406 B. WILLIS A THEORY OF CONTINENTAL STRUCTURE 



trict on the Portietli Parallel section. It does not follow, however, that 

 that limit was never passed by movements originating in the distant but 

 vast suboceanic mass. The folding which occurred during the late Cre- 

 taceous and Eocene throughout the Eocky Mountain province, even as 

 far east as the Front range of Colorado, is to be attributed either to 

 Pacific thrust or to pressure exerted from the region of the Great plains. 

 The latter 'involves the spreading of the negative element that lies be- 

 neath the plains, at least according to the hypothesis entertained, that 

 tangential movements in the earth's crust are due to displacement of the 

 lighter elements by lateral spread of the denser masses. It would seem, 

 however, that the relatively light and small negative element beneath the 

 Great plains is a very inadequate source of tangential thrust as compared 

 with the comparatively dense and enormous mass beneath the Pacific. 

 As already stated, I regard the latter as the source of the erogenic activ- 

 ity which has produced the Cordillera from Alaska to cape Horn, and this 

 conclusion necessarily includes the section in Colorado. 



Throughout the Cordillera the effects of tangential thrust in modify- 

 ing the isostatic relations which might otherwise exist among positive 

 and negative elements appear to be obvious, and I will not dwell upon 

 them. 



We may next turn to the zone of folding which crosses Arkansas and 

 Indian Territory and is represented by the novaculite area and the 

 Arbuckle-Wichita mountains. The structures are described by Bran- 

 ner,* Griswold,f and Taff,J who are agreed that the Paleozoic land area 

 lay to the south. Griswold states (page 213) that the direction of 

 pressure was from the south. Thus, if the gulf of Mexico and northern 

 Mexico be regarded as the site of a negative element from Avhich the 

 tangential thrust was exerted, this zone of folding occupies a position 

 within the Llano positive element which is homologous with that of the 

 Appalachian or that of the Great Basin zones. 



The effect of tangential thrxTst in transforming the deep Paleozoic 

 basin of the southern Great plains into an area liable to uplift is entirely 

 similar to the change in the tendencies of the Appalachian mediter- 

 ranean. 



In the preceding paragraphs the Atlantic, Pacific, and Mexican basins 

 are regarded as the sources of the tangential pressures which have 

 affected the intervening continent. The Arctic remains to be consid- 



* Ibid., Am. Jour. Sci., 4th series, vol. 4, 1897. 



t Arkansas Geological Survey Ann. Rept, 1890, Novaculites, by L. S. Griswold, 1892. 

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

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



