Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/288

Rh 27+" G-EOLOG-Y being 113}. Bischof calculated that, assuming the mean quantity of carbonate of lime in the Rhine to be 9'-16 in 100,000 of water, which is the proportion ascertained at Bonn, enough carbonate of lime is carried into the sea by this river for the annual formation of three hundred and thirty-two thousand millions of oyster shells of the usual size. The mineral next in abundance is sulphate of lime, which in some rivers constitutes nearly half of the dissolved mineral matter. Silica amounting to 4'88 parts in 100,000 of water has been found iii the Rhine, near Strasburg. The largest amount of alumina was 0'71 iii the Loire, near Orleans. The proportion of mineral matter in the Thames, 11ear London, amounts to about 33 in 100,000 parts of water, 15 of which (nearly half of the whole) consist of car- bonate of lime. It requires some reflection properly to appreciate the a_mouut of solid mineral matter which is every year carried in solution from the rocks of the land and ditfused by rivers into the sea. According to recent calculations by Mr T. Mellard Reade, 0.13., a total of _8,370,630 tons of solids in solution is every year removed by running water from the rocks of England and Vales, which is equivalent to a general lowering of the surface of the country from that cause alone at the rate of ‘O07?’ 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 923 tons per square mile, that of the Rhone at Avignon 232 tons per square mile, and that of the Danube at 72‘? 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 rocky matter per English square mile of surface.‘ ii. .l[echcmical.—The mechanical work of rivers is three- fold :—(l) to transport mud, sand, gravel, or blocks of stone from higher to lower levels; to use these loose n1ate- rials in eroding their channels; and (3) to deposit these materials where possible, and thus to make new geological formations. 1. Transporting l’ower.—It is one of the distinctions of river water, as compared with that of springs, that, as a rule, it is less transparent, that is, it contains more or less mineral matter in suspension. The same stream differs much at successive intervals in the amount of material thus trans- ported. In dry weather when the water is low it maybe tolerably clear; but a -sudden heavy shower or a season of wet weather will render it turbid. The mud thus so fre- quently noticeable in rivers is partly derived from the sur- face of the ground on either side, whence it is washed into the main streams by rain and brooks, but partly also by the abrasion of the water-channels through the operations of the streams themselves. In the mountain tributaries of a river we find the cha11nels choked with large fragments of rock disengaged from the cliffs and crags on either side. Traced downwards the blocks are seen to become gradually smaller. and more rounded. They are ground against each other and upon the rocky sides and bottom of the channel, getting more and more reduced as they descend, and at the same time abrading the rocks over or against which they are driven. Hence a great deal of debris is pro- duced, and is swept along by the onward and downward movement of the brooks and rivers. The ﬁner portions, such as mud and ﬁne sand, are carried in suspension, and impart the characteristic turbidity to rivers; the coarser sand and gravel are driven along the river bottom. The transporting eapacity of a stream depends (a) on the volume and velocity of the current, and (b) on the size, shape, and speciﬁc gravity of the sediment. (a) According to the calculations of Hopkins, the capacity of transport 1 Azldress to Liverpool Geological Society, 1-377. [111. DY.'..IICAL. increases as the sixth power of the velocity of the current; thus the motive power of the current is increased 64 times by the doubling of the velocity, 729 times by trebling, and 4096 times by quadrupling it. It has been found by experiment that “ ordinary sandy soil is moved by a cur- rent having a velocity of about half a mile an hour, :md that a current of about one mile per hour will move fine gravel, while heavy gravel resists a current of upwards of two miles per hour.” .Ir David Stevenson? gives the subjoincd table of the power of transport of different velocities of river currents. In. per Mile per Second. Hour. 3 = 0170 will just begin to work on fine clay. 6 =- 0340 will lift fine sand. 8 = 04545 will lift sand as coarse as linseed. 12 = 0'6819 will sweep along ﬁne gravel. 24 : 1'3638 will roll along rounded pebbles 1 inch in diameter. 36 : 2'0-15 will sweep along slippery angular stones of the size of an. egg. We must never lose sight, however, of the fact that it is not the surface velocity, nor even the mean velocity, of a river which can be taken as the measure of its power of transport, but the bottom velocity—that is, the rate at which the stream overcomes the friction of its channel. ((2) The average specific gravity of the stories in a river ranges between two and three times that of pure fresh water; hence these stones lose from a half to a third of their weight in air when borne along by the river. Iluge blocks which could not be moved by the same amount of energy applied to them on dry ground are swept along with ease when they have found their way into a strong river current. The shape of the fragments greatly affects their portability, when they are too large and heavy to be carried i11 mechanical suspension. Rounded stones are of course most easily moved ; ﬂat and angular ones are moved with comparative difficulty. Besides their ordinary powers of transport, rivers gain at times considerable additional force from several causes. Those liable to sudden and heavy falls of rain acquire by ﬂooding an enormous increase of transporting and excavating power. More Work may thus be done by a stream in a day than could be accomplished by it during years of its ordinary condition.3 Another source of increase to the action of rivers is provided when, from landslips, formed by earthquakes, by the undermining inﬂuence of springs, or otherwise, a stream is temporarily dammed back, and the barrier subsequently gives way. The bursting o11t of the arrested waters produces great de- struction in the valley. Blocks as big as houses may be set in motion, and carried down for considerable distances. Again, the transporting power of rivers is greatly augmented in countries where they freeze in winter. As the ice gathers along the banks it encloses gravel, sand, and even large blocks of rock, which, when thaw comes, are lifted up by the ice and carried down the stream. lround-ice likewise appears in cold latitudes 011 the bottoms of the rivers, whence, rising in cakes to the surface, it carries with it sand, mud, or stones lying on the bottom, which are then swept seaward. When rivers such as those of northern Russia and Siberia, ﬂowing from south to north, have the ice thawed in their higher courses before it breaks up farther down, much disaster is sometimes caused by the piling up of the ice, and then by the bursting of the impeded river through the temporary ice—barrier. In another way ice sometimes vastly increases the destructive 9 Canal and River Eazgineeo-ing, p. 315. 3 The extent to which heavy rains can alter the usual characters of rivers is forcibly exempliﬁed in the graphic account of The J[oray- shire Floods, by the late Sir T. Dick Lauder. In the year 1829 the rivers of that region rose 10, 18, and in one case even 50 feet above their common summer level, producing almost incredible havoc.