CRITICAL PERIODS IN THE HISTORY OF THE EARTH. 481 



ble conclusion only by admitting a more rapid rate of evolution during critical 

 periods. 



It is one of the chief glories of American geology to have first established 

 the Archaean as one of the primary" divisions of time. It is even yet reluct- 

 lantly admitted as such by many European, geologists. And yet it is seen 

 that from every point of view, whether of the rock system or of the life 

 system, it is by far the most widely and trenchantly separated of all the 

 eras. 



The next greatest lost interval (though far less than the preceding) is that 

 between the Palajozoic and the Mesozoic. Here we have the next most 

 general unconformity, indicating the next most wide-spread changes of 

 physical geograj)hy and climate, accompanied by the most sweeping changes 

 in organic forms, not only in species and genera, but also in families and 

 orders. This change is the more striking as it occurs in the midst of an 

 abundant life. It is the greatest and most general change in the forms of 

 organisms which has ever occurred in the history of the earth. It took 

 place, again, during a lost interval. A portion of the loss is recovered in the 

 Permian, but the most critical time, the time of most rapid change, namely, 

 that between the Permian and the Trias, is still missing. How we long to 

 find the steps of this great change ! What a flood of light would it shed on 

 the process of evolution ! But although the change in the organic kingdom 

 was, just here, so enormously great, yet the lost interval does not seem very 

 long, for in England the Trias and Permian seem to be conformable, though 

 probably with change from marine to fresh-water conditions. It is hen<^e 

 impossible to resist the conclusion that the steps were just here fewer and 

 longer and the progress more rapid than usual. As in human history, revo- 

 lutions are the times of the birth of new social ideas, upon which, during 

 the subsequent period of tranquility, society is readjusted in prosperity 

 and happiness on a higher plane, so also in geological history, critical pe- 

 riods are times of origin of new and higher organic forms, and the subse- 

 quent periods of tranquility are times of readjustment of equilibrium and 

 prosperous development of these forms. 



Like the previous lost interval, this was also a period of oscillation — a 

 period of great increase of land, which was again partly submerged to in- 

 augurate the Trias. It was, therefore, also a continental period. The land- 

 making commenced at the end of the Coal period, in this country with the 

 formation of the Appalachian Mountains, continued through the Permian, 

 and culminated in the lost interval, which is, in fact, for that very reason 

 lost. 



Far less in length of time and perhaps in the sweeping character of the 

 change of organisms, ]but far more important and interesting on account of 

 the high position of the animals involved, is the lost interval between the 

 Mesozoic and Csenozoic. The length of time lost here is comparatively 

 small. In America, in many parts of the "West, the uppermost Cretaceous 

 seems to pass into the lowermost Tertiary without the slightest break of 



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