418 PROCEEDr^GS OF THE GEOLOGICAL SOCIETY. [April 22, 



Trewen, (fcc. is at a short distance only above the uppermost belt of 

 volcanic rock, or that which extends from Trecarrel Bridge, by 

 Lewannick, to Laneast. The overl3'ing slates attain their greatest 

 thickness in this vicinity, north of Holloway Cross and Trenalt. 



Three miles to the north of South Petherwin, these lower rocks 

 are again brought up to the surface by a narrow anticlinal axis at 

 Yeolm Bridge, which extends westward by Underwood Farm, These 

 slates are sparingly fossiliferous ; but the few species that occur at 

 this locality are all South Pether\vin forms, with the exception of 

 Sanguinolaria ellijDtica and BeJleroplion hiuJcus, of which latter, I 

 believe, only a single very imperfect specimen has been met with, and 

 this may have come from the overlying Culm-measures*. 



The attempt to ascertain the relative position of the several beds of 

 ash is attended with considerable difficulty, owing to the paucity of 

 good exposures and the uncertainty regarding the true dip of the beds. 

 The belt of volcanic ash which extends from Tregue Cross by Penhale 

 to the south of Davidstow, appears to be the same as that which ranges 

 up from North Hill to the west of Alternan, broken off and carried 

 further to the northward by the granite of Brey Down ; but the 

 strip of Culm-measures which runs up from Coades Green to as far 

 as Trebant, obscures in great part the older rocks which dip away 

 from this igneous belt to the north-east; and tbe slates which fui'ther 

 on rise up from beneath these Culm-measures, and range by Alternan 

 and Tregue Cross, are altered by the granite. Crossing the country, 

 however, from Trevillans gate by St. Clether, we have apparently a 

 clear upward succession of strata, all the way to the Carbonaceous 

 rocks of Coose Moor. The volcanic belt of St. Clether consists of 

 an upj)er and a lower band separated by beds of more schistose ash 

 (along which the river runs), and has a north-easterly dip, at an angle 

 of about 20°. From this belt, a little further to the east, at Laneast, 

 we have again apparently an upward series to the Culm -measures 

 of Laneast Down, all the dips obtainable being north-easterly. The 

 intermediate belt of igneous rock, or that south of Laneast, is highly 

 calcareous, and has some beds of limestone at the top — a character 

 which it possesses in common with the volcanic rocks of Titch Beacon, 

 which strike up to an impure limestone at Grills, near Lesnewth, 

 and appear to be on the same horizon. 



Fig. 4. — Section from M'mwonnet hy Pollaphant to the Granite 

 near Neiuton. 



Camelford 

 PoUa- Inny Turnpike 



Newton. phant. Eiver. Eoad. 



s. I I ; i N. 



c a d d d a d e 



a. Devonian Slates, b. CuLn-measures. c. Granite, d. Volcanic Rocks. 

 e. Limestone. 



* PhilL Pal. Foss. p. 139, pi. 58. f. 203. 



