1868.] HOLL — SOUTH DEVON AND KAST CORNWALL. 41 1 



The relations between the Culm-measures and the lower rocks 

 north of South Petherwin are not altogether so clear as could be 

 wished. The Carbonaceous rocks dip to the north ; but the under- 

 lying slates are a good deal rolled, and their dip is not easily made 

 out. Following, however, the line of outcrop from the Landlake 

 quarry by St. Lavers to the south, the Culm-measures may be said 

 to arch over the older rocks brought up between South Petherwin 

 and Trekenna (see section fig. 3, p. 417), with minor east and west 

 undulations ; and in the deep synclinal trough of the Inny there are 

 some of the higher beds (the slates south of Beal's Mill), which 

 occupy the axis of the trough, corresponding probably in age with 

 those of the Lyd river. The volcanic rocks, however, are absent, 

 and the chert is for the most part replaced by grits. 



Near Bridestow, the chert-beds of Watergate thin out, and the 

 dark slates to the north and south of them are brought together, 

 and are continued by Sourton and Oakhampton Park round the 

 north side of the Dartmoor granite, which has pushed the Culm- 

 measure beds to the northward, and brought them into vertical or 

 highly inclined positions. It is not necessary, however, for the 

 objects of this communication, to follow these beds further in this 

 direction. I proceed, therefore, to notice briefly the relations existing 

 between the lower Culm-measures and the underlying rocks as they 

 occur on the east of Dartmoor, 



Along the margin of the granite from East Down, near Lustleigh, 

 to Skeriton, south of Holne, the Culm-measures consist of slates 

 and grits with chert-beds very similar to those which occur north of 

 Tavistock. The chert is perhaps not so abundant ; but a siliceous 

 rock, somewhat less flinty, is plentiful between Bickington and East 

 Down ; and volcanic admixtures occur near Ilsington, and become 

 frequent further north between Chudleigh and Dunsford. Near the 

 granite the rocks have become altered by it ; and the resulting meta- 

 morphic rocks resemble those on the west and north-west of Dart- 

 moor, showing their close similarity in mineral character. Near 

 Ilsington they contain plant-remains, and Goniatites have likewise 

 been found in them. 



We may safely assume, I think, these rocks to be the equivalents 

 in part of that portion of the Culm-measures which occurs south of 

 the Lyd river, and to occupy a position at or near the base of the 

 series. They dip, as a whole, away from the granite, to the east and 

 south-east, but are much undulated and contorted, and hence coun- 

 ter dips are frequent. The line between the Carbonaceous rocks 

 and the Devonian slates and limestones emerges from beneath the 

 Bovey beds at Black Pool, and extends by the New Inn southward 

 to Bickington. So far the line appears to be a line of fault, Culm- 

 measure grits and slate very highly inclined to the eastward being 

 brought against the Bickington limestone, dipping south-east at an 

 angle of about 2-5°. This fault appears to run into another short fault, 

 which extends north-west and south-east, and cuts off the Bickington 

 limestone from its continuation with theAshburton mass atLemonford. 

 From the factory north of Lemonford the line, as laid down on the 



