324 THE JOURNAL OF GEOLOGY. 



standard of comparison. Let us take, for instance, the work 

 of transportation which is performed by the Mississippi river. 



The efficiency of any transporting current is determined by 

 three factors, viz. : (i) the area of its transverse section, (2) 

 the velocity of its motion, and (3) its capacity for holding a 

 load. In the case of the Mississippi basin we may say that the 

 products of disintegration and erosion within its boundaries may 

 be removed by principally two agents, water and air. What is 

 removed by water all passes out through the channel of the 

 lower Mississippi. The size of this current in transverse section 

 is less than y^ of a square mile. It is evident that all the 

 materials removed by this river from its great basin, whether 

 taken from the Rocky mountains or from the Appalachian high- 

 lands, must pass through the same narrow circumscribed limits 

 of y^ of a square mile in the lower course of the river. Now, 

 the atmosphere may also be regarded as a current. The width 

 of this current will be the average width of the entire drainage 

 basin of the Mississippi, and in its height this current equals the 

 height of the atmosphere. Taking this to be ten miles, which 

 cannot very well be too much, and taking the average width of 

 the Mississippi basin as one thousand miles — it is at least one 

 hundred miles more — the transverse section of the atmospheric 

 current will be ten thousand square miles. The ratio of the 

 sizes of these two currents as shown in their sections is thus 

 I : 1,000,000, i. e., the cross section of the Mississippi current is 

 j^ Q Q^ Q QQ of that of the atmosphere. If velocity and capacity 

 for carrying a load were the same in both currents, the relative 

 transporting power of the greater one would be 1,000,000 times 

 that of the smaller. 



In respect to velocity the Mississippi is also less effective in 

 its work than the atmosphere above it. The average velocity of 

 the wind over the interior basin is not less than eight miles per 

 hour, while the average velocity of the lower Mississippi is about 

 .7 mile per hour. The ratio of the velocities is therefore repre- 

 sented by the fraction -^, which is a little less than y^. If^ 

 therefore, the two currents were equal as to their cross sec- 



