I,, ologic Time. — Walcott. 
359 
meters); and calcareous sediments covering the same area and 0,000 
feet thick correspond to 2 billion and 400 million mile-feet (1,890 
million kilometer-meters). In the calcareous sediments a liberal al- 
lowance of one-half may be made for arenaceous and argillaceous mat- 
ter in the limestone and partings, and analyses of ten clear limestones 
within the central part of the area give a little more than 75 per cent, 
of carbonate of lime. Applying these reductions we get 900 million 
mile-feet (711 million kilometer-meters) of pure carbonate of lime. 
Duration of Paleozoic Time in the Cordilleran Area. 
Estimates from mechanical sedimentation. The land area tribu- 
tary to the Corclilleran sea was larger before the depression of the con- 
tinent, towards the close of middle Cambrian time, than during subse- 
quent Paleozoic time. It included a portion of the region to the east- 
ward and probably a belt of land extending well towards the Pacific 
coast of the continental plateau. The interior (Mississippian) region, 
west of the 90th meridian, probably drained into the sea to the south, 
forming a Cambrian Mississippi river prior to middle Cambrian time. 
This limits the Cambrian drainage into the Cordilleran sea to an area 
estimated at 1,000,000 square miles. The average thickness of mechan- 
ical sediments deposited before upper Cambrian time is estimated at 
from 10,000 to 15,000 feet. Taking the minimum of 10,000 feet and the 
assumed drainage area of 1,600,000 square miles and the rate of denuda- 
tion at one foot in 1,000 years, it would have required 2,500,000 years 
to carry to the sea and distribute the 10,000 feet of sediment. This 
means the deposition of .048 of an inch per year, which is very small 
if the supposed conditions of denudation and transportation were as 
favorable as the character and mode of occurrence of the sediments 
indicate. If one-fourth of an inch per year is assumed as the rate of 
deposition, the 10,000 feet of sediment would have accumulated in 480,- 
000 years or, in round numbers, in 500,000 years, which increases the 
rate of denudation to one foot in 200 years.* 
In dealing with the post-middle Cambrian mechanical sediments we 
♦By Mr. Willis' method (ante, p. 358) the mechanical sediments of the Paleo- 
zoic age for the area under consideration correspond to six billion mile 
feet. Of this total the greater part, namely, two-thirds or four billion mile-feet, are of 
Cambrian age. Dividing this volume by the land area just given, 1,600.000 square 
miles, we get 2,500 feet as the depth of erosion during the formation of the Cambrian 
mechanical sediments. Assuming different rates of erosion we may obtain times dif- 
fering as follows : 
Cambrian Mechanical Sediments. 
Rate of erosion over land area 
of 1,600,000 square miles. 
Time in years 
for erosion 
of 2,500 feet. 
Rate of deposition over sea 
area of 400.000 square miles for 
strata 10,000 feet thick. 
1 foot in 8,000 years. 
1 foot in 1,000 years. 
1 foot in 200 years. 
7,500,000 
2,500,000 
500,000 
1 foot in 750 years, or .016 inch 
per annum. 
1 foot in 250 years, or .048 inch 
per annum. 
1 foot in 50 years, or .24 inch per 
annum. 
In view of the evidence of rapid accumulation contained in the strata themselves 
the most rapid rate of deposition here stated, namely, .24 inch per annum, is consid- 
ered as the most probable. 
