Metalliferous Secondary Rocks. 53 



strata cannot be ascertained, but discover in it the same 

 fossils, or at least the characteristic ones, which had been met 

 with in the other, we may thence conclude, with tolerable cer- 

 tainty, that it is the same limestone, and assume that it occurs 

 in the same order in which it was observed at the first point. 

 This procedure is certainly, in all points of view, of great uti- 

 lity ; but, as the accurate Delius says, we must beware, or at 

 least use every precaution, — that if, examine the relations of 

 superposition with ample detail, — ere we assume that a lime- 

 stone, which has been observed in one basin, is identical (in a 

 geological sense) with another which has been found in a place 

 which is remote from it, that is, which is separated by whole 

 groups of mountains ; for in this way many determinations 

 have been made which are not only uncertain, but are, in part, 

 plainly incorrect. The method of determining rocky masses 

 by their fossil remains possesses, it is true, two advantages, 

 which recommend it to a preference as compared with the la 

 borious investigation of the phenomena of superposition. The 

 first is its great convenience ; for at present many formations 

 are actually determined in the study.* The other is the ap- 

 pearance of learning which it carries along with it, in whose 

 place, however, owing to the recognition and employment of 

 the connection which prevails among the sciences, there is often 

 found an illogical mixture of them. But it is to be expected, 

 that those to whom is assigned and intrusted the important 

 and serious business of searching for valuable repositories of 

 useful minerals, will not sacrifice the success of their task either 

 to that convenience or to that semblance. 



Useful Minerals found in Secondary Mocks. — The useful mine- 

 rals which may be expected in the so-termed secondary rocks 

 are, with the exception of a few which seem to be confined to 

 isolated points, coal, rock-salt, ironstone, lead-glance, calamine, 

 and some copper-ores ; and the repositories in which they occur 

 are chiefly in the form of beds. 



The true coal-formation is easily recognised by its composi- 

 tion, and other characters. It contains the best and most pro- 

 ductive coals; but these are not always present where the coal- 



* Bulletin <!<■ la S<><H : n : Gte'ologique do France. 



