366 DYNAMICAL GEOLOGY. [Book II. — 
consist of carbonate of lime, the rest being chiefly sulphate of lime, 
with minor proportions of the other ordinary salts of river-water. 
Mr. Prestwich estimates that the quanitity of carbonate of lime 
removed from the limestone areas of the Thames basin amounts to 
140 tons annually from every square mile. This quantity, assuming 
a ton of chalk to measure 15 cubic feet, is equal to a loss of +2, of 
an inch from each square mile in a century or one foot in 13200 



’ 
years. According to monthly observations and estimates made — 
in the year 1866 at Lobositz near the exit of the Elbe from its 
Bohemian basin, this river may be regarded as carrying every year 
out of Bohemia from an area of 880 square German miles, or, in round 
numbers, 20,000 English square miles, 6,000,000,000 cubic metres 
of water containing 622,680,000 kilogrammes of dissolved and 
547,140,000 of suspended matter, or a total of 1169 millions of 
kilogrammes. Of this total 978 millions of kilogrammes consist of 
fixed and 192 millions of volatile (chiefly organic) matter. The pro- 
portions of some of the ingredients most important in agriculture 
were estimated as follows. In the yearly discharge of the Elbe there 
are carried out of Bohemia: lime, 140,380,000 kilogrammes; mag- 
nesia, 28,130,000; potash, 54,520,000; soda, 39,600,000; chloride 
of sodium, 25,320,000; sulphuric acid, 45,690,000; phosphoric acid, 
1,500,000.? 
Mr. T. Mellard Reade has estimated that a total of 8,370,630 
tons of solids-in solution is every year removed by running water 
from the rocks of England and Wales, which is equivalent to a 
general lowering of the surface of the country from that cause alone 
at the rate of ‘0077 of a foot in a century, or one foot in 12,978 
years. ‘The same writer computes the annual discharge of solids in 
solution by the Rhine to be equal to 92°3 tons per square mile, that 
of the Rhone at Avignon 232 tons per square mile, and that of the 
Danube at 72°7 tons per square mile; and he supposes that on an 
average over the whole world there may be every year dissolved by rain 
about 100 tons of rocky matter per English square mile of surface.’ 
If the average proportion of mineral matter in solution in river- 
water be taken as 2 parts in every 10,000 by weight, then it is — 
obvious that in every 5UV00 years the rivers of the globe must carry 
to the sea their own weight of dissolved rock. 
iis Mechanical.—The mechanical work of rivers is threefold :— 
(1) to transport mud, sand, gravel, or blocks of stone from higher to 
lower levels; (2) to use these loose materials in eroding their 
channels; and (3) to deposit these materials where possible, and thus _ 
to make new geological formations. 
1 Prestwich, Q. J. Geol. Soc. xxviii. p. Ixvii. 
? Breitenlohner, Verhand. Geol. Reichsanst., Vienna, 1876, p. 172. Taking the 
978,000,000 kilogrammes to be mineral matter in solution and suspension, this is equal 
to an annual loss of about 48 tons per English square mile. But it includes all the 
materials discharged by the drainage of an abundant population, 
3 Address, Liverpool Geol, Soc. 1877. 
