ii2 THE WORLD'S LUMBER ROOM. 



came the iron oxide which coats each separate grain of the 

 red sandstone. 



Sandstones consist mainly of grains of silica, which are 

 generally intermixed with small particles ot other minerals, 

 and are cemented either by carbonate of lime, iron, or silica. 

 Where both grains and cement are of silica, the sandstone 

 would seem to have been formed by the agency of heat. 

 Intensely hot steam, for instance, may have penetrated the 

 mass of porous sand, partly melting each grain which, as it 

 cooled, would be cemented to its neighbours. 



The heat of molten lava would also have a similar effect, 

 for the sand used to line furnaces is found at the end of 

 a fortnight to be in part converted into a compact, close- 

 grained stone, simply by the heat ; and quartz rock, in which 

 the grains can hardly be distinguished even with the aid of a 

 microscope, was probably also formed by heat. 



Pure quartz consists simply of silica, and crystallises in 

 six-sided prisms. Cornish, Bristol, and other so-called 

 " diamonds," are small bright colourless crystals of quartz, 

 the purest variety of which is the rock-crystal used by spec- 

 tacle makers, while the Scotch cairngorm, purple amethyst, 

 ehrysoprase, chalcedony, carnelian, onyx, heliotrope or 

 blood-stone, and the precious opal, all consist of silica, va- 

 riously coloured by other minerals and metals. We cannot, 

 indeed, say that any of these stones have ever actually existed 

 in the form of sand, but neither does there seem to be any 

 reason why sand should not, in process of time, be trans- 

 formed into any one of them. 



But to return to the changes which we can see for our- 

 selves to have taken place. 



