Causes of Shape of Unsymmetrical Hills. 



453 



dation on a soft series of sand and clay-beds. Water enters by fissures and cracks 

 along the upper surface of the alternate beds of clay and sand, when it reaches the 

 air, where there is little or no weight of rock to keep the sand and clay from 

 sliding, motion ensues, and the outer surface of rock or sand is dislodged. With 

 eight times the quantity of water flowing along these unstable surfaces the de- 

 structive effect might be as the cubes, or nearly 500 times as much as at present. 



(22) I have shown what I believe to be the typical form of a hill, drawn from a 

 photograph of Mount Tabor, Fig. 19. This hill has, I believe, a base of seven 

 miles, and the surface has been eroded into the binomial curve, which is, 1 

 consider, the form of greatest stability. It is the form which gives the nearest 

 possible approach to uniform motion of water on its surface. 



May Hill, seen from Gloucester, is a good example of this form, which is 

 common to the hardest and softest rocks, and even to clay and sand. 

 „,.»._. Watershed. 



FISH BACK HILL. 



Binomial Curve 

 See Fij IS. 



.TVLDR 



Hills of drift have been drawn by Jamieson and others not quite so regular or 

 uniform as those in Sweden. 



The drift hills, 40 feet high, on Hirwain Common, 200 feet above the 

 river and two miles from Aberdare on the Neath-road, contain blocks of mill- 

 stone grit many tons' weight, and rolled pieces of old red sandstone transported 

 on ice. The hills are in form like Figs. 19 and 21, but not quite so regular. 



Watershed, 



f/c: 21^ 



A.TYtOR.DEl. 



Longitudinal Section* tha"AB 



FISH BACK HILL 



JZiiwmiaZ curve. 

 jSee RcfL5± 



120 <iS IO 



X 10 &S 1Z0 SM SS3 33D 



(23) If hills of glacial drift on Hirwain Common and in Sweden, Green Sand 

 hills near Leighton Buzzard, and Sandown in the Isle of Wight, and in fact in all 

 formations and countries, assume this form, it is because the formation of sloping 

 surfaces in all hills and valleys is really the same natural process to be observed 

 in an exaggerated form in most waterfalls. See Fig. 23, p. 456. The law for 

 waterfalls is, twenty-seven times the water would produce as much destruction 

 in a clay as now occurs in a year. Water filling the brook at Ecclesbourn, pass- 

 ng through the fissures and carrying away the soft clay or shale (which in most 

 waterfalls underlies massive jointed rocks) produces instability ; and as soon as 

 the surface of the clay is washed away, the rock is undermined and falls down. 



