158 ELEMENTARY GEOLOGY 
only deepens the river bed, but that which seeps in from 
the ground, serves to lower the general surface; and 
this is one of the ways in which river valleys are 
broadened. 
We look at the clear river water and hardly imagine 
that it holds a load of mineral in solution; yet analysis 
proves this, and in the course of a year a good-sized 
river conveys a prodigious burden of dissolved substances 
to the sea. For instance, each year the Arkansas River 
carries past Little Rock, about 6,800,000 tons of dis- 
solved mineral, and this is by no means exceptional. 
The substances that are found to be most abundant in 
this river, are salt, carbonate of lime, carbonate of soda, 
and silica. 
It has been estimated upon a fairly satisfactory basis, 
that over 8,000,000 tons of mineral matter are carried 
to the sea each year from the British Isles. At this 
rate (assuming the material to have come equally from 
all parts of the islands), solution alone would lower the 
whole surface of the British Isles one foot in about 
13,000 years. So in the course of ages this work of 
water is an important one. 
Mechanical Action. — Cutting Tools. Pure water, 
flowing over a hard rock, produces no perceptible effect 
upon its channel by mechanical action. The tools 
with which rivers deepen their valleys are the rock frag- 
ments which they carry (Plate 5). As these are dragged 
