Theory of Isostacy. 
471 
sippi river. 1 It has been calculated that if the present rate of 
erosion continued, and no uplift occurred, North America would 
be reduced to sea level by erosion in 3,000,000 years, and Europe 
in 2,000,000 years. 2 * * * * * * 9 
However, it must be remembered that as the elevation of the 
land areas is lessened by erosion, the rate of degradation rapidly 
decreases, for the speed of erosion largely depends upon the 
amount of precipitation and the declivity of the slopes. At a 
low elevation both the quantity of water in the streams and the 
declivity of the streams are less than at high elevations. Con¬ 
sequently to actually reduce the continents to the level of the sea 
by erosion, even if no further uplift occurs, would undoubtedly 
require a far longer time than indicated by calculations based 
upon the present rate of erosion. 
However, under present conditions it is plain that as a secon¬ 
dary result of the movements of the atmosphere and hydro¬ 
sphere there is a horizontal movement of great magnitude of 
earth material from the continent to the ocean, combined with 
a relatively small but important vertical movement. The hori¬ 
zontal movement is from less than a mile to thousamds of miles. 
The vertical movement is from less than a foot to thousands of 
feet. Gravity is the force which causes the transfer, both hori¬ 
zontal and vertical. It is to be noted that the transfer is cu- 
1 According to the report (1894) of Messrs. Humphrey and Abbot, the 
engineers charged with the investigation by the United States Govern¬ 
ment, the amount of mud carried in suspension and solution by the Mis¬ 
sissippi river is 812,500,000,000 lbs. per year. The amount rolled along 
the bottom is 126,360,000,000 lbs. per year. Thus the yearly contribution 
of the Mississippi to the Gulf of Mexico is 938,860,000,000 lbs. of mud. 
The number of freight cars in the United States, according to the report 
of the Railroad Commissioner for 1894, is 1,191,866. If each of these carry 
30,000 lbs., the amount of one load of all these cars would be 35,755,980,000 
lbs. A round trip of 3,000 miles, 1,500 miles each way, would be made in 
about 6 days, giving about 60 trips per year. The cars would thus carry 
in a year about 2,145,358,800,000 lbs. Thus the amount carried by the 
cars would be a little more than twice the amount transported by the Mis¬ 
sissippi. 
9 Einleitung in die Geologieals historische Wissenschaft, by J. Walther: 
Theil III, 1894, P. 580, Text-book of geology, by Archibald Geikie: 3rd ed., 
1892 , pp. 462-5. 
