POSITI\'E ELEMENTS OF NORTH AMERICA 395 



on the southeast; on the north it probahly extends to the folded zone of 

 Paleozoics in Indian Territory;* on the west it appears to be separated 

 by the zone of folding in central New Mexico from the similar elements 

 in Colorado and Arizona. 



Northeastward from the Llano element lies the Ozark, which is of a 

 similar neutral, slightly positive, character. The sum of its unconform- 

 ities comprises a pre-Cambrian period, the Siluro-Devonian in part, and 

 the post-Paleozoic. The region lies midway between Isle Wisconsin and 

 the Llano district, and, as was pointed out by Weller,! forms part of a 

 zone which constituted a barrier to faunal migration during early De- 

 vonian time. Whether it be regarded as a continuous belt of land or as 

 an archipelago among shallows capable of separating ocean currents, and 

 consequently of keeping faunal provinces distinct, is not here important. 

 The fact which bears on continental structure is that a zone of neutral 

 character, with loci which appear as slightly positive elements, extends 

 from Wisconsin to Texas. 



An analysis of western North America with reference to positive and 

 negative elements is complicated by the effects of Pacific thrust. I regard 

 that vast ocean basin as the source of tangential pressure, which has 

 stamped its bordering continents with a geologic history that is peculiar 

 to the Pacific, in contrast to the sequences of events that are specially 

 marked by movements that originated beneath the Atlantic or the Medi- 

 terranean-Himalayan zone. I The development of tlie Cordillera of 

 North America, comprising all the mountain chains west of the Great 

 plains and involving folding, intrusion, extrusion, and warping, I regard 

 as a more or less direct effect of disturbances originating beneath the 

 Pacific. These effects are superimposed upon the positive or negative 

 movements of the continental elements and greatly obscure them. How- 

 ever, the analysis may be made, with appropriate reservations. 



Colorado, Wyoming, and Arizona constitute an area in which the pre- 

 Cambrian and Paleozoic sequences of sediments are comparatively in- 

 complete. Except for the Grand Canyon section in the extreme south- 

 west, the pre-Cambrian consists largely of gneisses and schists which re- 

 semble the oldest rocks of the Canadian protaxis, together with intensely 

 metamorphosed sandstones and shales — ^that is, shore deposits. These 

 rocks were deeply denuded before the invasion of the Upper Cambrian 



*J. C. Branner : The former extension of the Appalachians across Mississippi, Louis- 

 iana, and Texas. Am. .Jour. Sci., 4th series, vol. 4, 1897, p. 368. 



tChicago Jour, of Geol., vol. 3, 1895, p. 905. 



t Bailey Willis: Research in China, vol. ii, Systematic (Jeology. Publication no. 54, 

 Carnegie Institution of Washington, 1907. 



