486 Geological Society. 



this formation, whilst the marly sandstones and conglomerates of 

 Herefordshire are abnormal exceptions in it, we see the reason why 

 their slaty continental equivalents, like the greater part of the South 

 Devon slates, have been referred to the undivided Wernerian forma- 

 tion of grauwarke. 



Mr. De la Beche in his map of Devon and Cornwall, published 

 in 1839, has adopted divisions of the strata, similar to those of Pro- 

 fessor Sedgwick and Mr. Murchison, as to their order of sequence ; 

 applying, provisionally, to the culmiferous rocks the name of Car- 

 bonaceous series, and to the Devonian and Cornish slates the appel- 

 lation of Greywacke. 



We know also on the authority of Mr. De la Beche that tin mines 

 are worked in carbonaceous rocks at Owlescomb near Ashburton, 

 on the east side of the Dartmoor granite, and on its west side at 

 Wheal Jewel near Tavistock. He further informs us that one of 

 the richest tin mines now worked in Cornwall, namely the Charles- 

 town mine, east of St. Austle, is in a fossiliferous rock containing 

 Encrinites and corals, and that the same corals occur also near tin 

 mines at St. Just ; and in the neighbourhood of Liskeard the Rev. 

 D. Williams has found slates which contain vegetable impressions, 

 dipping under other slates which are intersected by lodes of tin and 

 copper. 



From these new facts, we learn that the killas and other slate 

 rocks of Cornwall and the south of Devon do not possess the high 

 antiquity which has till lately been imputed to them ; and that tin 

 occurs, as copper, lead and silver have long been known to do, not 

 only in slate rocks that contain organic remains, but even in the 

 coal formation. 



Mr. Greenough, in the new edition of his map of England, repre- 

 sents nearly the same boundaries and order of succession in Devon 

 and Cornwall as we find in the maps of Mr. De la Beche and Messrs. 

 Sedgwick and Murchison ; but in his memoir connected with the 

 map, adopting the name of Carbonaceous series for the culmife- 

 rous rocks, he substitutes that of Upper killas for the Devonian 

 system of Sedgwick and Murchison, (including under that term 

 the old red sandstone of Herefordshire,) and Lower killas for the 

 slates inferior to the Silurian system, which they have termed Cam- 

 brian. 



Mr. Greenough, in his memoir, also shows by quotations from Dr. 

 MacCulloch, that the undisputed old red sandstone of the north of 

 Scotland exhibits, at intervals, the same great changes of mineral 

 character, that occur in the strata intermediate between the Carbo- 

 naceous and Silurian systems in the west of England and on the 

 borders of Wales; and justly infers the inadequacy of any one term 

 to characterize formations which vary so much in lithological com- 

 position, that at one place they present the condition of a fine- 

 grained silky slate, at another of sandstone, and at a third that of 

 coarse gravel and conglomerate rock. 



Thus, with respect to the slate rocks of Devon, Cornwall and 

 Wales, the difficulties are reduced to those of an unsettled nomen- 



