442 PEOCEEDZN'GS OF THE GEOLOGICAL SOCIETY. [April 22, 



as regards both the range of limestones and the base of the measures, 

 with the similar volcanic rocks of the Brent Tor district. There is 

 more grit, perhaps, on the east side of the Dartmoor granite than on 

 the west, though even this may be questioned ; but in the abundance of 

 chert-beds, and in their general petrological aspect, they are precisely 

 similar. jSTow the base of these lo^-er Culm-measures does not every- 

 where rest on the same part of the underlying Devonian rocks. The 

 beds which underlie them at Penter's Cross and ^"hitechurch Down 

 are considerably lower than those on which they rest at Petherwin and 

 Trewen ; while on the east they lie directly on the denuded surfaces 

 of the Torbay limestones, which are probably more than t^vo thousand 

 feet higher in the series ; and if the Culm-measures of the Ilsington 

 and Holne district are not, as I believe, brought into contact with the 

 older rocks by faults, then they rest on successively lower and lower 

 beds as we pass from Bickington to Skeriton, near Dean Church. Nor 

 has this want of conformability altogether escaped observation ; for it 

 is noticed by Prof. Sedgwick as occurring "near Launceston"*, and by 

 God win- Austen in the jS'ewton Bushell districtf. As already stated, 

 on the west of Dartmoor the great undulations into which the up- 

 thrust of the granite has thrown the beds has affected both systems.; 

 but nevertheless the minor contortions and crumpliugs of the higher 

 series have no relationship to those of the lower, and, moreover, 

 the angles at which the beds of the two formations rest are commonly 

 altogether different. !N"ow this unconformability on the southern side 

 of the Culm trough is so considerable that it throws doubt upon the 

 reality of the apparent regular succession on the north, and leads to 

 the suspicion that the conformability which is there supposed to exist 

 may be more apparent than real. The late Mr. Thomas Weaver 

 did not, in fact, consider the Culm-measures to rest conformably on 

 the underlying rocks ; for he says, " The Wavellite schists and sand- 

 stone (7), and culmiferous shales (8), though apparently in some 

 places in parallel (conformable) position with the Trilobite slates % 

 (6), do, when thoroughly examined upon the line of outcrop in the 

 district, form a break with No. 6, and are unconformable thereto "§. 

 That there is some difficulty in detecting any want of conformability 

 is due to the similarity in appearance of the slaty rocks of the two 

 series and the absence of those interstratified beds of volcanic ash 

 which are so serviceable in enabling us to follow the lines of strike 

 in the south. But if, as I believe, the Culm-measures have been laid 

 down on the denuded surface of the older rocks, then there is a break 

 in the sequence and a lapse of time to be accounted for. 



It is obvious that the true position of the Plymouth and Torbay 

 limestones in the general mass of the South-Devon rocks is a matter 

 of great importance to the correct interjDretation of the structure of 

 the country. Looking at the map, the question might suggest itself 

 whether or not the limestones which range by Bickington and Ash- 

 burton to Dean Prior might not be the same as those of Ogwell, 

 Ipplepen, and Dartington, thrown over a broad antichnal axis of the 



•'^^ Pi-oc. Geol. Soc. vol. ii. p. 681. t L. c. p. 458. X 1. e. the Pilton group. 

 § "Geological Kelations of North Devon," Proc. Geol. Soc. vol. ii. p. 589. 



