CLIMATES OF GEOLOGIC TIME. 289 



and especially of the animal world, is the most imperfect of all paleontologic records 

 until we come to Tertiary time. The known mammal history is a vast one and, although 

 very difficult to interpret from the climatic standpoint, we have in the work of Deperet 

 (1909) and Osborn (1910) glimpses into the many temperature fluctuations, faunal isola- 

 tions, and intercontinental radiations of Tertiary time. The history of the Tertiary is 

 the last one of at least three previous and similar records (Mesozoic, later and earlier 

 Paleozoic) of vastly longer eras, taking us back to a time when the lands were without 

 visible hfe. 



In conclusion, it is seemingly clear that the variability in the storage of solar radiation 

 by the earth's atmospheric blanket and by oceanic waters, and the consequent climatic 

 variations of the past and present are due in the main to topographic changes in the 

 earth's crust. These telluric changes alter the configuration of the continents and oceans, 

 the air currents (moist or dry), the oceanic currents (warm, mild, or cool), and the volcanic 

 ash-content of the atmosphere. 



On the other hand, a great deal has been written about the supply and consumption 

 of the carbonic acid of the air as the primary cause for the storage of warmth by the 

 atmospheric blanket. A greater supply of carbon dioxide is said to cause increase of tem- 

 perature, and a marked subtraction of it will bring on a glacial climate. This aspect of 

 the climatic problem is altogether too large and important to be entered upon here. It is 

 permissible to state, however, that the glacial climates are irregular in their geologic 

 appearance, are variable latitudinally, as is seen in the geographic distribution of the tillites 

 between the poles and the eciuatorial region, and finally that they appear in geologic time 

 as if suddenly introduced. These differences do not seem to the writer to be conditioned 

 in the main by a greater or smaller amount of carbon dioxide in the atmosphere, for if this 

 gas is so strong a controlling factor, it would seem that at least the glacial chmates should 

 not be of such quick development. On the other hand, an enormous amount of carbon 

 dioxide was consumed in the vast limestones and coals of the Cretacic, with no glacial 

 chmate as a result; though it must be admitted that the great limestone and vaster coal 

 accumulations of the Pennsylvanic were quickly followed by the Permic glaciation. Again 

 it may be stated that the Pleistocene cold period was preceded in the Miocene and Phocene 

 by far smaller areas of known accumulations of limestone and coal than during either the 

 Pennsylvanic or Cretacic, and yet a severe glacial climate followed. 



Briefly, then, we may conclude that the markedly varying climates of the past seem to 

 be due primarily to periodic changes in the topographic form of the earth's surface, plus 

 variations in the amount of heat stored by the oceans. The causation for the warmer 

 interglacial climates is the most diflficult of all to explain, and it is here that factors other 

 than those mentioned may enter. 



Granting all this, there still seems to lie back of all these theories a greater question 

 connected with the major changes in paleometeorology. This is: What is it that forces 

 the earth's topography to change v,'ith varying intensity at irregularly rhythmic intervals? 

 This difficult and elusive problem the older geologists solved with a great deal of assurance 

 by saying that such change was due to a cooling earth, resulting in periodic shrinkage; 

 but the amount of shrinkage that would necessarily have taken place to account for all 

 the wrinkUngs and overthrustings of the earth's crust during geologic time would be far 

 greater than that which has apparently occurred. Further, a coohng earth is yet to be 

 demonstrated. Again, some paleogeographers seem to see a periodic heaping up of the 

 oceanic waters in the equatorial region and a pulsatory flowing away later toward the poles. 

 If these observations are not misleading, are we not forced to conclude that the earth's 

 shape changes periodically in response to gravitative forces that alter the body-form? 



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