System cannot be of the age of the Old Red Sandstoiie. 129 



fossils, and intimately resembles some of the culm-beds and 

 building slates quarried at St. Thomas, below Launceston. 

 This was first pointed out to me by Mr. Pattison, who con- 

 siders the slate, I believe, to be the same as the Yeolm Bridge 

 flags. This eastern extremity of the Petherwin ridge is, how- 

 ever, distinctly arched over, and undulates on to Bottonnel, 

 and just behind Trebollet Shop, about two miles to the south, 

 where it is abrupted by a protruded axis of Coddon grit and 

 floriferous (see section. No. 3), which ranges continuously in 

 hills and downs, from Inny Foot and Mount Pleasant on 

 the Tamar, to the westof Alternon, between which and Camel- 

 ford it is concealed beneath the wild moors under Brown 

 Willy, &c. 



The abundant alternations seen about Linkinghorne and 

 South Hill are very satisfactorily accounted for by their being 

 in the actual confines of the killas in mass, and the great spur 

 of No. 9, whicii, as hereafter stated, extends uninterruptedly 

 from the culm-field to Stoke Climsland. About a furlong or less 

 to the N.E., the E. and the S.E.of the Landlake lime-quarries, 

 the Coddon grit in its full force is exposed and extends to the 

 Tamar and many miles beyond, so that remembering its 

 re-appearance at Congdon and near Trevasper, on the west, 

 the evidences per se, are here at least quite as good for sup- 

 posing it to be the mineral axis of the arch of killas, as 

 for supposing the Petherwin slates to rise from below^ it. 

 If, however, we examine the cutting at Landue, on the new 

 road from Launceston to Callington, about two miles S.E. of 

 the Landlake quarries, and not more than half a mile due east 

 of the killas in mass, we find it to consist of the lower culm- 

 beds ; there is no true killas there, but the carbonaceous 

 slates appear sometimes to have been neutralized by an ad- 

 mixture of killas matter (composite beds), and they contain 

 thin calcareous plates with several of the Landlake fossils. 

 The unconformity once imagined here, has been nearly all 

 cut out ; in truth, when I first saw it, I recognised it as nothing 

 more than one of the many thousand complicated involutions 

 which everywhere almost, in a greater or less degree, charac- 

 terize the culm rocks, — multiplied unconformities among 

 themselves. 



If, again, we take a north and south traverse from a mile 

 east of Werrington on the north of Launceston, we observe 

 the culm rocks, either No. 8 or 9, to be exposed continuously 

 from the great body of the culm-field to the south of Stoke 

 Climsland, and at Linkinghorne and South Hill, within three 

 miles of Callington, where we observe them reijcaledly alter- 

 7iatin^ with killas, till finally they are altogether overlaid by 



Phil. Mail. S. 3. Vol.20. No. 129. Feb. 1842. K 



