CLAY, MARLE, AND SAND. 607 



shale. Their geological and loeal importance, 

 no less than their economical uses, render them 

 objects of considerable interest to a geologist, 

 but an examination of these subjects is not 

 here admissible. The clays of the coal fields, 

 which contain ironstone, the blue clay of Lon- 

 don, and Fuller's earth, are among the most in- 

 teresting. 



The marles occur in similar situations, and 

 are, most particularly, interstratified with lime- 

 stone ; being often solid, and thus forming marie 

 slates. Those of Gloucestershire and Lincoln- 

 shire are among the most interesting in the 

 English strata. In the foreign, it yet remains to 

 be ascertained what the true nature of many of 

 these rocks are ; as, in some, which are said to 

 occur with primary limestone, organic remains 

 are found. 



The positions of the sands are similar, and 

 they are frequently also intermixed with extensive 

 consolidated portions forming sandstones, which 

 it has been necessary to notice already under that 

 head . Among the English strata, the ferruginous 

 tmd the green sand, are the most conspicuous 



