400 B. WILLIS A THEORY OF CONTINENTAL STRUCTURE 



thicknesses we may place that of 5,000, determined by Walcott for the 

 Kanab section, Arizona, which lies on a positive element. The condition 

 which determined the accumulation of 30,000 feet of strata in the Great 

 Basin region was, I take it, the subsidence of that area between the Colo- 

 rado element on the east and the Pacific element on the west. This 

 movement was a decidedly negative one. 



In northern Idaho, western Montana, and Britisli Columbia, there is a 

 thickness of 37,000 feet of late Proterozoic (Algonkian) strata,* and to 

 this we must add the Paleozoic, which comprises! at least 17,000 feet 

 above the base of the Castle Mountain group. 



This region of profound subsidence lies between the Pacific land, 

 from which the bulk of the sediments was derived, and Laurentia. In 

 comparison with them its negative character stands out distinctly. 



The negative elements of the central portion of the continent present 

 very diverse aspects. 



Between Missouri and Colorado the region of the Great plains is the 

 surface of a comparatively thick mass of Paleozoic and Mesozoic sedi- 

 ments. Along its southern margin the Paleozoic measures 18,000 to 

 23,000 feet in thickness, J including the Middle Cambrian and the Car- 

 boniferous. The strata rest on pre-Cambrian granites, which appear to 

 constitute the northern limit of the Llano positive element. The extent 

 of the Great Plains element northward from Indian Territory and Okla- 

 homa is not readily defined. The Paleozoic sections of the Colorado 

 Front, the Black Hills, and the Bighorn^ range from 1,000 to 3,000 feet 

 only and represent the sediments deposited on the margin of the Colorado 

 positive element. Adding the Mesozoic, we get a total of 8,000 feet, 

 much of which falls in the upper Cretaceous and therefore in a period of 

 tangential pressure, when subsidence and elevation can not be attributed 

 without question to isostatic vertical adjustment. Nevertheless it ap- 

 pears probable that a trough separated Colorado from Ozarkia and Lau- 

 rentia during the Paleozoic and Mesozoic and connected with the nega- 

 tive element of British Columbia. 



East of the Ozarkia-Isle Wisconsin zone the coal basin of Illinois pre- 

 sents an area of moderate depression which is bounded on the east by the 

 Cincinnati-Nashville axis. In Michigan is a similar basin. I regard 



* C. D. Walcott : Algonkian formations of northwestern Montana. Bull. Geol. Soc. 

 Am., vol. 17, 1906, pp. 1-29. 



t R. G. McConnell : Report on the geological, structure of a portion of the Rocky 

 mountains. Canada Geol. and Nat. Hist. Survey Rept., 1886, pt. 2, 1887. 



t J. A. Taff : Preliminary report on the geology of the Arbuckle and Wichita moun- 

 tains. Professional paper no. 31, U. S. Geol. Survey, 1904. 



§ N. H. Darton : Geology and underground waters of the Great plains. Professional 

 paper no. 32, V. S. Geol. Survey, pi. xii, p. 42. 



