14 



THE GEOLOGIST. 



his paper, however, in the ' Memoirs of the Geological Surrey of 

 Great Britain,' the same author speaks of them in a somewhat more 

 decided tone as follows : — '•' Among the pebbles of the new red sand- 

 stone conglomerate nearest to Dartmoor, granite from it is scarce, 

 some varieties having been only found on the north, by Tawton and 

 Sampford Courtney."* 



Geologists, however, have by no means all concurred in this 

 opinion respecting the so-called " granite pebbles." Thus we find 

 Mr. Godwin-Austen — and probably few geologists are so intimately 

 acquainted with the district — in his paper already quoted, expressing 

 himself thus : — " In the study of detritic formations the identification 

 of mineral fragments becomes 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 granite 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 Hal dons 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 dis- 

 trict ; but here again, although fragments of all the older rocks occur 

 in the conglomerate beds at the base of the greensand, granitic peb- 

 bles are altogether wanting ; nor do we meet with them until we 

 arrive, in ascending order, at those superficial accumulations 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." t 



Sir Charles Lyell says, " The granite of Cornwall is probably of 

 the same date," (as that of Dartmoor) "and therefore as modern as 

 the carboniferous strata, if not much newer ."J This expression is 

 evidently very guardedly indefinite, much more so probably than 

 would have been the case had that distinguished author been satisfied 

 that the pebbles in question were really granite. 



Happening, a few years ago, to be at North Tawton, I mentioned 

 the subject to Mr. William Vicary, then resident there. He imme- 

 diately took me to the conglomerate, and in a few minutes extracted 

 two or three pebbles, which we both regarded as certainly of Dart- 

 moor derivation. I am not sure that either of us would have con- 

 tended that they were true granite, if by that term we are to under- 

 stand a mass made up solely of distinct crystals of felspar, quartz, 

 and mica ; nor, thus defined, would any one be prepared to call every - 



* Memoirs Geol. Survey, vol. i. p. 228. 



f Geol. Trans., 2nd series, vol. vi. part ii. p. 4?8. 



% ' Manual of Elementary Geology,' 5th ed. p. 587. 



