508 C. SCHUCHERT CHRONOLOGY ON BASIS OF PALEOGEOGRAPHY 



basal Jurassic limestone, followed above by 4,000 feet of slates. Evi- 

 dently the Upper Jurassic material was derived from a high land, and in 

 places these formations are seen to rest nnconformably on the Triassic. 



Toward the close of the Middle Jurassic the northern Pacific, with a 

 cool-water fauna, began to spread widely over Alaska and British Colum- 

 bia, and, as the Logan Sea, continued into the States of Montana, Idaho. 

 Wyoming, Colorado, and Utah. In the Great Plains region the deposits 

 of the Logan Sea have an average thickness varying between 200 and 400 

 feet, but increasing to the west to upward of 1,000 feet and in south- 

 western Wyoming to 3,500 feet. The cross-bedded sandstones, the change- 

 able sediments, and the general prevalence of oysters indicate that the 

 sea was a shallow one, and. further, that it flowed over a warped land 

 eroded to a low relief. 



Volcanic activity began again locally along the Pacific border of North 

 America early in the Jurassic and continued throughout the period, be- 

 coming more marked toward its close than at any time during the Trias- 

 sic. The eruptions were in part submarine. 



Toward the close of the Jurassic the Sierra Nevadas, the Coast Eange 

 of California, and the Humboldt Eange of Nevada were elevated; also the 

 Cascade and Klamath Mountains farther north. The making of the 

 Sierra Nevada Mountains at this time was pointed out by Whitney in 

 1864 and further described by Dana. The marked significance of this 

 deformation has been emphasized more recently by Lawson, who regards 

 it as having the importance of a revolution, and Smith has given it the 

 name Cordilleran Eevolution. Last year Blackwelder called it the Ne- 

 vadian movement, but it seems better to retain the older implied term of 

 Sierra Nevada, just as we speak of the Appalachian and Laramide revoln 

 tions. That the Sierra Nevada movement was of wide extent and that it 

 was of greater importance than the average disturbances closing the 

 periods is admitted, for it is probable that mountains were made extend- 

 ing from Mexico into southern Alaska, and yet it had not the importance 

 of a revolution, when mountains were made in nearly all of the continents. 

 For these reasons I prefer to call it the Sierra Nevada Disturbance. 



The Sierra Nevada deformation also shut out the Arctic-Pacific inter- 

 communication and prevented further wide overlaps of the Pacific Ocean 

 over Canada and the United States. With the rising of these mountains 

 also began the formation of two new troughs or geosynclines. The smaller 

 one, which was clearly developed in latest Jurassic time, Le Conte has 

 named the Shastan Sea, and of this the present Great Valley of California 

 is the structural remnant. The other, of far greater extent. I have re- 

 cently named the Coloradoan geosyncline, but it was not in full develop- 

 ment until Cretaceous time. 



