43 



constituent parts are, becomes, therefore, an object of necessary ex- 

 amination. 



Of the first stratum of mould, sufficient has been already said 



for our present purpose. 



Clay, which I have reckoned the second stratum, has for its basis, 

 the earth called ALUM IN E, or the earth of clay, which constitutes 

 its real argillaceous part. This earth readily combines with acids, 

 and forms the salt called alum, by its union with the sulphuric acid 

 or the acid of sulphur. It hardens to a very considerable degree 

 in the fire; but does not fuse, unless combined with lime, or certain 

 salts containing the phosphoric acid. The other ingredient, in the 

 formation of clay, is silicious sand, of which I shall presently speak 

 more fully. From the various proportions of these two substances ; 

 from the different degrees of their- respective purity ; and, particu- 

 larly, from the admixture of iron, and of some of the other earths, 

 proceeds a considerable variety in clays. Those which are most 

 likely to come under our observation are, the lithomarga, formed by 

 the finer clays in various degrees of induration ; and fullers earth, 

 an argillaceous substance, deriving a saponaceous softness from its 

 containing a small portion of magnesia. 



Trap, rowley rag, toad-stone, or wacken, are stones in which argil- 

 laceous matter very much predominates. These, from the manner 

 in which they are diffused through the subterranean regions, and 

 from the peculiar structure which they possess, have been errone- 

 ously supposed to be of volcanic origin. 



Schistus or slate, is also a mixture, in which alumine very much 

 predominates. It also contains the earth of flint and magnesia. It 

 is found to vary considerably in colour and texture, according to 

 the degrees of purity, and the different proportions of its constituent 

 parts. 



SILICA, or silicious earth, the earth of flints, is the chief consti- 



