500 r. H. KXOWLTOX EVOLUTION OF GEOLOGIC CLIMATES 



Page 



Sources of heat 540 



In general 540 



Heat from a bigger sun 541 



Heat from the earth itself 542 



Radioactive heat 544 



Ocean temperatures 545 



Evidences of glaciation 547 



Physiological action of increased cloudiness on plant life 548 



Obstacles to the acceptance of earth heat 550 



Review of hypotheses proposed to account for the initiation of glacial 



activities 554 



In general 554 



Shifting the position of the poles 554 



Continental elevation hypothesis 555 



Changes in atmospheric circulation h5i)othesis 556 



Hypothesis of the variation in the atmospheric content of carbon- 

 dioxide 557 



Transportation theory 558 



Reversal of deep-sea circulation 559 



Volcanic dust as a climatic factor 561 



Views of Barrel! 563 



Summar3' and conclusion 564 



Ils^TRODUCTIOis- 



In present-day acceptance the term climate has come to be applied to 

 the atmospheric conditions or weather normal to a given locality or re- 

 gion, especially as affecting life, health, comfort, and the multifarious 

 activities of terrestrial existence. Although there are very considerable 

 areas of the earth's surface that exhibit similar climatic conditions, it 

 needs little reflection to demonstrate that the climate of the earth is by 

 no means uniform throughout. If, for instance, we travel either north 

 or south from the equator, we pass successively through a so-called torrid 

 zone, a temperate zone, and, in polar lands, a frigid zone, each with minor 

 but distinctive modifications. In other words, the present distribution 

 of climate is zonal. 



This zonal distribution of climate on the earth, as we know it at the 

 present time, is a phenomenon that appears to have had its origin, or at 

 least its most marked accentuation, during and subsequent to the Pleisto- 

 cene or so-called Ice Age. Its persistence at the present time is due to 

 the very good reason that we are living in what is either an interglacial 

 cycle, of which there already have been several, or possibly in a period 

 that may represent the permanent waning of the Ice Age. Although as 



