THE BORDER-LINE BETWEEN PALEOZOIC AND 

 MESOZOIC IN WESTERN AMERICA 



There have been in recent years in America many contro- 

 versies as to the Silurian-Devonian and the Devonian-Carbonif- 

 erous boundaries; but American geologists have always felt 

 secure as to the line of demarcation between the Paleozoic and 

 the Mesozoic. This has always been thought to be marked by a 

 grand chasm, a hiatus in stratigraphy and a break in life, 

 accompanying a great change in physical geography, all of 

 which is true of the region east of the Rocky Mountains. 



But later discoveries in the Great Basin region have shown 

 that the gap is at least partly filled out by marine sediments, 

 and that the hiatus is not universal in America. The most 

 important of these discoveries was the findingof marine Permian 

 in northern Texas, with forms suggestive of the Mesozoic types 

 of life, and the finding in southeastern Idaho of marine Lower 

 Trias, with a fauna reminiscent of the Paleozoic. 



Recently there has been evident among geologists a tendenc}' 

 to revert to first conditions in determining the boundaries of 

 geologic systems. They have been inclined to use unconformi- 

 ties as the dividing lines between geologic groups, and to 

 look upon geologic systems as realities, and not mere names for 

 the convenience of stratigraphers. This must mean that they 

 regard the geologic events that delimited the systems as of 

 world-wide effectiveness, or even of cosmic origin. But almost 

 every trangression of the sea has been found to be balanced by 

 a retreat elsewhere, every uplift to have its correlative subsi- 

 dence. And instill other parts of the earth there may have been 

 neither uplift nor subsidence. Even the Appalachian revolution 

 did not effect the western Carboniferous Sea, and this is gener- 

 ally admitted to have been one of the greatest events in the 

 dynamic history of North America. 



In defining the upper Paleozoic and the lower Mesozoic no 



512 



