308 GEOLOGY. 



land were undergoing decay, even though the products of decay were 

 not removed. Under these conditions, thick mantles of residual earths 

 accumulate, representing the excess of rock decay over transporta- 

 tion. When, after such intervals of rock decay, uplift of the land 

 gives the streams transportive power, they find a large amount of dis- 

 integrated material ready for removal. Under such conditions, land 

 surfaces are rapidly degraded, and sediments rapidly accumulated 

 about them. 



Concerning the Ordovician strata of the western portion of North 

 America much less is known, since most of the region has not been 

 studied in detail; but it is safe to assume that the same processes, 

 governed by the same principles, were there in operation. Ordovician 

 strata are known in the Black Hills of South Dakota, 1 in Indian Ter- 

 ritory, 2 Texas, 3 at various points in New Mexico, Arizona, California, 4 

 Utah, 5 Nevada, 4 Wyoming, 6 Montana, Colorado, 7 and British America, 8 

 and are doubtless present over considerable areas where now com- 

 pletely concealed. 



Looked upon as a whole, the sediments of the Ordovician period 

 were as wide-spread as the Ordovician seas themselves. To them, all 

 preceding formations of rock so situated as to be subject to erosion, 

 contributed, and their development meant the destruction of an equiv- 

 alent body of older rock. The old material which entered into the 

 new system was brought from the land by streams, worn from 

 its shores by waves, or blown to the sea by winds ; and where terrig- 

 enous sediments failed, or where they were relatively unimportant, 

 the shells and other secretions of animals and plants accumulated, 

 giving rise to sedimentary rocks of organic origin. Even these had 

 their ultimate source in the older formations, for the mineral matter 

 extracted from the sea to make the shells had been dissolved from 



1 Todd, Geology of South Dakota; and Jaggar, 21st Ann. Rept. U. S. Geol. Surv., 

 Pt. Ill, p. 178. 



2 Taff, Atoka folio, U. S. Geol. Surv. and Professional Paper No. 31. 



3 Richardson, Bull. 9, Univ. of Texas Min. Surv., p. 32. The Ordovician is here 

 unconformable on the Cambrian; idem., p. 27. 



4 Spurr, Bull. 208, U. S. Geol. Surv. 



5 King, Geol. Surv. of the 40th Parallel, Vol. I. 



6 Absaroka and Yellowstone folios, TJ. S. Geol. Surv. 



7 King, op. cit. and Anthracite-Crested Butte, Pike's Peak and Ten Mile folios, 

 U. S. Geol. Surv. 



8 Dawson, Bull. Geol. Soc. Am., Vol. XII, p. 68. 



