HISTORICAL GEOLOGY 107 



short but decisive periods of cooled waters and great mortality, 

 followed by quick evolution, and the rise of new stocks. The 

 times of less warmth are the miotherm and those of greater 

 heat the pliotherm periods of Eamsay. 



On the land the story of the climatic changes is different, but 

 in general the equability of the temperature simulates that of 

 the oceanic areas. In other words, the lands also had long- 

 enduring times of mild to warm climates. Into the problem 

 of land climates, however, enter other factors that are absent 

 in the oceanic regions, and these have great influence upon the 

 climates of the continents. Most important of these is the peri- 

 odic warm-water inundation of the continents by the oceans, 

 causing insular climates that are milder and moister. With the 

 vanishing of the floods somewhat cooler and certainly drier 

 climates are produced. The effects of these periodic floods must 

 not be underestimated, for the North American continent was 

 variably submerged at least seventeen times, and over an area 

 of from 154.000 to 4,000,000 square miles. 



When to these factors is added the effect upon the climate 

 caused by the periodic rising of mountain chains, it is at once 

 apparent that the lands must have had constantly varying 

 climates. In general the temperature fluctuations seem to have 

 been slight, but geographically the climates varied between mild 

 to warm pluvial, and mild to cool arid. The arid factor has 

 been of the greatest import to the organic world of the lands. 

 Further, when to all of these causes is added the fact that dur- 

 ing emergent periods the formerly isolated lands were connected 

 by land bridges, permitting intermigration of the land floras 

 and faunas, with the introduction of their parasites and parasitic 

 diseases, we learn that while the climatic environment is of fun- 

 damental importance it is not the only cause for the more rapid 

 evolution of terrestrial life . . . 



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 difficult 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 with varying intensity at irregularly 

 rhythmic intervals? . . . Are we not forced to conclude that 

 the earth's shape changes periodically in response to gravitative 

 forces that alter the body-form?" 



