CHAPTER VII. 



THE BAGSHOT SANDS. 



It is now time, however, to say something of the sands which 

 give so marked a character to the summit of the hill. They 

 are so well and conspicuously displayed, and therefore so well 

 known, that a description of their appearance is scarcely neces- 

 sary. Their prevailing colour is yellow of various shades, but, 

 like the clay, they undergo a change in this respect with ex- 

 posure to weather — not, however, by increased oxidation, but by 

 having their colouring matter removed. The yellow sand is 

 coloured, as is, indeed, all brown and red sand, by each grain 

 being coated over by a thin film of iron-oxide, the grain itself 

 being a particle of white quartz or colourless rock-crystal ; in 

 each case, however, silica — a grain, in fact, which is a fragment 

 of some old granitic or quartz-bearing rock or quartz rock itself, 

 some quartz vein, or some previously existing bed of sand or 

 sand-stone. When, therefore, this thin film or coat of colour- 

 ing matter is worn off, or taken off by exposure to the weather, 

 the sand becomes white or whitish. In some places there is 

 sufficient iron-oxide to cement the sand together into a ferru- 

 ginous concretion, producing irregular-shaped masses of varying 

 hardness. Occasionally, too, some thin clayey seams are to be 

 found interstratified with the sands. The grains themselves 

 are more or less rounded or sub-angular, as it is termed, giving 

 evidence of some amount of wearing action on a sea-shore. 



