LITHOLOGY. 89 s 



books recommend, that an acquaintance with the appear- 

 ance and characteristics of all ordinary rocks and 

 minerals should be formed by careful study of cabinet 

 specimens. 



All the stratified rocks, except those organically and 

 chemically formed, were deposited as clay, pure or im- 

 pure as sand or gravel, which differ from each other 

 only in the size of their particles or as an admixture 

 of the two, sometimes with the addition of lime or other 

 secondary ingredient. 



Clay is simply mud derived from the waste of pre- 

 existing rocks. Generally, it consists of finely-com- 

 minuted silica, with about one-fourth of alumina, 

 and as a rule may be considered a deep-sea deposit. 

 When indurated by pressure more or less verti- 

 cal to the lines of bedding, it becomes divisible into 

 laminae, and is termed Shale. 



When indurated by pressure in any other direc- 

 tion, it is cleaved, or becomes divisible into laminae 

 at right angles to the pressure, and is termed Slate. 

 (See note below.) 



In whatever form clay occurs, whether unaltered, 

 as shale, or as clay-slate, it gives off when breathed 

 on an earthy or slaty odour which is unmistakable. 

 Sand, also derived from the waste of existing 

 rocks, consists mainly of silica, with a slight admix- 

 ture of other minerals. When finely-bedded, it has 

 been deposited in tranquil water ;* when the lines of 

 stratification intersect each other at various angles, 

 it is false-bedded, and has been formed in a current, 



* Except in the case of " blown sand " see Jukes' " Manual,'* 

 p. 378. 



