METHODS — GEOLOGICAL 23 



limit the range of marine organisms, as climate and physical 

 barriers circumscribe the spread of terrestrial animals. Pro- 

 fessor Perrin Smith has shown that in the Mesozoic era Bering 

 Strait was repeatedly opened and closed, and that each opening 

 and closing was indicated by the geographical relationships of 

 the successive assemblages of marine animals that are found 

 in the Mesozoic rocks of California and Nevada. When 

 the Strait was open, the coast-line between North America 

 and Asia was interrupted and the North Pacific was cooled by 

 the influx of water from the Arctic Sea. At such times, 

 sea-animals from the Russian and Siberian coasts extended their 

 range along the American side as far south as Mexico, and no 

 forms from the eastern and southern shores of Asia accompanied 

 them. On the other hand, when the Strait was closed, the 

 Arctic forms were shut out and the continuous coast-line and 

 warmer water enabled the Japanese, Indian, and even Mediter- 

 ranean animals to extend their range to the Pacific coast of 

 North America. A comparison of the marine fishes of the 

 two sides of the Isthmus of Panama shows an amount and 

 degree of difference between the two series as might be expected 

 from the length of time that they have been separated by the 

 upheaval of the land. 



In working out the geographical conditions for any particular 

 epoch of the earth's history, it is possible to go much farther 

 than merely gaining an approximate estimate of the distribu- 

 tion of land and sea ; many other important facts may be 

 gathered from a minute examination of the rocks in combina- 

 tion with a genetic study of topographical forms. By this 

 physiographical method, as it is called, the history of several 

 of the great mountain-ranges has been elaborated in great detail. 

 It is quite practicable to give a geological date for the initial 

 upheaval and to determine whether one or many such series 

 of movements have been involved in bringing about the present 

 state of things. Similarly, the history of plains and plateaus, 

 hills and valleys, lake and river systems, may be ascertained, 



