n8 THE WORLDS LUMBER ROOM. 



state in Nature. Clays owe their peculiar plasticity to the 

 presence of combined water.* 



The purest form in which silica is found is quartz, but 

 even that contains a trace of alumina, and sometimes of 

 iron as well ; and the purest form of alumina is the blue 

 sapphire, which is crystallised alumina, coloured by iron, 

 and containing also a minute quantity of silica. Neither 

 silica nor alumina, therefore, is ever absolutely pure. 



These two, with potash or soda, constitute the felspar 

 which makes up so large a portion, not only of the granites, 

 but of all the lavas, ancient and modern; and decayed 

 felspar is simply day> which varies in colour and con- 

 sistency as it contains more or less silica, and more or 

 less iron. 



The purest clays those used for the manufacture of 

 porcelain are more than half silica, and none are entirely 

 free from iron, while many contain soda, potash, mag- 

 nesia, &c. 



The brick-making clays when burnt come out red, blue, 

 or black, when they contain much iron, brown when they 

 contain magnesia, white and dun-coloured when they con- 

 tain lime. More than a thousand million common bricks 

 are made in England every year. 



But Nature, too, has her potteries and brick-kilns, 

 where she bakes her clay to bring out its colours, and one 

 of the largest of these is in the " Bad Lands " of the Little 



* Oxygen constitutes nearly half the weight ot the earth, and is found 

 combined with many metals and minerals. Soda, alumina, and lime are all 

 compounds of oxygen, with the metals, sodium, aluminium, and calcium, 

 none of which occur in nature except in union with oxygen. 



