OVERLYING ROCKS. 479 



is only used as a general term, and in a geological 

 sense ; having commonly been applied in so 

 vague a manner, to all the rocks of a dark colour 

 which are not porphyry, basalt, or decided green- 

 stone, as to have ceased to form any distinction, 

 and having indeed become a source of confusion. 

 To distinguish two varieties of claystone, is per- 

 haps an unnecessary refinement ; but it is not 

 productive of any inconvenience. 



Characters derived from colour alone, have 

 been uniformly rejected ; and it will be found, 

 that a dependance on this vague criterion, has 

 hitherto formed a principal source of confusion 

 in describing these rocks. It is, in itself, an ex- 

 tremely variable circumstance in every rock, and 

 in none more than these ; in which it often differs, 

 in substances in other respects identical. Un- 

 questionably, it depends on differences of chemi- 

 cal composition ; but as it is not considered as a 

 permanent distinction by the best informed mi- 

 neralogists, it merits no consideration here. The 

 habitual neglect of a mark so variable and so 

 empirical, will teach the student to turn his 



