478 Mr. Austen on the 



extremity of an axis of crystalline rocks, which, graduallydecreasing in elevation and 

 superficial importance, extends westward as far as the Scilly Islands. The general 

 direction of the axis is about south-west to north-east, and we should naturally 

 expect to find, in the undulations of the surface of these erupted masses, a general 

 parallelism in conformity with this direction ; such, however, is not the case, the 

 whole surface showing north and south lines of hills, valleys and water-courses ; a 

 circumstance which suggests that the present outline and configuration of Dart- 

 moor are the result of disturbing causes which have acted subsequently to its 

 projection. 



The Dartmoor mass of granite may be estimated as occupying about 200 square 

 miles of surface • at Cawsand it attains an elevation of 2000 feet, but near Bovey 

 it is exposed in the vicinity of secondary, and even much more recent formations 

 httle above the level of the sea ; yet over the whole of this area, there is not 

 the slightest appearance that any stratified formations were deposited; its deep 

 valleys would certainly have presented some few traces of them, even allowing for 

 extraordinary denudation, had the present outline of surface existed prior to only 

 the cretaceous series. 



In the study of detritic formations, the identification of mineral fragments be- 

 comes of almost equal importance with that of organic remains ; in the present 

 instance they afford only negative testimony ; but as, from the absence of volcanic 

 fragments in the conglomerate eocene beds of central France, we infer the relative 

 age of the volcanic outbursts of that region, so, as no granitic pebbles have been 

 found among the various materials of which the new red conglomerate is composed, 

 we may conclude, that at the period of its accumulation the granite of Dartmoor 

 could not have been exposed, particularly when we bear in mind that the two 

 formations are at present separated only by the valley of the Teign. 



The beds of the greensand of the Haldons and the Bovey valley, in the thin 

 mica, sharp quartzose crystals and seams of felspar clay, suggest that they may 

 have resulted from a decomposed granitic district ; but here again, although frag- 

 ments of all the older rocks occur in the conglomerate beds at the base of the 

 greensand, granitic pebbles are altogether wanting ; nor do we meet with them 

 until we arrive, in ascending order, at those superficial accumulations (p. 447) 

 which cap the Haldons, when they appear in great abundance, associated with 

 rolled flints, and worn like marine shingle. Possibly, then, the rise of the granite 

 of Dartmoor in its present form may belong to a period comparatively recent, nor 

 would such an inference be at variance with much that is already known : the 

 granite of Arran, according to Professor Sedgwick and Mr. Murchison*, is more 



* Geol. Trans., 2nd Series, vol, ii. pi. 3. p. 353, 1828 ; vol. iii. part 1. p. 34, 1829. 



