DEVONIAN. 139 



4. CORNWALL. 



The stratigraphical relations of the slaty rocks of Cornwall have 

 yet to be determined with precision. 



Fossil shells were as early as 1817 noted by the Rev. J. J. 

 Conybeare ^ in the slates of Tintagel. These include Spirifera 

 giganha, often distorted by the cleavage.^ Sedgwick, Murchison, 

 De la Beche, and more recently Mr. J. H. Collins, have described 

 the rocks. The greater part of the area is made up of slaty rocks, 

 with the exception of the tracts where granitic and igneous rocks 

 are exposed. These slaty rocks, known under the general name of 

 Killas or Clay-slate, are usually grey or bluish-grey in colour, 

 sometimes green, brown, or violet. Quartz-veins occur here and 

 there. At Penzance and St. Ives hornblendic slates are developed.^ 

 (See p. 46,) 



Mr. Champernowne thinks that the Liskeard group of Sedgwick and Murchison 

 may represent the Middle Devonian beds of Plymouth and Torquay — the limestone 

 itself being absent or feebly represented, and this appears to have Ijeen the view 

 taken by Sedgwick.^ The Liskeard group, however, in part included the Fowey 

 Beds now considered by Mr. Collins to be Silurian. 



The separation of the Cornish rocks has been retarded, as Mr. 

 Collins has pointed out, by the assumption that all the rocks 

 called "Killas" by the miner, are of one age. He has made the 

 following divisions : — 



Devonian and Silurian ? 



Ladock Beds. ) 



Fowey Beds. 



Gorran Haven Beds. I ^ , • , a./:. ,^^\ 



r> a -D J 1 Cambrian (see pp. 00, 75). 



Ponsanooth Beds. ) \ ft- > 1 ji 



Fowey Bids. — This great series of strata, named from Fowey, 

 south-east of St. Austell, is described by Mr. J. H. Collins as 

 comprising dull greyish-brown slates. In some parts, as in several 

 quarries in the parish of St. Veep, these pass into a yellowish or 

 reddish sandstone, containing fossils. Westwards the Fowey beds 

 are cut off by the Hensbarrow granite, but they appear to rest on 

 the older rocks of Mevagissey. They occupy the country round 

 Lostwithiel, Bodmin Road Station, St. Veep and Polruan ; and 

 attain a thickness estimated at 10,000 feet. 



Large numbers of fish-remains were found by Mr. C. W. 

 Peach in 18+1, and subsequently by Mr. Pengelly and others in 

 these beds at Lantivet Bay, Polperro and Looe ; they include 

 Pteraspis, Cephalaspis } Phyllohpis concentriais, Holopiychius, Onchiis, 

 etc., also Brachiopods, Encrinites, Trilobites, Echinoderms, and 



' Trans. G. S. iv. 424. 



* See also J. Phillips, Rep. Brit. Assoc, for 1856, pp. 388, 391. 



^ See J. A. Phillips, Q. J. xxxi. 321 ; T. G. Bonney, Q.J. xxxix. 20. 



* G. Mag. 1881, p. 414 ; Q. J. viii. 3. 



