250 W. H. HOCBS ORIGIN OF OCEAN BASINS 



of the earth naturally suggests the great revolutions which occurred in 

 the Permian and Algonkian times. It is noteworthy that each of the three 

 periods appears to have been followed closely by a more or less extended 

 continental glaciation, the Permian in "Gondwana land" and Brazil, 

 the Algonkian in China, South Africa, and Ontario. The changes in 

 extent and in depth of the ocean basins and the elevation of plateaus and 

 mountain barriers react on climate through affecting the humidity of the 

 atmosphere, through regulating the amount, character, and distribution 

 of precipitation, and through changes in the ocean currents, thus offering 

 at least a possible explanation of the glaciations. 



In conclusion, it may be pointed otit that there is in modern geological 

 thought a tendency for the pendulum to swing backward to complete its 

 cycle by returning toward the position which it had before the promulga- 

 tion of the well known doctrines of Lyell. 



