of the Older Stratified Rocks of Devonshire andCortmall. 255 



separated the culm measures as a distinct formation, nor had 

 he ascertained their place in the general section ; for he had 

 so far mistaken their relations, on their northern limits, as to 

 place them not over (as he now does after our example), but 

 under the first three groups of the older system ; and their 

 southern limits he had never ascertained. In short, to work 

 out this point it was necessary he should do what he had not 

 done — to separate the black or culm limestones from the other 

 limestones of Devon; for without this it was impossible for 

 any one to take the first step. As soon as our new views were 

 announced, he suppressed the map as originally coloured, 

 and revisited Devonshire to make himself acquainted with the 

 fresh data, which he had now no difficulty in doing. Major 

 Harding had, indeed, pointed out to one of us the existence 

 of the culm limestone even as long ago as 1835, at the time 

 when Mr. De la Beche's map was first exhibited. Great 

 therefcre was our surprise when we perceived that no distinc- 

 tions were drawn in the ordnance geological map between 

 this very remarkable flat-bedded limestone, and those lime- 

 stones of a slaty character which predominate in other parts 

 of Devon ; the more so as the fossils of the one are entirely 

 distinct from those of the other. So palpable, however, is 

 the line of demarcation between the rocks containing the black 

 or culm limestone and those containing the slaty limestone, 

 that Major Harding, though then very slightly acquainted 

 with geological phaenomena, had traced the boundaries of the 

 two classes of deposit from Barnstaple by Swimbridge and 

 Venn before we entered Devonshire, calling the one mountain 

 and the other transition limestone, and had thus prepared an 

 excellent point of departure for our examination. On the 

 south side of the trough (as we afterwards showed it to be), 

 Mr. De la Beche referred the inferior part of the culm series 

 to the oldest system of Devon and Cornwall ; because it al- 

 ternated, like the Cornish killas, with certain contempora- 

 neous trap rocks. We pointed out to him the imperfection 

 of this reasoning, because similar alternations take place 

 among rocks of many ages, and therefore by themselves prove 

 nothing. Now, in 1839, Mr. De la Beche publishes his Re- 

 |)ort, accompanied by an index map, in both of which he 

 adopts our view respecting the right mapping of the culm 

 measures as a distinct formation, which he calls " carbona- 

 ceous rocks:" and although before our visit he had always 

 insisted on these rocks Ibrming an integral pare of the grey- 

 wacke series, he now gives valid reasons for their separation, 

 yet without acknowledgement, and with no other indication 

 that we were the agents who produced this change in his 



