CLAY, SAND, MARL. 251 



yet the most fissile and tenacious varieties serve for 

 roofing-slate and flags ; especially when it occurs in 

 the lowest sandstone. It is the repository of metals, 

 like the other strata which it accompanies ; bull is it- 

 self an iron ore, when highly ferruginous ; being the 

 ironstone so extensively used in the manufactories of 

 Britain. 



Clay, Sand, Marl. 



In terminating this account of the secondary strata, 

 I must notice these substances ; which, though not 

 rocky, form a portion of the structure of the earth. 

 Sometimes the result of disintegration, they seem at 

 others to be waiting for that induration which occa- 

 sionally takes place in certain portions without alfect- 

 ing the rest. Thus the clays are found intermixed 

 with shales, the sands with sandstones, and the marls 

 with earthy limestones. 



The general position of these among the strata is 

 already understood ; and it has been seen that they 

 occur throughout the whole series. In general, they 

 are more important in an ceconomical view, than in- 

 teresting to a geologist ; while many of them are un- 

 distinguishable by mineralogical characters. The ex- 

 tent of a few, however, renders them objects of greater 

 interest. Such are the clays of the coal-beds, contain- 

 ing ironstone, the niarl of Gloucestershire and Lincoln- 

 shire, the fen clay, fullers' earth, the extensive forma- 

 tion of plastic clay, and the no less conspicuous blue 

 clay which contains septaria, forming the uppermost 

 material in the London district. Among the sands, 

 the most remarkable are situated high in the series ; 

 and these are the ferruginous sand, and the green sand, 

 being portions of the general deposits already described. 

 The others occur among all the several other strata 



