GEOLOGIC TIME. 



315 



If ail av^erage is taken of the mechanical sediment deposited subse- 

 quent to the close of ^liddle Cambrian time, it will be found to be about 

 5,000 feet for the entire area, which, I think, does away with any neces- 

 sity for assuming- an additional hypothetical land area for the source 

 of the mechanical sediment. The tine sand composing the quartzites 

 and the silt forming the shales, as well as the fine conglomerate of later 

 deposits, were derived from the adjoining laud areas, and, in all proba- 

 bility, currents swept through from the ocean to the south or north, 

 distributing the mud and sand contributed from the rivers and streams 

 along the shores. 



Chemical sediments. — The present supply of the carbonate of lime, 

 silica, etc., contained in sea water is derived from waters poured into 

 the sea by rivers and streams. The Cordilleran sea undoubtedly received 

 a large contribution from the adjoining land areas, but a considerable 

 amount was possibly deri\ed from an oceanic current that circulated 

 through it, as the southern equatorial currentof the Atlantic now sweeps 

 through the Caribbean. From the vast deposits of carbonate of lime 

 it might be assumed, a priori, that the waters of a Mississippi or Amazon 

 were poured into it, but there is not any evidence of the existence of 

 ■ such a river, although the tributary area may have been very large in 

 Cambrian and Carboniferous time if the drainage of the country west 

 of Hudson Bay was to the westwa7^d. 



Conditions of deposition. — With free communication into the open 

 ocean on the south, and probably on the north, during most of Paleo- 

 zoic time strong currents must have circulated through the Cordilleran 

 sea. The broad distribution of mechanical sediments of a uniform 

 character clearly shows this to have been the case, especially in pre- 

 Sihirian time. The present known distribution of the mechanical sedi- 

 ments indicates that they were mainly brought into the sea from the 

 west,* although a vast amount was derived from the land on the east- 

 ern side in pre-Ordovician time ; they were quite evenly distributed 

 over the sea bed, excejjt where local accumulations of silt and sand 

 occurred near the larger sources of supply, or in the direction of pow- 

 erful currents within the sea. 



The conditions of the deposition of the carbonate of lime are less 

 clearly understood than those governing mechanical sediments, and I 

 shall enter upon the discussion of them at considerable length. There 

 are three methods by which it is usually considered it may be depos- 



* Geo/. Expl. Fortieth Parallel, 1878, vol. i, p. 247. 



