210 ENQLISH CHALK DOWNS. 



rounded surfaces. Under other conditions there 

 is no depth of soil for the trees to root in ; and so 

 the native vegetation of tlje downs, appearing 

 wherever tliey liave not been cleared for forming 

 sheep-walks, consists almost exclusively of juniper, 

 yew, gorse, and blackthorn — all of them shrubs 

 that require but little foothold to assist them in 

 fastening on the bare rock. Few or no rivers 

 flow over the chalk, save in the deepest and nv)st 

 basin-like hollows; elscwheie, as in the countr}'' 

 of the Downs and Wolds, the streams that inter- 

 sect them have long since cut themselves narrow 

 and precipitous gorges in tlie soft chalk, by which 

 they burst through the barrier ridges. Thus the 

 Thames has carved out for itself the picturesque 

 dale between Mapleduiham and Ilcnley, across 

 the Chilterns and the Berkshire Downs ; and thus 

 its tributaries, the Mole and the Wey, have forced 

 their road through the long line that extends 

 across the country from Salisbury Plain to the 

 great white cliffs that overhang Ramsgate. Our 

 southern coast scenery itself has owed its origin 

 to much the same assemblage of circumstances ; 

 here, low tertiary levels slide easily under the 

 bed of the Channel ; there, the end of the Downs 

 topples over precipitously into the sea in a shorn, 

 white cliff; and yonder, again, the whole thick- 

 ness of the chalk has been denuded off by the 

 wearing agency of wind and weather, and the 



