326 



University of California. 



[Vol. i. 



the Laramie and the Glacial, there is good reason to believe that it 

 was of much longer duration than either of the others. The read- 

 justments of the earth's crust were much greater and it was much 

 longer time before the equilibrium was settled and the organic 

 kingdom again in a prosperous condition. The unconformity in 

 this case was still greater and more general, and the loss of record, 

 therefore, greater. The change in life-forms was simply enormous, 

 the greatest that has ever occurred in the history of the earth. In 

 Arctogaea the unconformity is almost universal, though not every- 

 where on exactly the same horizon. In the Southern Hemisphere, 

 including India, Australia, and South Africa, there is little or no 

 unconformity, but none the less are the changes of life-forms great 

 and rapid at this time, doubtless by change of climate and by migra- 

 tions of conquering Mesozoic forms from elsewhere. 



The mountain-monument of this great period of change, in 

 America, is the Appalachian Range. It is therefore called by Dana 

 the Appalachian Revolution. But everywhere it was a period of 

 great changes, and many other mountains originated at this time. 



Observe again that the time intervening between this and the 

 post-Cretaceous or Rocky Mountain Revolution, viz., the Mesozoic, 

 was a time of remarkable quiet and wonderful prosperity of the 

 organic kingdom. It is true that here in America and elsewhere 

 there were important changes in physical geography and some 

 mountain making at the end of the Jurassic, but these changes were 

 far less general and therefore far less potent in determining changes 

 in the organic kingdom. 



Among the causes of change in organic forms, one of the most 

 potent is oscillation of temperature. Now the most decided evi- 

 dences of glaciation known in any period except the glacial, are 

 found in the Permian, and that, too, in many widely-separated 

 regions. This is especially true of Australia and South Africa^ 

 where the glaciation seems to have been severe and undoubtedly 

 connected with the great elevation and enlargement of the Antarc- 

 tic continent, already spoken of. We have in these facts a probable 

 key to the origin of the great reptiles of the Mesozoic. May they 

 not have come by migration from the Southern Hemisphere, which 

 seems to have been the home (Karoo beds) of the earliest and most 



