37 
impact. Hence it is heavily charged with matter in suspension 
(mud or sediment), but owing to the short time during which 
it comes into contact with the rocks on which it falls, it con¬ 
tains but little matter in solution. On the other hand, the 
river at its normal level is fed to a great extent by springs, 
that is by water which has soaked into the earth. Such water 
has had time to dissolve a considerable amount of the soluble 
constituents of the rocks with which it has thus been brought 
into contact. This is most probably the reason why the clear 
water at summer level yields more solid matter than the turbid 
flood-water. But though a given volume of clear water may 
contain a somewhat larger amount of solid matter than an 
equal volume cf flood water, it must not be supposed that the 
river is doing more geological work when low than when in 
flood. In the latter condition the river flows at a much greater 
rate, hence a much larger volume of water comes down in any 
given time, and consequently a larger amount (though a lower 
percentage) of solid matter is brought down. In addition to 
this, too, the actual amount of water in the river is greatly 
increased. 
In order to estimate the amount of solid matter brought 
down by the river, we must first ascertain the volume of water 
that flows past any given point, and for this we require the 
cross section at that point and the rate of flow. Owing to the 
kindness of Mr. Copperthwaite and of Mr. Greer, I have been 
enabled to get several sections of the river taken within a few 
yards of the place from which was drawn the water for our 
experiments. From these we get a generalized section of 167 
square metres (about 200 square yards). Taking an average 
of the rates of flow observed with the river at summer level, 
91 metres per eight minutes (100 yards in eight minutes), we 
find that 15,197 cubic metres (about 20,000 cubic yards) flow 
past every eight minutes, or about 2,736,000 cubic metres 
(3,600,000 pubic yards) per 24 hours. This volume of water 
brings down 610,128 kilogrammes (about 600 tons) of solid 
matter. As the river was at (or about) summer level for 140 
days in the year during which the observations were taken, 
the amount brought down under those conditions was 
85,417,920 kilogrammes or 84,000 tons. 
