1851.] SEDGWICK SLATE ROCKS OF DEVON AND CORNWALL. 17 



slates of Devonshire. Again, in a quarry by the road-side at Common 

 Wood, a little further north, fossils are extremely abundant. Perhaps 

 no large portion of these Devonian slates is entirely devoid of fossils ; 

 but they abound most on certain lines along which are found streaks 

 or masses of limestone ; which, I may remark, seldom burns into white 

 lime, and appears in several cases to be magnesian. Many of these 

 junks of limestone have been laid down by Sir H. De la Beche, and 

 many more may hereafter be discovered. Our friend Mr. Box in- 

 formed us that he had found masses of limestones, similar to those 

 above noticed, at the following places between the Looe River and the 

 Hamoaze, viz. Shuttleback, Common Wood, Treloy, Tremain, and 

 St. Germains. 



All the fossils from this neighbourhood in the collection of Mr. 

 Hicks are the counterpart of the series from Plymouth ; and nearly 

 the same remark applies to the collection of Mr. Giles at Liskeard. 

 A few species which are new or have been imperfectly described will 

 be noticed, as before stated, by Prof. M'Coy in the next "Fasci- 

 culus of the Cambridge Palaeozoic Fossils." 



That the calcareous slates immediately north of Looe are a part of 

 the Plymouth Group, and nearly on the strike of the Plymouth 

 limestone, cannot admit of doubt. The beds near Liskeard are con- 

 siderably lower in the series of deposits ; and it was not without 

 hesitation that I designated them by the distinctive name of Liskeard 

 Group (see above, p. 5). They are certainly neither Silurian nor 

 Cambrian, and may be regarded as a lower subdivision of the great 

 Plymouth Group. 



I wish the Society to bear in mind that some conclusions arrived 

 at in this paper are hypothetical, and that the opinions I have stated 

 repecting the age and distribution of the groups in the south-western 

 parts of Cornwall are chiefly based on observations made in the sum- 

 mer of 1836, or during previous years. To the south-west of St. 

 Austell Bay, hardly any Devonian fossils have yet been discovered. 

 But in Devonshire the middle or Dartmouth Group is almost devoid 

 of fossils ; and if (as I think is true) this group be prolonged into 

 the south-western extremity of Cornwall, the fact will at once account 

 for the almost entire absence of fossils ; not to mention the highly 

 altered character of the slates in the great mining districts of Western 

 Cornwall. Again, even in the Plymouth Group the quantity of cal- 

 careous matter is less developed in Cornwall than it is in Devonshire ; 

 and this fact explains the less abundance, and less perfect preservation, 

 of the fossils in the range of this group towards the south-west. 



By way of conclusion we may, I think, rationalize and explain 

 the actual collocation of the great mineral masses in Devonshire 

 and Cornwall, without being accused of indulging in an unreasonable 

 spirit of hypothesis. 



1 . What was the condition, before the period of the Old Red Sand- 

 stone, of the physical region now occupied by the sea, both on the 

 north and south sides of Devonshire and Cornwall, it would perhaps be 

 idle to conjecture ; but we have a positive proof that some old rocks 

 did exist along what is now a part of the S.E. coast of Cornwall : and 



VOL. VIII. — PART I. C 



