SCIENTIFIC TALKS.

Otago Witness, Issue 1950, 4 April 1889, Page 37

SCIENTIFIC TALKS.

BrF. G S.

THE GEOLOGICAL ACTION OE WATER. -No. IV. In the three last papers on the, above subject, it has been shown that the ,earfch is gradually tut surely, being washed' down from a higher to a lower level; We have seen that • every : rivulet is 1 ceaselessly 'carrying on a scouring abtion, and wearing away the surface of the ground ; that each ; drop of- rain has its appointed share in thistoork, and that the frost •and snow combine to disintegrateatid demolish the rugged rocks ooff f which the crust of the earth is composed; And we have even paused occasionally to consider' what becomes of the vast quanties of material which , are thus removed. So long, however, as this detritus is above sea-level, it is never safe . from further molestation. Hard as are the rocks of which the great mountains are built , up, they are day by day and year by year ! attacked, by forces which, though apparently 1 trivai and insignificant, are in fact irresistible. Truly has ibbeen written '> The mills of God grind slowly. ! But they grind exceeding Email, 1 Naturally the ocean is the great repository of j the waste of the land, and until the materials I which are washed down find their resting ; place below the limit of wave or current 'action, they are continually liable to be ; sorted and removed, and redeposited. But it , must not be imagined thatjbhis redeposition iis carried on in a haphazard or a methodjless manner. By the rushing torrent 'large rocks are hurled along; by the swiftly j flowing river, stones and gravel ; and by the ' sluggish stream fine mud and sediment are (carried.- So surely as a diminution, of ■ velocity ,takes place, is part of the burden jlaiddow'h.: sometimes, for instance, a river sin its course passes through a lake ; muddy !and turbid it enters, pellucid and clear it | makes it exit. The Rhone, for example, ! flows into the Lake of G-eneya as a muddy ,' stream, charged with the mud of all the [glaciers that feed it ; at the lower end where [it quits the lake it is as pure and limpid as .the waters of a spring. Thus a process of i filtration is carried on, and the lake gradually j fills up, being converted first, into a swamp i and then into flat alluvial land, through | which the river, meanders on its course. Most of our inland Otago plains have been formed in this manner, as the section of their strata distinctly shows.

j When our river 'arrives at the sea, there (is still matter in suspension, but as soon as jthe velocity of the current is checked deposition takes place, and a bar is formed, suoh las we know so well at tho mouths of our 'New Zealand, rivers. Some,, great water ,oourses carry" their' sediment .many miles to | sea, and the Amazon is said by this means to j discolour .th^ waters of the ocean for a dis? Stance of 300 miles ' from land. TSTear the jshore the larger and coarser fragments are I {dropped; further out the smaller and, more i'easily, carried stones ; yet further, the sand; I until we arrive at the extreme limit of ; carrying capacity, where only the finest silt 'is retained. But besides the inorganic refuse ! of the land, shells, timber, the carcasses of : animals, and the works of man are borne j along ; and all those which are covered up (sufficiently soon to arrest the progress of de- | jcomposition may betpreserved for countless I iages, to -form among the continents of the j 'future rriaterial for the study and delight of jgeologists, by whom the present age will be ■regarded as, among the early or middle jstage&of Sftr^planet's life. All the countless treasures. lost irom ships, the golden coins ''and ornaments, the fragments of pottery, land other indestructible work, will convey to that student in a long-distant age the history of those races were living when their possessions were buried. | We have mentioned the fact that calculations have been made to determine how long, jat the present rate, our existing earth would ;last if the destroying forces alone acted upon |it ; and it may have occurred to the reader jto ask how, if this action has been going on from the commencement of geological history up to the present time. There are still 'mountain ranges of great height, vast tablelands and plains far above sea level, and so great a bulk of land above the waters. The geologist will tell us, in answer, that in the 'all wise adjustments of nature there are other forces, no less powerful and persistent, by whose aid the total submergence of the land is prevented.. Slowly but , surely the internal forces are raising up the crust of the earth in -various regions; gradually but' with unremitting'perseverance they are carrying on a warfare against tb,ose agencies which', were they to remain unchecked, would eventually bury the solid ground beneath the waters of a universal ocean. I In ' the first oi this series of papers, we f tated that the action of water was believed by geologists to have been going on since the Earliest geological periods. The reason for this belief is that almost" the' oldest rocks of Which ,thS crust of the earth' is'composed. are formed of solidified mud .and sand, due evidently to the action of water, and that throughout the' whole of the geological record, the world as we now know it is, with the exception of comparatively insignificant masses of igneous rock, built up of sedimentary deposits, teeming with fossil remains of the ever changing denizens af the land and water "at different epoch?. Deep as we have gone' in the ' crust of the globe, we have not found the limits of : these deposits ; far above the sea level, on the tops of the 'highest mountain ranges, are rocks whose aqueous origin is unchallenged. And not only_J3o, b-ut.when in. the Himalayas, 16,500 ft above the sea, we find sedimentary deposits belonging to the eocene,; or lowest division of^ the kainozoio'or "recent life" period, we know that "the upraising of these gigantic mountain masses'* fias' been in— geologically gpeaking^q^ite, modern tames. In the Appalachairi chain the "sedimentary rocks attain a thickness of 40,000 ft and in the. Eocky Mountains, of ,60,000 ft, or upwards of 11 miles. Any comprehension of the vast cycles of time, necessary for the deposition ,

