Geologic Time. — Walcott. ; !">1 
Wasatch. 
Central 
Nevada. 
Southwest 
Nevada. 
Montana. 
Alberta. 
Mechanical Sediment 
•Chemical Sediment 
Ratio 
10,000 
10.40(1 
1 
1 
7,800 
15450 
\ 
2,500 
13,000 
1 
3 
1,000 
4,000 
l 
I 
4,600 
15,000 
l 
a 
If an average is taken of the mechanical sediment deposited subse- 
quent to the close of middle Cambrian time, it will be found to be 
about 5,000 feet for the entire area, which, I think, does away with any 
necessity to assume an additional hypothetical land area for the source 
of the mechanical sediment. The fine sand composing the quartzites 
and the silt forming the shales, as well as the fine conglomerate of later 
deposits, were derived from the adjoining land 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 re- 
ceived a large contribution from the adjoining land areas, but a con 
siderable amount was possibly derived from an oceanic current that 
circulated through it as the southern equatorial current of the At- 
lantic 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 evi- 
dence of the existence of such a river, although the tributary area may 
have been very large in Cambrian and Carboniferous time, if the drain- 
age of the country west of Hudson's Bay was to the westward. 
Conditions of deposition. With free communication into the open 
oeean on the south, and probably on the north, during most of Pale- 
ozoic time strong currents must have circulated through the Cordiller 
an sea. The broad distribution of mechanical sediments of a uni- 
form character clearly shows this to have been the case, especially in 
pre-Silurian time. The present known distribution of the mechanical 
sediments indicate that they were mainly brought into the sea from 
the west,* although a vast amount was derived from the land on the 
eastern side in pre-Ordovician time; they were quite evenly distrib- 
uted over the sea bed, except where local accumulations of silt and 
sand occurred near the larger sources of supply, or in the direction of 
powerful 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 th<- discussion of them at considerable length. There 
-are three methods by which it usually is considered that it may be de- 
posited: 1. Agency of organisms: 2. Chemical precipitation: 3. By 
mechanical methods. 
*(Jeol. Exj.1. Fortieth Parallel, vol. 1. 187>. p. £47. 
