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Show 172 TRANSPORTING POWER the new direction, volume, and v~locity, acquired by rivers and torrents may modify the former surface,-wbat effects an im· portant difference in the mean temperature of the climate, or the greater intensity of beat and cold at difl'erent seasons, may produce,-what pre-existing valleys, under a new configura· tion of the land, may cease to give passage to large bodies of water, or may become entirely dried up,-how far the relative level of certain districts in the more modern period may become precisely the reverse of those which prevailed at the more ancient era. When these and other essential elements of the problem are all duly appreciated, the reader will not be sur· prised to learn, that amongst geologists who have neglected them there has prevailed a great contrariety of opinion on these topics. Some writers of distinguished talent have gone so far as to contend, that the origin of the greater number of existing valleys was simply due to the agency of one cause, and that it was consummated in a brief period of time. But without discussing the merits of the general question, we may observe that we agree with the author before cited, that the sinuosity of deep valleys is one among many proofs that they have been shaped out progressively, and not by the simultaneous action of one or many causes; and when we consider other agents of change, we shall have opportunities of pointing out a multitude of striking facts in confirmation of the gradual nature of the process to which the inequalities of hill and valley owe their origin. In regard to the transporting power of water, we are often surprised at the facility wherewith streams of a small size, and which descend a slight declivity, bear along coarse sand and gravel; for we usually estimate the weight of rocks in air, and do not reflect sufficiently on their comparative buoyancy when submerged in a denser fluid. The specific gravity of many rocks is not more than twice that of water, and very rarely more than thrice, so that almost aU the fragments propelled by a stream have lost a third, and many of them half of what we usually term their weight. It has been proved by experiment, in contradiction to the theories of the early writers on hydrostatics, to be a universal law, regulating the motion of running water, that the velocity at the bottom of the stream is everywhere less than in any part above it, and is greatest at the surface. Also that the suppr· (>F RUNNING WATER ficial pat•ticles in th 'dd ' 173 t h ose at the sides eT mhi' le of th e stream move swi'ft th . . IS retard t' f er an currents IS produced b f . . a Ion o the lowest and I t al fi · 1 Y r1chon d h a er Cient y great, the soil com '. an w en the velocity is suf way. A velocity of thr . phosmg the sides and bottom . • b ffi · ee me es gtves e su Cient to tear up fine cla p~r s~cond is ascertained to s::md,-twelve inches per yd,-six Inches per second fi P . d · sccon fin 1 , ne eiW sehc on tl ' · stones of th e . ' e grave ,-and thr ~ SIZe of an eO'O' ;~· ee leet en 1IS mechanical power of r oo · · we "7 prepared for the transporta~·nnmg water is considered, grave' s~nd, and mud, b the Ion of large quantities of scend with great velocity f torrents and rivers which d • But a question naturally a~se:om the mountainous region: of the vaUeys and 1 . ' how the more tranq 'I . . . d P ams, flowinO' UI rivers ?toun ' can remove the P d" . o on comparatively 1 I Into them b h . ro Igwus burden h. I . . eve th y t etr numerous tribut . w tc Its dtscharged ey are enabled to conve aries, and by what means they had not th. y the whole mass to th choked IS power' their channels e sea. If . up, and the lower vall would be annuall mountam-chains would b e conti eyIsI and districts ca dJ' om. m. Oy' ments of rock and stei·I'l dnua y strewed over with fr o by e san B t h. ag-twoa gen~rallaw regulating the ~ond~ct t /" evi! is prevented eqt~a streams do not occu o runmng water, that proportio~J, therefore, as the p'b al bed .of double surface. In space wh1ch it · w 0 e flmd mass · water; nnd hen~:·~[;;: ~eCI·e~ses relatively to t~:r~~~:%.th~ rhetarded by friction aO'ain:t smthallebr proportion of the who~e cci ta nne] . h 'fhe porl!. on t5h us unimpee d odt tom an d st· des of the y, sot at the main current . e moves with great velo- ;::~::;;• notwithstanding tha;s t~•:l;ccelerated in the lower d • It not unfrequentl h pe of the channel i j::;.nstrate by examples, t6at al:J'oe~s, as w~ shall afterward: viouiOIn , have only the surface wh. h arge nvers, after their 8 Y; and ev · tc one of th 1 in an en m some cases their . em lad pre· tiful =~j~;ter bed than each of them 61~~~:~ waters are confined mad .. ment, the water which d . Ie ~re. By this beau-e contmualJy to occu 1 rams t 1e mterior countr . and thus the most valuabl!~a;tss ;oom as i~ approaches the:.:~ o our contments, the rich delta s,' * Encycl, Brit ·- Art· RI' vers, |