of such enormous masses of rock, exceeds by . far the limit of man's feeble intellect. It is clear that if such great effects are produced, the traces of denudation' must be ' plain and conspicuous, and in reality it 'is to this action that the present surface features of the landare mainly: due. Certainly the great mountain- ranges have been upheaved and lifted, above the surface of the surrounding country, but to subsequent aqueous agencies are due .the valleys and plains, the 'river gorges and rolling downs, and consequently, by reason, of their < greater power of resistance, the outlines of the mountains themselves. No grander example of the excavating power of water can be given than the grand Canons of Colorado. For nearly 300 miles of its course the river flows through a gorge sometimes not more . than 50 yard 3 in width, and varying from 3000 ft to 6000 ft in depth. The rocks on each side are approximately horizontal, and from side to side we can trace the same strata as clearly as' we could trace the remnants of a floor in a half -ruined house ; as in the one ; case we should feel justified in assuming that the timbers, of which portions remain, had at one time extended across the intervening space, so in the other we have a right to assume that, before the flowing waters cut that gigantic chasm, the rocks were continuous and unbroken. It may be wondered that the sides have not, during the long period necessary for excavating such a channel, been worn back into a slope, such as we ordinarily see, but the plain through which the network of rivers flow is a rainless, desolate tract of country, and consequently the bottom of the gorge only has been subjected to aqueous agencies. After considering the various methods in which the surface 6f the globe is affected by water action it is only natural, before leaving the subject, to pause a moment in order to refleot what would be the condition of the , world if no aqueous denudation had taken place. This brings vs 1 ' into a somewhat difficult train of reasoning, for we must know, in the first place, what was the primeval state of the earth. Supposing, for , example, that we take the f andamental frocks, granite and gneiss, and imagine a world built up of rugged masseshuge hills of bald, bleak, desolate granite ; unworn and unweathered, everlasting in its ; barren misery, and' frowning down upon a ', lower region of naked volcanic rocke. No I alluvial plains of waving corn; no deep i swampy soil to delight the heart of the husbandman ; no sloping river banks and gullies {where the rich soil is found. Inhospitable, .sterile, and unfit for the habitation of man or beast. If we call the astronomer to our .aid, and gaze upon the .moon, we see an example' of what has been described. Steep, i pinnacled volcanoes, plains of scoria and ash, i unchanged since the day when they were ■ poured} forth from a world full of geological .energy, and now condemned' to hang for ; ever lifeless in space, and gaze upon the fertile and ever-changing features of our earth.

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