364 DYNAMICAL GEOLOGY. [Boo III. _ 
son states that the velocity of such rivers as the Thames, the Tay, — 
or the Clyde may be found to vary from about one mile per hour as 
a minimum to about three miles per hour as a maximum velocity.! 
It may be remarked, in concluding this part of the subject, that 
elevations and depressions of land must have a powerful influence 
upon the slope of rivers. The upraising of the axis of a country, 
by increasing the slope, augments the rate of flow, which, on — 
the contrary, is diminished by a depression of the axis or by an. 
elevation of the maritime regions. 
IV. Geological Action.—Like all other forms of moving water, 
streams have both a chemical and mechanical action. The latter 
receives most attention, as it undoubtedly is the more important ; 
but the former ought not to be omitted in any survey of the general 
waste of the earth’s surface. 
i. Chemical.—The water of rivers must possess the powers of 
a chemical solvent like rain and springs, though its actual work in 
this respect can be less easily measured, seeing that river water is 
directly derived from rain and springs, and necessarily contains in 
solution mineral substances supplied to it by them. Nevertheless, 
that streams dissolve chemically the rocks of their channels can be 
strikingly seen in limestone districts, where the base of the cliffs of 
river ravines may be found eaten away into tunnels, arches, and 
overhanging projections, presenting in their smooth surfaces a great 
contrast to the angular jointed faces of the same rock where exposed 
to the influence only of the weather on the higher parts of the 
- cliff. Daubrée endeavoured to illustrate the chemical action of 
rivers upon their transported pebbles by exposing angular fragments 
of felspar to prolonged friction in revolving cylinders of sandstone 
containing distilled water. He found that they underwent con- | 
siderable decomposition, as was shown by the presence of silica 
of potash, rendering the water alkaline. Three kilogrammes of 
felspar fragments made to revolve in an iron cylinder for a period 
of 192 hours, which was equal to a journey of 460 kilometres 
(287 miles), yielded 2°720 kilogrammes of mud, while the five litres 
of water in which they were kept moving contained 12:60 grammes 
of potash or 2°52 grammes per litre.” 
The mineral matter held in solution in river-water is, doubtless, 
partly derived from this mechanical trituration of rocks and de- 
tritus; for Daubrée’s experiments show that minerals which resist 
the action of acid may be slowly decomposed by mere mechanical 
trituration, such as takes place along the bed of a river. But in 
sluggish streams the main supply of mineral solution is doubtless 
furnished by springs. | 
The proportion of mineral matter in river-water varies with the 
season, even for the same stream. It reaches its maximum when 
the water is mainly derived from springs, as in very dry weather 
1 “ Reclamation of Land,” p. 18. 
2 “Géologie Expérimentale,” p. 271. 

