1868.] HOLL — SOUTH dp.von and east coenwall. 415 



The country to the west of the Tamer is thrown down by the par- 

 tially metalliferous faults of Calstock and Colete. A fault whicli 

 ranges east and west by New Bridge, on the Notter, with upcast on 

 the south, repeats some of the rocks between it and Callington ; but 

 the dip is to the south, and higher beds come in about Pillaton 

 Down and St. Mellion, south of which a second nearly east and west 

 fault throws down the country, and with it a patch of Culm-mea- 

 sures, at Pentre Cross. In a similar manner faults have brought 

 down the area around Liukinghorn and South Hill ; but lower beds 

 occur on the west of the Notter, between it and the granite at Ca- 

 radon. 



The elvans, which are associated with these lower slates as we ap- 

 proach either of the great granitic masses of Dartmoor or the Camel- 

 ford Hills, although sometimes parallel to the strike, have no relation 

 to the plane of the bedding. Some of them may have been contempo- 

 raneous with the outburst of the granite, filling in fissures made 

 by the same disturbing cause, as in the vicinity of St. Neot's and 

 Blisland ; but others, as near Redruth and Penryn, are seen to tra- 

 verse the granite as well as the adjacent rocks, and must therefore 

 be somewhat more recent. It is othermse, however, with the volcanic 

 rocks which are associated with the slates, and which occur more 

 or less abundantly on certain horizons. They lie in the plane of 

 the bedding, and were contemporaneous with the rocks among which 

 they occur. Their structure is often schistose or vesicular ; and 

 many of them are rich in lime, which in the vesicular varieties 

 has been infiltrated into the cavities. With this ash, whether schis- 

 tose or vesicular, there is often intermingled more or less fused 

 rock, in such a manner that it is very difficult, as observed by Sir 

 Henry De la Eeche, " to see where the one variety of igneous pro- 

 duct ends, and the other commences " *. It is seldom that even 

 the compact portions have produced any very obvious effects upon 

 the subjacent slates. 



As the rocks between Hingstown Down and the southern edge 

 of the Culm-measures range by North Hill towards Alternan, they 

 dip away from the granite at angles which do not appear to ex- 

 ceed 10°, and are often less ; and in doing so they pass under the 

 Culm-measures which extend by Coades Green to within a mile 

 of the Penpont "Waters, capping the higher grounds as they rise 

 to the north-west. These lower rocks form a belt of country consist- 

 ing of slates and ash-beds, which strike north-west and south-east, 

 the angle of dip varying from 5° to 10° or 15°. At Holloway Cross 

 and Trewen, westward of the line of fault, near Kneller's House, 

 these ash-beds pass under overlying slates, which dip to the north- 

 east with the same low undulating dip (under 15°), and include the 

 limestone and fossiliferous slates of Trenalt and Tall Petherwin ; 

 and as the ground rises some 300 feet or so, we come rapidly upon 

 higher beds, which are finally overlain by the Culm-measures. 

 At Treguddick Mill some of the slates which underlie the ash are 

 brought up by a short, sharp anticlinal axis, which throws the volca- 

 * Mem. Geol. Survey, vol. i. p. 82. 



VOL. XXIV. — PART I. 2 G 



