396 B. WILLIS A THEORY OF CONTINENTAL STRUCTURE 



sea.* All the middle Paleozoic is wanting or is represented by slight and 

 local sedimentation. The general Carboniferous transgression was fol- 

 lowed by conditions of erosion and by deposition of continental or littoral 

 formations during the Permo-Triassic and the late Mesozoic. Tertiary 

 history has been largely one of elevation. Since the Carboniferous, how- 

 ever, the movements may be attributed in considerable degree to tan- 

 gential compression rather than to positive or negative vertical adjust- 

 ment. The sum of unconformities and the sum of sediments are not so 

 different for this district of the Middle West as to stamp it in itself 

 decidedly with a positive or negative character, but when compared with 

 the definitely negative element which bounds it on the west, the Great 

 Basin region, and considered with reference to the folds of the Laramide 

 compression, it assumes a more distinctly positive character. 



For the eastern boundary of this positive element, which we may desig- 

 nate as the Colorado, we may take the Front range from Wyoming south 

 to Santa Fe. Thence the outline runs southwest to include most of Ari- 

 zona, and, turning north, sweeps across southeastern California and 

 Nevada toward Salt lake. The northern end includes the Belt moun- 

 tains of Montana, east of Butte and south of Great Falls. 



In western Nevada the King survey determined the existence of a 

 Paleozoic landmass on the succession of Triassic strata unconformably on 

 supposed Archean.f This particular line of evidence is invalidated by 

 Loouderback'sJ determination of the intrusive nature of the supposed 

 Archean into Triassic and possibly Jurassic sediments of the Humboldt 

 range, for the descriptions of other occurrences resemble that which Lou- 

 derback has revised. Nevertheless the character of pre-Cambrian and 

 Paleozoic sediments in the Great Basin region, in western Montana and 

 Idaho and in British Columbia, indicates that there was a western land- 

 mass, which prior to the granitic intrusions and metamorphisni of the 

 Mesozoic might have been distinguished by unconformities. Cambrian 

 sediments in Nevada pass from a marine to a littoral phase from east to 

 west, as observed by Walcott. Lindgreng and Eansome|| describe exi)o- 

 sures of pre-Cambrian schists in central Idaho and the Cceur d'Alene dis- 

 trict. Daly, in his reports on tlie international boundary survey,^ has 



*C. W. Cross on the pre-Cambrian of Colorado, in Bulletin on the Archean and Algon- 

 kian, by C. R. Van Hise and C. K. Leith. revised edition in press. 



tClarence King: Exploration of Fortieth Parallel, vol. 1, Systematic Geology, p. 247. 



t G. D. Louderback : Basin range structure of the Humboldt region, Nevada. Bull. 

 Geol. Soc. Am., vol. 15, 1904, pp. 289-.346. 



§ W. Lindgren : A geological reconnaissance across the Bitter Root range and Clear- 

 water mountains, Idaho. Professional paper no. 27, U. S. Geol, Survey, 1904. 



II Current manuscript report on Coeur d'Alene region. 



H Summer reports, Canadian Geol. Survey, 1902-1905. 



