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Show 238 GRAVEL. of the bridge and fill a bucket of water. The strata through which the Isla has cut become hard more gradually than those at the Esk; so that the floods in the dell at Airlie do not rise to such a height as those in that at the Burn ; though in late harvests, which are generally those that are more than usually rainy, hoth rivers do considerable damage to the crops and fields in the plains below. But those rivers, notwithstanding the headlong impetuosity of their floods, and the traces of their devastation in those channels, have done very little within the period of their recorded history; and probably the "linns," or cascades, where the hard strata have resisted the action of the water, are nearly in the same places as they were when Agricola led his Romans through that part of the country. Those linns, too, do not fall over granite, but over secondary rocks of some description or other -partly hard pudding-stone and partly schistus; so that in the formation farther up, whether of the stratified or the granular stone, the rivers could have had very little to do. Even in cases where there are no rock founda-tions with which to contend, it is quite impossible to account for the form of the present surface, by any action of the waters now existing, or by any action not carried on entirely under water. Any situation in an alluvial country will suffice for enabling one to understand that. Go, for instance, to any of the heights near London, which command a view of that part of the valley of the Thames, examine the position of the gravel and clay hills on both sides, and then say whether, trifling as they are, they could have been formed by any action of the Thames and of the ocean jointly, working at the surface, even when the sea may have flowed as far in as Teddington, or even farther. What action of the river, and of the resisting sea jointly, could have raised up Richmond Hill, and all the successive CAPPING CLAY. 239 swP-lls ~r " caps" of gravel, by Wimbledon, Clap .. ham, Bnxton, and so onward till one comes to the chaik formation near New Cross 1 So also the heights of ~aling, Kensington, Prim'rose 'Hm, Hampstead, Highgate, Pentonville, and the other swells, towards Finchley common and the flats on t~e river Lea. It is true that where the estuary of a river so meets the set of the tide as to form a con~ tnnt ed~y it:I the waters, and a permanent whirlwind m the air, hills of sand are in some instances collected,. as high as any that have been named, or even h1gher. There are instances of them at the m.outh of the Tay, below Dundee ; and at that of the Fmdhorn, below F?rres ; and on some of the sandy shores of the Contment. On that of Jut]and, for instance, they are very numerous, and formed without any river, by the action of the sea-eddies alone. So also in the sandy deserts, there are hills of sand fo~med by whir~winds or eddies of the atmosphere, Without any assistance from water, for there is no water ~here. But these cases will not explain the formatiOn of the emin~nce~ in the valley of the Thames. T~ese contam flmt pebbles, which are ra.ther too wmghty for being built into hills hy the wmds ; and they also contain beds of clay a substance. which imbibes too much water, and f~rms too much !n the state. of a paste, for drifting much with the wmds. Besides., the " London clay" is obviously a gr~dual depos1te from water which has stood over the highest points where it is found· and even though we consider the flint gravel as th~ debris of chalk rocks, out of which all the lime has been was~ed except that which suffices to give a binding quality to th~ gravel, we must allow it to have been rolled .about m the water till the flints abraded each other mto smoothness, and the dust thence produced formed. the connecting powder of the gravel. It is Impossible to say how long it may have taken to roWld the nodules and praduce the powder : but the |