1868.] HOLL — sourii dkvon and east Cornwall. 401 



the relations of the beds on the south side of the Culm-measures ; and 

 any one who is acquainted with the literature of the subject will, I 

 think, admit the fact. Indeed the late Sir Henry Dc la Beche, 

 subsequently to his last essay in the first volume of the Memoirs 

 of the Geological Survey, regarded the succession of the rocks as 

 still unravelled, and expressed his intention of taking an early 

 opportunity of revising this portion of his labours* . 



Many of the difficulties which stood in the way of the pioneers of 

 Devonian geology have long since been cleared away, more especially 

 by the publication of the Geological Survey-maps of Devon and Corn- 

 wall, the general accuracy of which is all the more remarkable as it was 

 the first attempt at anything in equal detail in this country ; and a 

 great step was made towards a better understanding of the structure 

 of the Devonian country when Prof. Sedgwick and Sir Koderick 

 Murchison, in identifying the Culm-measures with the Carboni- 

 ferous system, separated them from the underlying slaty rocks with 

 which, under the name of Grauwacke, they had been previously 

 united f. Nor need the general order of succession, as established 

 by them thirty years ago, in the first of their memoirs, be greatly 

 disturbed. But there are many matters of detail, not comprehended 

 in their memoir, which are still open to inquiry, notwithstanding the 

 elaborate and able report subsequently published by the Geological 

 Survey J. To this work the following communication must be 

 regardedj as supplementary, its object being to enter, in a general 

 manner only, on the consideration of certain points in the physical 

 structure of that portion of the country which lies to the south of the 

 Carbonaceous rocks of Central Devon and adjacent parts of Cornwall. 



II. Carboi^^aceous Rocks or Culm-measures. 



The Culm-measures, as they occur on the southern side of the 

 great synchnal trough of Central Devon, consist of argillaceous slates, 

 with seams of grit and chert, and include beds of volcanic ash and 

 limestone, the latter for the most part of dark colours. These lime- 

 stones are precisely similar to those at the northern edge of the 

 trough, near Bampton and South Molton ; and the grit and chert 

 beds resemble those of Coddon Hill ; but the interstratified volcanic 

 rocks, which are not met with in the north, occur here in consider- 

 able abundance ; and as they are likewise abundant in the underlying 



* See Sir Charles Lemon's Presidential Address, 1848. Trans. Eoy. Geol. 

 Soc. of Cornwall, 35th Annual Eeport, p. 13. 



t Trans. Geol. Soc. 2ndser. vol. v. p. 633. This first great reform in the classi- 

 fication of the rocks of Devonshire was made by Professor Sedgwick and Sir 

 Roderick Murchison in 1836, and was communicated to the British Association 

 in that year at Bristol. See Report of British Association 1836, Proc. of Sec- 

 tions, p. 95. All geologists, including Sir H. De la Beche, had previously con- 

 sidered the whole of the great Carboniferous tract of Devon to be part, and 

 even a lower part, of that Grauwacke series which Sedgwick and Murchison 

 assigned to the true Devonian, and which they had proved to underlie, and be 

 wholly distinct from, the Culm strata. See also Trans. Geol. Soc. 2nd ser. vol. v. 

 p. 701., where the term Devonian was proposed. 



I Geolo.2;ical Report on Cornwall. Devon, and West Somerset, p. 56 e( feq. 



2 f2 



