4 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV. 5, 



Bearing these three groups in mind (for they were previously 

 made out by Sir R. I. Murchison and myself), I entered Cornwall 

 in the autumn of 1836, and traced what I have called the Plymouth 

 Group to Looe, Polperro, and Fowey, and thence round to St. Austell 

 Bay ; and I collected fossils from all the places above-named. Nothing 

 which I have seen or learnt during the past summer, has changed my 

 views respecting the age of the rocks spread along this portion of the 

 coast of Cornwall ; of course excepting the old Cambrian rocks (above 

 alluded to) which run down to the headlands called the Dodman and 

 Nare Head. 



I did not during the past summer examine the coast from Veryan 

 to Falmouth Bay ; but I persevere in the opinion I formed in 1836, 

 that all the slate-rocks S.W. of Gerrans Bay are nearly on the parallel 

 of the Plymouth Group, and therefore true Devonian rocks ; and to 

 the same group I refer all the slate-rocks to the west of Falmouth 

 Bay, between the granite and the great plateau of the Lizard. This 

 may, however, be thought a somewhat rash conclusion, as I have not 

 carefully examined the sections through the slate-rocks immediately 

 north of the Lizard plateau since the year 1819. 



While following the fossiliferous Plymouth group along the south- 

 eastern coast of Cornwall in 1836, I had no assistance from any pub- 

 lished work ; and in this part of my task I preceded Sir H. De la 

 Beche ; but while terminating my tour along the north-western coast, 

 I derived the greatest assistance from the notes and sections which 

 he kindly communicated to me, thereby enabling me in a few days 

 to visit the best fossil-bearing rocks, and to gain a knowledge of the 

 country which might have cost me several months had I laboured 

 without his help. All the rocks of this north-western coast from St. 

 Ives Bay to a headland considerably to the north of Cligga Point, 

 I then supposed to belong to the Middle Devonian Group {Dart- 

 mouth slate) ; and so far as I made out, were without fossils. 



But, guided by Sir H. De la Beche' s notes, I found a highly fos- 

 siliferous group extending from New Quay to Mawgan Porth and Pad- 

 stow, and thence to Tintagel ; and not merely ranging near the coast, 

 but running far up the interior of the country, until in some instances 

 it approached a central boss of granite, and became metamorphic. 

 Thus, for example, in following a section a few miles south-east of 

 Tintagel, I found the rocks passing into a highly crystalline chiastolite- 

 slate ; yet among them were some clear impressions of the well-known 

 long-winged Spirifers of Tintagel. Beyond Tintagel, I found the 

 same group wrapped round the northern end of the great protruding 

 boss of granite, and thence prolonged, with a great change of strike, 

 into the Petherwin Group above noticed. 



Hence it followed, that although a line drawn through the great 

 central bosses of granite might, in a certain sense, be called the mineral 

 axis of Cornwall, yet it by no means represented the range of the oldest 

 slates of the county ; for the great zone of slates on the S.E. coast 

 was apparently on the strike of the old Plymouth Group ; while the 

 fossiliferous zone on the N.W. coast, last described, was the westward 

 prolongation of the Petherwin, or newest Devonian Group. 



