mildness and moistness of climate permit in both of those very 

 rainy regions extreme luxuriance of vegetation. To account for mild polar 

 climates these conditions would have to be more wide-spread and reach 30° 

 farther from the equator. The results would be helped out by a perpetual 



