1851.] SEDGWICK SLATE ROCKS OF DEVON AND CORNWALL. 9 



discussed before, and have been illustrated not only by several distinct 

 memoirs*, but by the two excellent and very elaborate volumes 

 published by Sir H. De la Beche and Professor Phillips f ; I must now 

 shortly notice the more recent discoveries of Mr. Peach, and of some 

 other good Cornish observers. In general we have, both in Cornwall 

 and Devonshire, a considerable approach to symmetry in the positions 

 of the great mineral masses. For example (not to mention three or 

 four minor protruded granitic masses) we have in these counties five 

 great bosses of granite ; and as a general rule the immediately sur- 

 rounding slate-rocks, whatever be their age, dip from the protruding 

 granite in a symmetrical form. In the southern parts of Cornwall the 

 mineral axis, defined by a line drawn through the several centres of 

 the greatest bosses of granite, strikes in a direction about N.E. by E. ; 

 but in the northern part of Cornwall, and in Devonshire, the mineral 

 axis, defined in the above manner, strikes nearly east and west. It is 

 also true, that in both counties, when we are removed from the im- 

 mediate disturbing influence of the granite, the slaty masses have a 

 strike approximatively parallel to the two directions just indicated. 



The great mass of granite that forms the moors north of St. Austell 

 has broken through the slate-rocks not far from the point where the 

 granitic axis of Devonshire and Cornwall has been bent, or perhaps 

 broken. Some great disturbing forces have modified the symmetry 

 of this part of Cornwall, affecting, I believe, the whole transverse sec- 

 tion of the country from the headlands near Fowey to the headlands 

 south of Padstow. However this may be, there is an unusual want 

 of symmetry in the position of the rocks south of the St. Austell 

 granite. In the great open mine-work of Carglaze, about two miles 

 N.E. of St. Austell, we find a very remarkable, veined schorlaceous 

 granite. On it repose the slate-rocks (or killas), in the usual symmetry 

 and conformity to its surface ; and near their base they show such 

 beautiful laminations of schorl and earthy feldspar, that one formation 

 seems to pass almost insensibly into the other. When I first saw 

 this pheenomenon in 1819, 1 thought, what I by no means think now, 

 that it almost proved the truth of the Wernerian hypothesis J. Here 

 then we have the usual collocation of the slate-rocks. 



If, however, we continue our examination of the line of junction 

 near St. Austell, we find the killas penetrated by elvans, traversed by 

 mineral veins, and presenting that semi-metamorphic structure so 

 commonly seen near the granite ; and some of the great mineral 

 masses, instead of rising towards the steep granitic hills, appear to 



* Especially the following: — "On the Physical Structure of Devonshire," by Prof. 

 Sedgvsick and R. I. Murchison, Trans. Geol. Soc. N. S. vol. v. p. 633. " Notes on 

 the Age of the Limestones of South Devonshire," by W. Lonsdale, op. cit. vol, v. 

 p. 72L This memoir contains an elaborate and critical enumeration of papers and 

 works referring to the subject. •' On the Geology of the South-east of Devonshire," 

 by R. A. C. Austen, op. cit. vol. vi. p. 433. See also the Transactions of the Royal 

 Geological Society of Cornwall. 



t Report on the Geology of Cornwall, Devon, and West Somerset. 8vo, London, 

 1839 ; and Figures and Descriptions of the Palaeozoic Fossils of Cornwall, Devon, 

 and West Somerset. 8vo, London, 1841. 



X See Trans. Cambridge Phil. Soc. vol. i. p. 108. 



