CLAY, SAND, MARL. 



already noticed, and require no additional remarks 

 here. The reader must be referred, for a fuller account 

 of all these substances, to the topographical and mi- 

 neralogical works in which they are described. But a 

 few remarks on their present condition will not be 

 useless. 



It is evident that the upper clays, such as those of 

 the London district, are now in the same state in which 

 they were deposited; and that, although they might 

 have produced rocks, under other circumstances, they 

 have never existed in that form since their deposition. 

 This seems equally true of many of the inferior strata 

 of the same nature ; however difficult it maybe to 

 explain why, in certain situations, the same material 

 should have remained uncompacted into stone, while, 

 in others, it has become shale. Yet the remarks in a 

 former chapter have shown that many of these clays 

 have once been in the state of rocks, which have been 

 decomposed within the earth, so as to assume their 

 present form. This, indeed, is proved in Cornwall, 

 where the ordinary argillaceous schist is found con- 

 verted into clay at great depths, retaining the indica- 

 tions of its former state ; as are granite, gneiss, and 

 the traps in innumerable situations. The same re- 

 marks may be made on the sands : but it would be 

 merely to repeat the same reasoning ; and I need only 

 remind the reader of the proofs of extensive decom- 

 position among these, formerly given. Neither is it 

 necessary to extend this reasoning to the marls ; as, 

 in some cases, they are obviously in the state in which 

 they were first formed; while, in others, it is equally 

 apparent that they are the result of decomposition. 



But the term Marl has also been applied to a 

 schistose rock, so as to have been the cause of much 

 inconvenience. That of Marl-slate has particular- 



