Geology of the South-east of Devonshire. 



459 



be no question as to the geological position and equivalents of the arenaceous and 

 conglomerate beds of Rydon and Ugbrooke, as over the whole of central Devon, 

 and throughout the narrow baud south of Dartmoor, the argillo-carbonaceous cha- 

 racter of the culm series is remarkably uniform, but the mineralogical resemblance 

 is not so great as to warrant their being considered portions of one formation. The 

 central trough of Devon is believed by Prof. Sedgwick and Mr. Murchison* to be of 

 the same age as the South Wales coal-field, on the evidence of mineral character. 

 The inquiries of Mr. Lonsdale established the place which the South Devon lime- 

 stones should hold in the scale of British formations ; by means of these alone can 

 be determined the age of the carbonaceous system. If along the line from Dean 

 Prior to the Haldons the culm measures pass beneath the slates and limestones 

 to the south, they must have a much greater antiquity than has been assigned to 

 them ; but if, like the beds of Ugbrooke and Rydon, they are superior to them, 

 they may, as suggested by Professor Sedgwick, be the equivalents of the coal- 

 measures. In the north of Devon, along a line extending from Fremington to 

 Hockley Bridge, north of Bampton, the carbonaceous rocks pass downwards into 

 the older system, which composes the range of Exmoor. At Boscastle, on the 

 opposite side of the trough, the coast-section exhibits a similar passage. From this 

 point inland, the line of separation is best marked by the distinct vegetation and 

 barren tracts of the carbonaceous grit beds ; but no good section is to be found of 

 the relative positions of the two deposits till we reach Landue Mill, near which a 

 road-cutting in 1837 exposed the following section, where the carbonaceous beds, 



Fig. 11. 



Carbonaceous beds {a) resting unconformably on older strata (6) near Landue Mill. 



of inconsiderable thickness, rest unconformably upon slate and shales, very rich in 

 fossils : from this point again the junction-line presents no section, though it can 

 be easily traced upon the surface, until it abuts against the granite of Dartmoor, 

 north-east of Tavistock. 



Crossing the granitic tract of Dartmoor, and in the direction of the line of bear- 

 ing of the carbonaceous rocks from the coast, we meet them again about Holne, 

 perfectly identical in character and appearance with the beds about Tavistock ; but 

 on this side also there is the same difficulty as to sections. On the road from Buck- 

 fastleigh, south of Ashburton, to Holne, and in the ascent to Hembury Castle, the 



* See vol. iv. p. 669 et seq. 



