Absh'act of Lecture. 435 



PAGES 



from observation, reduced in a branch one-half of the size, to 1 "6 metres 

 per second, and again to 1*27 metres per second, in a branch one- 

 quarter the size of the main stream ; all the streams at the same slope. 

 This gives an idea of the effect of weight in increasing velocity at the 

 same slope. If the Nile was divided higher than at present, the rise 

 would occur earlier and be higher, as the stream would be slow. The 

 reverse would be the case in the Rhine. If that river was prevented 

 from dividing at its delta it would flow in a swifter course to the sea, 

 and keep a deep mouth open in the sea so as to admit large ships . . . 452 



22. Mount Tabor, Fig. 19, from a photograph. The outline is compared 

 with a dotted line representing a binomial curve, obtained from the 

 co-efficients of (a + b) l0 . Every symmetrical hill has a convex top 

 and a concave bottom, and is steepest in the middle. This form, so 

 constant in nature, is caused by the watershed being flat, so that little 

 water flows upon the high lands, above ground. Springs escape at the 

 middle heights of the valleys and widen valleys. The action of 

 sub-aerial fluvial and pluvial denudation is not so much to lower 

 watersheds as to widen valleys. Thus, in a great denudation of this 

 kind the maximum height of the mountains and watersheds and the 

 depth of the valleys may be constant.* The valleys could be enor- 

 mously widened, and the mountain masses thereby emasculated, with- 

 out the average height of the land been materially lowered. This, of 

 course, supposes that as much material is removed by denudation from 

 any level above the mean as from another below the mean, or rather, 

 that the relative removals balance each other, leaving the mean and 

 maximum heights of the land untouched. For this reason the land 

 can never be all removed below the sea level ... 



A representation of the binomial curve or curve of denudation, showing 

 how it is set out. Fig. 20. A symmetrical hill curve ... ... 452 



23. Representation of form of hills in different position, to show that all 

 the surface curves can be represented by binomial curves. The 

 Fishback Hill, Fig. 21, is a very common form, as naturally the 

 watershed is not often exactly intermediate between two lines of 

 drainage. Figs. 21 A, 21 B and 21 C, are sections in different posi- 

 tions of the Fishback Hills ... ... ... ... ... 453,454 



24. Hills of Lower Greensand, Fig. 2 ID, and hills of glacial drift in Sweden, 

 can also be represented by binomial curves. Fig. 2IE ... ... 454 



25. Figs. 23 and 24 represent the case of water passing through fissures in 

 hard rock and washing away the clay below, and causing the formation 

 of Ecclesbourn, which is the type of all valleys. The hard rocks 

 falling into the brooks of any district pave them, and prevent the 

 sands being washed away. This is the cause of Crowborough Beacon 

 standing 900 feet above the sea : without the iron-sand rock, this 

 hill would have been washed away by its brooks. The position of 

 rock and clay at Ecclesbourn, on a very small scale, is that of Niagara 

 on a very large one. The slopes of surface or contour of the land is 

 terraced or shaped into alternate walls and slopes, in consequence of the 

 water escaping over the clays, levelling the wet surfaces of clay, but 

 leaving the rock or sand standing up at a steeper angle than the clay, 

 because less water passes over it and its stability is greater. This is the 

 form of the buttress in architecture ; that is, the wall and slope used is 

 the form best adapted to resist atmospheric action — it is the form of 



* Mr. Croll has stated without apparently considering the geological evidence, that all land 

 can be reduced to the level of the sea, and he calculates the time by, what I consider, an 

 unsafe method. He has copied my method of 1853, of computing denudation in other 

 respects, without adding any improvement. 



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