Earths. 187 



They are thought purest when they are trans- 

 parent and colourless ; but we find flinty matter 

 very pure when tinged of a dark colour, provided 

 it burns white in a violent heat. This, added 

 to its hardness, are sufficient characteristics: 

 but it is much oftener met with impure, and 

 hence its varieties. 



It is found in strata of different kinds. Those 

 which contain it purest are freestone, gravel, 

 and sand : the whitest sorts are the purest. 

 Black sand is not uncommon in America, and is 

 generally supposed to contain gold ; but the se- 

 paration is too difficult to answer the expense. 



Gravel consists principally of the same kind 

 of matter, but in larger and more irregular 

 masses, and, on the whole, is seldom so pure as 

 the flinty matter in sand. 



The stones admired under the name of peb- 

 bles are of this nature, and owe their beauty to 

 the other materials they contain. Their external 

 surface is rough, but their internal structure beau- 

 tiful when polished. In all the white milky peb- 

 bles there is an alternation of whiteness and dark- 

 ness, as in the large ones called agates. In most 

 pebbles the layers are of different colours. From 

 this structure it has been supposed that they have 

 been formed by successive incrustations of flinty 

 matter, which has been more or less coloured. 

 Some have the same appearance externally, and 

 are called pebble crystals ; they are either colour- 

 less, or are all over of the same colour, and from 



