CHAPTER III. 



HAMPSTEAD HILL ITS MATERIALS — THE LONDON CLAY. 



Oe the material of which the great bulk of Hampstead Hill is 

 composed, it will be necessary to speak somewhat in detail, 

 since, although its general features are commonly known from 

 the many excavations for building and other purposes which daily 

 reveal the stiff tenacious character of this great mass of clay, 

 yet its composition, peculiarities, and especially its extraordinary 

 and most suggestive contents, are not by any means so well 

 known as they ought to be. And there is one simple character 

 of importance that is not at all revealed by ordinary excavations, 

 and that is its colour. This may seem a curious and unwarrant- 

 able assertion, but it is nevertheless true, since the colour of the 

 clay exposed in excavations that only penetrate the bed near the 

 surface is not the colour of the mass of the clay. The colour 

 usually seen is, as everybody knows, a yellowish brown, but this is 

 due to a change that has taken place from contiguity to the surface 

 which has allowed the oxygen of air and water to act on the 

 iron matter disseminated through the clay in a manner precisely 

 analogous to the familiar rusting of iron by water or dampness. 

 Thus it is that only the uppermost beds are of the well-known 

 brown colour, while the deeper beds that have been protected 

 from this oxidizing influence are of quite a different hue. The 

 true unaltered colour of this lower clay is a greyish blue, which 



