Geology of the South-east of Devonshire. 



^77 



Mr. Murchison, at the meeting of the British Association at Bristol in 1836*, that it must have been 

 erupted subsequently to the completion of the carbonaceous deposits, as along the northern slope of 

 Dartmoor, about Oakhampton, beds of that part of the North Devon series had been greatly altered and 

 disturbed by it. The same phaenomena occur on the south. At Higher Alvvay, near Bovey, a branch- 

 ing vein of granite of considerable thickness extends a quarter of a mile from the main mass ; and an- 

 other vein may be seen at Lower Alway. At Ilsington, south-west of Bovey, a small mass protrudes through 

 the carbonaceous shales, and the whole line of contact, along the southern flank of the moor, is an 

 altered rock. In the upper part of the valley of the Dart, in the Holne Chase, numerous instances of 

 granitic veins may be observed in the sides of the roads which have been cut through the woods, and 

 the range of several smaller ones may be traced across the road at Ash, above Spitchwick. 



The observation of Prof. Sedgwick and Mr. Murchison above quoted, as to the age of the Dartmoor 

 granite, applies necessarily to that schorly portion alone which comes in contact with sedimentary de- 

 posits, for the entire mass is not of the same age. Fig. 15. represents a section exposed near Murcheton 



Fig. 15. 



Section near Murcheton. Porphyritic granite (a) intruded among consolidated granite (A). 



and shows the manner in which the usual porphyritic granite has intruded itself among such as had already 

 become compact and jointed, and containing schorl ; and other instances occur in the same neighbour- 

 hood. Again, this porphyritic and micaceous granite is traversed by elvans of a compact, fine-grained 

 stone, presenting no distinct crystallization of any of its constituents, and they have evidently been pro- 

 truded posterior to the consolidation of the rocks in which they occur : good examples may be seen in 

 the neighbourhood of Lustleigh, about six miles to the westward of Chudleigh. The Dartmoor region, 

 like every other composed of plutonic rocks erupted on a great scale, presents fewer facilities for geolo- 

 gical observations than inhabited and more intersected districts, but the facts, here noticed, warrant the 

 conclusion that it contains granite of three distinct ages. 



In Lower Normandy, on the opposite coast of France, where the geological fea- 

 tures present so many points of resemblance with those of the west of England, the 

 granite is an intrusive rock, and is generally supposed to be of no great antiquity; 

 it is also intersected by trap dykes ; in Devonshire, on the contrary, every dyke is 

 cut off by the granite. 



Dartmoor, as a physical region, presents two very distinct features : 1st, the 

 great dome-shaped masses, of which Cawsand is the best illustration ; and 2ndly, 

 the long vertical walls, with lines of ruin and confusion, constituting the wild 

 scenery about Lustleigh, and along the road from Bovey to Moreton. Viewed in 

 connection with the Cornish masses of erupted granite, Dartmoor is the eastern 



* See Athenaeum 1836, p. 612, explanation of Section. 

 VOL. VI. SECOND SERIES. 3 Q 



