674 Professor Sedgwick and R. I. Murchison^ Esq., on the 



sity (whatever may be its age) of mapping the whole culm series by a colour distinct from that of 

 the older slates. 



The group we have been describing is of great thickness, and the calcareous portions are several 

 times repeated on the same section ; and not only in the shales, but also among the limestone bands, 

 are thin laminee of culm, but in no instance so thick as to be of economical use. It is difficult to esti- 

 mate the whole thickness of this portion of the lower culm group, because the beds are repeated by 

 contortions, which in some instances are so violent as to exhibit many salient and re-entering angles 

 like those of a fortification agate. On the whole, however, the limestones and associated black shales 

 plunge away from the older rocks, and gradually disappear under the upper culm-measures. On 

 the south side of the culm-trough, the shales both above and below the black limestone are occa- 

 sionally used for roofing slate. Thus, on the banks of the Lyd, near Coryton, there is a line of 

 slate quarries apparently under the neighbouring range of the black calcareous rock, which, 

 though excessively contorted on the small scale, (a single bed exhibiting five or six zig-zags in one 

 escarpment,) dips on the whole about magnetic north ; and at Cory Park, immediately over the 

 same contorted limestone, there is a very good striped slate derived from a true cleavage-plane. 

 The beds dip magnetic north 35°, and are cut by fine vertical dip-joints : but the cleavage-planes dip 

 magnetic south about 25° or 30°. Here, again, the cleavage-planes are less inclined than the beds — 

 a phenomenon, perhaps, unknown in the north of England. We observed no beds of like quality 

 in any part of the culmiferous rocks along their northern edge. 



We think that the southern calcareous bands are in general thicker than 

 the northern ; but the changes of mineral structure (excepting the meta- 

 morphic portion near the north end of Dartmoor) are more striking on the 

 northern boundary. For example, near Bampton, there are several quarries, 

 where the shales become indurated, have a conchoidal fracture, and a red 

 colour, and sometimes pass into chert ; and near the same quarries the calca- 

 reous shales are associated with manganese, and the limestone bands are more 

 crystalline, and become of a lighter colour. These localities also exhibit con- 

 tortions of great complication ; but in that respect they are not anomalous. 



Lastly, at Holcombe Rogus, we have a most extraordinary combination of 

 sharp anticlinal ridges and contorted beds of limestone of great thickness, in 

 which the black shale, which generally forms about three-fourths of the 

 escarpments that are worked for lime, has almost disappeared, or at least be- 

 come quite subordinate*. 



* In nearly all the great quarries of black limestone, some of which are of great extent and 

 grandeur, both on the north and south boundary of the culm system, not only are there consider- 

 able portions which are rejected as mere waste; but even in the richer portions with the true lime- 

 stone bands, there is seldom more than a third or fourth part which is actually burnt for lime. The 

 alternating bands of dark indurated shale (frequently riddled through in all directions by white 

 veins) are used for flagstones or coping-stones. To prevent misapprehension, we may also state 

 that the culm-limestone (though far more persistent than the limestone j«n^s of the lower groups) 

 often comes to an edge, or passes into mere calcareous shale. Hence quarries for economical use 

 cannot be opened indifferently on every part of the strike ; and indeed along the coast of Cornwall 

 the limestone has disappeared from the section. 



