26 PHYSIOGRAPHY. [chap. 



remains wet and sticky. The water absorbed by the sands 

 drains through them until it reaches the clay beneath, when 

 it either oozes out in an irregular sheet of water, or is dis- 

 charged through definite channels as springs. The line 

 of springs consequently serves to mark the junction between 

 the two kinds of soil. As the surface of the clay receives 

 the drainage of the sands it is constantly kept wet, and 

 thus forms swampy ground, often marked by the growth of 

 rushes. It sometimes happens that the upper part of the 

 London clay is more or less sandy ; and, consequently, it is 

 not always easy to say precisely where the London clay 

 ends and the Bagshot sands begin. The springs are. 



Fic. 7. — Section of Hampstead Heath. Vertical scale twice the horizontal scale. 



however, thrown out as soon as the water reaches an 

 impervious bed. 



The sands of the Bagshot series as exposed on Hampstead 

 Heath are commonly of yellow and brownish colours. 

 These colours are due to the presence of a peculiar com- 

 pound of iron, oxygen,^ and water, known to chemists 

 as hydrous peroxide of iron or ferric hydrate. As the rain- 

 water slowly filters through these ferruginous sands it dis- 

 solves more or less of this iron-compound, and carrying 

 itotT in a soluble form, acquires medicinal properties. Thus 

 the spring in Well-walk at Hampstead is locally valued as a 

 tonic ; while its inky taste, and the foxy-red sediment 



^ For description of the gas called oxygen, sec pp. 78, 79, So. 



