166 



GEOLOGY. 



these faults has not been determined, but they occurred during the 

 general period of disturbance inaugurated at the close of the Upper 

 Cretaceous. The position of the Cretaceous near Livingston, Mont., 

 is shown in Fig. 405, while the effect of faulting on outcrops in the 

 plains of Colorado is shown in Fig. 398. 



CD C 





7V 



Fig. 405. — Section showing position of Cretaceous beds at one point in the vicinity 

 of Livingston, Montana. ^R=Archean; €, Cambrian (Gallatin and Flathead 

 formations) ; D, Devonian (Jefferson formation) ; C, Carboniferous (Quadrant 

 and Madison formations); J, Jurassic (Ellis formation); Kd, Dakota formation; 

 Kc, Colorado series; Km, Montana series, and Kl, Laramie series; bbr, basic igne- 

 ous rock, and apt, acidic rock. Length of section about 11 miles. (Livingston, 

 Mont., folio, U. S. Geol. Surv.) 



With present data it is impossible to interpret all the deformations at this 

 time in a strictly inductive way, and differences of opinion remain appropriate. 

 A composite interpretation may, however, be indicated. The facts that have 

 just been given relative to folding and overthrust seem to indicate clearly a lateral 

 movement of the crust, attended by a sub-crustal shear, and a folding and 

 faulting of the crustal zone. Using the methods of estimate previously set forth 

 (Vol. II, p. 125), the thickness of crust thus sheared was three or four miles. The 

 fault throw given above (15,000 feet) is what would naturally follow if a crust 

 three miles thick were thrust over the normal surface. Dawson's estimates 

 of shortening and height of the folded portions are closely in harmony with this 

 very instructive faulting phenomenon. 



So far as the American continents are concerned, the folding-faulting move- 

 ment, here interpreted as a shear movement of a shell three or four miles thick, 

 was essentially confined to the western border, but it extended the length of both 

 North and South America. This is probably typical of the great mountain- 

 making movements of post-Cambrian times. Folding seems to have been con- 

 centrated along one great belt in each continent for a given continuous direc- 

 tion. This folding is thought to imply shrinking of the earth-body. Daw- 

 son's estimate of the shortening involved in the Laramide range alone implies 

 a descent of the surface of four miles. If the shortening involved in the parallel 

 ranges west of the Laramide range be added, the descent of the surface was 

 probably as great as the extreme upward folding of the range, as maintained 

 by Suess. 



The shrinkage which is implied by this folding was probably first and chiefly 

 felt by the ocean basins, for reasons set forth previously. The primary effect of 

 this is thought to have been some increase in their capacity as basins, and hence 



