II. J SPRINGS. ::5 



the first outlet, as at D. Such a flow of water thrown out 

 from a rock constitutes a spring. 



Springs of this simple character, which issue at the 

 junction of permeable and impermeable strata, are extremely- 

 common. If the reader who lives in London will take the 

 trouble to walk across Hampstead Heath soon after rain 

 has fallen, he can observe for himself exactly the condition 

 of things which has just been described. The highest 

 ground at Hampstead rises to about the height of St. 

 Paul's, or rather more than 400 feet above the sea-level, 

 and consists of loose sand which forms irregular ground 

 overgrown with ferns and gorse. This sand is similar to 

 the sand which is spread out in large mass over the widely- 

 separated district of Bagshot Heath in Surrey, whence it is 

 termed Bagshot sand. It is, however, only the highest 

 ground on Hampstead Heath that is formed of such sand, 

 which indeed does not attain, in this locality, more than 

 eighty feet in thickness. Beneath the sand is the same 

 stiff brown clay which underlies the whole metropolis, and is 

 consequently known as the London clay. Hampstead Heath 

 is therefore formed of London clay capped by Lower 

 Bagshot sand, as shown in Fig. 7. This is taken from a 

 section drawn by the Geological Survey, but in order to 

 bring the section within the width of the page, and yet pre- 

 serve its characters, the vertical scale has been exaggerated. 

 On a natural scale the heights in the figure would require to 

 be reduced by one half, and the capping of sand would 

 consequently appear of insignificant thickness. Where one 

 kind of soil ends and the other kind begins, is pretty sharply 

 mapped out by the effect of rain. Any one who walks over 

 the Heath after a shower will not fail to observe that 

 the sandy soil remains almost perfectly dry, the rain having 

 been at once sucked in, while the clay, only a few yards off. 



