39 Dr. Mac Culloch on 



arrangement, but that it is not founded, as far as any evidence 

 yet goes, on the real order of nature, while it is of far less 

 utility even as an artificial arrangement. 



To examine, in the first place, the simple classification, or 

 that into primitive and secondary. 



The primitive rocks are distinguished by the following cir- 

 cumstances. Whatever geographical places they may occupy 

 in nature, they are the lowest in geological position ; or else 

 those rocks which steadily maintain the lowest places in the 

 order of relative superposition, up to a certain point, are con- 

 sidered as primitive. It is further stated, as characteristic of 

 this class, that the strata are always elevated at high angles, 

 and that they follow in consecutive parallel order. It is also 

 said that their nature or texture is chemical, or that they bear 

 no marks of mechanical origin ; and, lastly, that they do not 

 contain organic remains. This class, thus determined, is 

 found to comprise a certain number of rocks, of which the 

 mineral characters are, to a great degree, peculiar and suffi- 

 ciently constant ; and thus also, reversely, these mineral com- 

 pounds are considered, wherever they may be found, as pri- 

 mitive rocks. 



The secondary class is, of course, the receptacle of all those 

 not included in the other. It is supposed that these are cha- 

 racterized by prevailing low angles of elevation, by the general, 

 or frequent, or necessary mechanical nature of their texture 

 or nature, and by their containing organic remains. It is also 

 supposed that their order is parallel and consecutive among 

 each other, but that this order is not parallel to that of the 

 substances in the primitive class. Hence, therefore, the boun- 

 dary between the two is placed at that point where the change 

 of order takes place ; or the reverse position of approximate 

 strata indicates that the lowest of these is the last, or upper- 

 most, of the primitive, and that the other is the first, or lower- 

 most, of the secondary class. If, in certain cases, it happens 

 that this reverse position does not exist, then the boundary is 

 determined by the mineral characters of the approximate 

 rocks ; it being observed that a conglomerate or sandstone is 

 the lowest rock, whenever the series of the secondary strata is 

 complete. If, again, even that rock should be absent, then 



