506 C. SCHUCHERT PALEOGEOGRAPHY OF NORTH AMERICA 



(see table, page 499). By adding together all the percentages of the 

 periods represented, North America will be found to have the sum of 559 

 against 695 for the United States. 



The many pulsations of ancient Tethys across Europe, Africa, and 

 Asia, and the far rarer Arctic invasions, furnish evidence of the same 

 fact — that the equatorial waters are the more persistent. Tethys extended 

 from Spain to India and China, and today is represented by the Mediter- 

 ranean. These facts were also noted by Suess (II: 252). 



On discussing the foregoing conclusions with Professor Barrell, he said 

 that the relation between equatorial protuberance and periodic shrinkage 

 could be given quantitatively, and it would then be seen whether or not the 

 postulate had any real value. It will be seen from his statement that the 

 amount of water periodically taken from the polar regions and heaped in 

 the equatorial belt is in quantity more than sufficient for the periodic 

 withdrawal of the continental seas. He writes as follows : 



Influence of earth shrinkage on the distribution of continental seas. 

 By Joseph Barrell. — The great movements which have broken up the 

 earth's history into periods are, as Professor Schuchert indicates in this 

 paper, in general characterized by broad continental emergences either 

 slight or pronounced in character. Evidences of circumferential short- 

 ening are always local, and may or may not be observed, distinguishing 

 especially those greater movements which have been classed as revolu- 

 tions. But even the slight disturbances, when recorded by the recession 

 of continental seas from quiescent continental areas, are thought to be- 

 due principally to subsidence of the oceanic bottoms, lowering in turn the 

 general surface level of the waters of the ocean. All movements, there- 

 fore, which mark off periods are in all probability due to earth shrinkage,, 

 which for each of the greater revolutions may, according to Yan Hise, 

 amount to a radial shortening of from 8 to 16 miles. The momentum of 

 the earth remaining constant, it follows that as a result of such shrinkage 

 a greater speed of rotation must ensue. 



According to recent studies by Chamberlin and others, 132 tidal friction 

 has been a negligible factor during the known history of the earth; but 

 even were it important enough and able to produce an appreciable secular 

 slowing of the earth's rotation, temporary accelerations would result, 

 accompanying the comparatively rapid shrinkage during the transition 

 epochs which separate the successive periods. Chamberlin and his co- 

 workers have shown that the figure of the earth could not have been 

 greatly altered by this means, but Professor Schuchert has raised the- 



132 The tidal and other problems. The Carnegie Institution of Washington, 1909. 



