Physical Structure and older stratified Deposits of Devonshire . 673 



«»-neiss, &c. &c. Nor is this all. The granite sends out veins into the altered 

 beds^ of which we found an example, not merely in the metamorphic beds of 

 contact, but in a black, indurated shale, at a considerable distance from the 

 junction, in one of the ravines south-west of Oakhampton. Moreover, there 

 is an Elvan dyke (precisely like the Cornish Elvan) still further north, in 

 the central portion of the culm measures, far above the group now under 

 notice*. 



Phenomena like these, where cause and effect are so clearly brought toge- 

 ther, are of no small interest. The metamorphic group is one of the most 

 instructive we have ever examined, inasmuch as we can trace it through all its 

 gradations : and the facts we have stated prove to demonstration, that the gra- 

 nite is not older than the stratified series. And that it is not contemporaneous 

 with the altered group, which is made up of a long series of mechanical de- 

 posits, is too obvious to require a proof. There remains, therefore, only one 

 conclusion, (sanctioned by all the changes of position and structure, as well as 

 by the protruded veins,) that the granite in mass is a rock of protrusion, and of 

 a date posterior to the development of, at least, all the lower divisions of the 

 culm measures. 



We must now briefly describe the black shales and limestones which form 

 the upper division of our present group, and are, indeed, its most characteristic 

 and important members. These beds have so many characters in common, 

 that the same descriptions will serve, with a few exceptions, for the black lime- 

 stones both on the north and south side of the great culm trough ; and on both 

 lines it is extensively worked in a succession of large open quarries, in several 

 of which are vertical sections more than 100 feet deep. 



This upperdivision is regularly bedded, and essentiallyconiposedof black carbonaceous shale, tra- 

 versed by numerous irregular white veins of calc-spar, and alternating with beds of black limestone. 

 The limestone is also filled with white veins ; and being of a rather earthy structure, is only distin- 

 guished from the calcareous shale by close inspection. Among the more calcareous bands some are 

 fossiliferous, containing a great abundance of at least two genera of bivalve shells ; one a Posidonia, 

 the other of a genus not ascertained, but regarded, by those who have examined it, as a marine 

 shell. In the same part of the series are Gonialites of at least two species, both of which are 

 unquestionably marine, and (according to Professor Phillips) identical with Goniatites of the 

 Yorkshire coal-field. And there is a negative fact also of great value. This culm limestone 

 contains no fossils resembling those of the groups inferior to the culm series. The beds in ques- 

 tion are therefore not more distinguished by their structure from all the calcareous strata of the 

 inferior groups, than they are by their fossils. Yet the black culm-limestones and the calcareous 

 slates of the older system, sometimes range almost side by side. This shows the absolute neces- 



* This Elvan was first laid down by Mr. De la Beche. 



