26 HAMPSTEAD HILL. 



sapphire. So we find that the substance of these splendid 

 gems of almost adamantine hardness, is the basis of the soft 

 and despised clay which, in wet weather at all events, we so 

 much dislike our feet to come in contact with. The pure 

 silicate of alumina is kaolin, a fine white clay called ** china-clay " 

 from being used for the manufacture of porcelain, and this, 

 when mingled with impalpable sllicious powder, with such 

 compounds of iron as have been mentioned, and with other 

 substances in a finely divided condition, and permeated 

 throughout with a certain amount of water, constitutes ordi- 

 nary clay ; and such is therefore the composition of the London 

 Clay of Hampstead. 



But besides the substances that make up the body of the clay 

 itself, and only separable by chemical analysis or separately visible 

 by microscopic aid, there are some which form considerable 

 embedded masses, but quite separate and distinct from the clay, 

 and some in such quantities as to be of economic value. One 

 of these distinct substances appears in the form of large, roughly 

 circular masses, sometimes as much as three feet in diameter, 

 and of considerable thickness in the centre, but gradually 

 diminishing towards the circumference, thus forming bi-convex 

 lens-like bodies. These are called " septaria " or '* cement 

 stones," the latter name being given to them because they are 

 employed for the manufacture of the so-called *' Roman cement," 

 which was so much used in pre-Ruskin days for stuccoing the 

 outsides of brick-built houses, their value for this purpose 

 being due to the material of which they are composed being 

 an admixture of calcareous and clayey matter. They are formed 

 by the natural concentration and aggregation of the calcareous 

 matter contained in the surrounding clay ; and, on account of 

 internal shrinkage, radiating fissures or cracks from the centre 

 with transverse minor cracks are produced, and these become 



