Subjacent to the Boulder-clay. 



249 



" The commer- 

 cial value of the 

 material has only 

 lately been dis- 

 covered. It is 

 now extensively 

 worked for various 

 purposes. The best 

 sandy varieties are 

 used for building 

 and lining smelt- 

 ing furnaces, the 

 coarser, for the g; 

 making of fire- 2^ 

 bricks, and the ' % 

 plastic clay when •* •" 

 carefully selected e" 

 will probably be ^^ 

 found applicable 't % 

 to the making of o^ 

 earthenware. It w § 

 is well adapted |p 

 to these various ? ^ 

 purposes and is ^|^ 

 composed almost a " 

 exclusively of 5'g 

 silica and alu- | & 

 mina. f'| 



"Below Caldon 

 Low this deposit 

 betrays its exist- 

 ence by deep sink- 

 ings on the surface 

 of the ground, or 

 by ' Swallows,' as 

 the inhabitants of 

 the neighbour- 

 hood term them. 

 These are minia- 

 ture valleys that 

 converge towards 

 deep central de- 

 pressions in 

 which, without 

 any apparent hole, 

 the surface water 

 disappears sud- 

 denly into the 

 ground. This is, 

 no doubt, owing 



