466 Denudation is Instability, 



The quantity of water flowing is an important element in computing the change 

 of surface, whether in a cliff or a waterfall. The stream at Black Gang Chine 

 (page 460) is very small at the present time, but if eight times the quantity of rain 

 fell, and the velocity was doubled, the power of the surface stream would be 

 much increased. This is, however, only one element. The water from each 

 flat watershed descends under ground and escapes in springs, and springs in a 

 wet period have been the great denuding agent. The action of denudation by 

 underground watercourses has never been sufficiently estimated or properly 

 calculated. 



The contour of surface depends upon the resistance of the strata to the rain- 

 fall. The terrestrial surface of the earth is always a succession of terraces and 

 flats, or rather walls and slopes. This is what we call "contour." In some 

 districts, where the walls are very conspicuous, we call the ground terraced. 

 Where the walls are low they often escape observation. Water produces, 

 equally, lofty and nearly vertical walls in some permeable beds, and low surface 

 slopes in other impermeable beds, or it produces a succession of infinitely 

 small walls and slopes, which to the eye appear like a plane or curved surface. 



These are questions of mechanical stability or instability. (Denudation is 

 Instability. See page 487, Geological Magazine, 1872.) The load, the quantity 

 of water, and the direction of the strata, are elements which affect the coefficient 

 of instability. Spring water is the most destructive agent, as the water flows 

 out of the ground, at times under pressure, underholeing, or undermining the most 

 solid rocks, and letting immense masses of rock and sand fall down from the 

 sides of the valley into the bottom, to be carried away by rivers. Anyone who 

 has seen the slips all along the top out-crops of the impermeable fire-clays in a 

 Welsh valley in the coal measure series, when there is ten inches of rain in a 

 month, will be able to estimate the enormous denudation in the pluvial period 

 (seepage 487, Geol. Mag., 1872), when there was 300 inches of rain in England 

 in the low districts, and probably two or three times as much on the mountains. 



I shall continue these observations at a meeting of the Geologists' Associa- 

 tion, June 4th next.* 



any other independent geologist but Lyell hold these opinions. Sir R. Murchison in his 

 address to the Royal Geographical Society in 1863, said almost all geologists were agreed as 

 to man having made his appearance in a very recent and in a glacial period. 

 * Quarterly Journal, 1868, p. 394. Geological Magazine, 1872. 



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