1851.] SEDGWICK— SLATE ROCKS OF DEVON AND CORNWALL. 5 



Had any one inquired what was the date of the slates in the cen- 

 tral portions of the county, hetween one boss of granite and another, 

 I should have replied, that many of these rocks were too much mine- 

 ralized and penetrated by veins, to give any certain indication of 

 their age ; but that considered collectively, they were nearly on a 

 parallel with the Middle Devonian or Dartmouth Group. 



Such were the general results of my tour in the autumn of 1836. 

 The slate-rocks of Cornwall and Devon, whatever might be their age, 

 formed one connected and inseparable system ; and this conclusion 

 was in a subsequent year embodied in a single sentence, in a paper on 

 the " Devonian System," published by Sir R. I. Murchison and my- 

 self, wherein we affirmed that all or nearly all the slate-rocks of 

 Cornwall were of the Devonian age, and therefore on the parallel of 

 the Old Red Sandstone. Why, the reader may ask, was the saving 

 clause introduced into this conclusion ? I must honestly state that 

 I did not at that time anticipate the probability of finding, on the 

 south-eastern coast of Cornwall, any older rocks than those of the 

 Plymouth Group ; but I did believe it probable that some older 

 rocks might be brought to the surface near the central portion of 

 the county, by the elevation of the great bosses of granite ; and on 

 that account the saving clause (just alluded to) was introduced into 

 our Devonian Paper. 



After revising the evidence during the past summer, I now think 

 that the south-eastern promontories of Cornwall (such as the Dod- 

 man, Nare Head, and St. Anthony's Point) are exactly the spots 

 where we ought to look for rocks older than the Plymouth Group ; 

 and that we have at present no good reason for expecting any great 

 expansion of older rocks in the central parts of Cornwall. For this 

 change of opinion I shall give some reasons in the sequel. 



If the slate-rocks spread out along the N.W. coast of Cornwall be- 

 long (as above stated) to the Dartmouth and the Petherwin Groups, 

 we cannot, it may be said, expect any great development, on the 

 north side of the granitic axis, of rocks as old as, or older than the 

 Plymouth Group. While making this remark, I wish to keep in 

 reserve one possible exception. There is a great spread of slate-rocks 

 between Bodmin and the mouth of the Padstow River, and the sec- 

 tions are complicated. The peculiarity of these sections was pointed 

 out to me by Sir H. De la Beche in 1836, and it is I believe on his 

 information that Sir R. I. Murchison has conjectured, that some old 

 Silurian rocks 7nay have been elevated into the sterile ridges called 

 Pydar Downs, which rise a few miles to the south of the Padstow 

 River, nearly on a line drawn from Bodmin to Mawgan Porth. Now 

 it is a matter of fact that a very old Devonian group with many fos- 

 sils, collected and partly described by Mr. Giles* and Mr. Peach f, 

 exists in the neighbourhood of Liskeard. It might be called the 

 Lower Plymouth Group ; or perhaps, more conveniently, and to 

 avoid ambiguity, the Liskeard Group. This Liskeard Group is pro- 

 longed to the neighbourhood of Bodmin ; but in many places in that 



* Trans. R. Geol. Soc. Cornwall, 1849, p. 93, and 1850, p. 155. 

 t Loc. cit. 1849, p. 103. 



