510 Alex Somervail — Origin of Dartmoor Granite. 



extraordinary degree. There is likewise a cleavage structure more 

 or less developed throughout these rocks. With regard to the con- 

 glomerate series, the very reverse of all this is the case, and as a rule 

 they repose at low angles and seem to have suffered little disturbance 

 in comparison with the older members of the Culm. 



The Lower Culm system has lately received close attention from 

 Messrs. Hinde & Fox in their valuable memoir recently published 

 in the Quart. Journ. Geol. Soc.^ The Eadiolarian cherts and 

 associated beds there described are conclusively shown to have 

 been deposited in a sea of great depth, far removed from the 

 detrital washings of the land ; yet there is the clearest of evidence 

 that this deep-sea bottom was elevated by earth-crust-movements, 

 and even ultimately eroded and wasted, to supply the materials 

 which abundantly occur in the conglomerates just referred to. 

 These conglomerates, in all the various localities where they occur, 

 contain abundant fragments of the Eadiolarian cherts, besides those 

 of the other associated rocks of the Lower Culm series. 



It has been suggested that the fragments of the Eadiolarian cherts 

 may have been brought up to the surface by volcanic explosions. 

 These fragments, however, are as a rule well rounded and water- 

 Avorn, and they are not accompanied by any true tuff-like matter in 

 the conglomerates. The only intelligible explanation of the frag- 

 ments of the cherts in the contents of the conglomerates is the long 

 interval separating the former from the latter ; the granting of 

 sufficient time to account for the phenomena of elevation, waste, and 

 reconstruction. 



It is most interesting at this point, to note that during the 

 formation and elevation of the Lower Culm there are the clearest 

 proofs of contemporaneous volcanic action. Belonging strictly to 

 this period, there are in the immediate localities concerned numerous 

 examples of eruptive, explosive, and effusive volcanic products, 

 which might have extended over a long period previous to the 

 formation of the conglomerate series. These volcanic outbursts had 

 apparently entirely ceased long before the formation of the con- 

 glomerates began to be deposited. It is to the latter portion of this 

 interval — sufficiently long — that the writer would refer the eruption 

 of the Dartmoor granite. 



On studying with attention the geological map of the area, it 

 will be perceived that during the early or Lower Culm period the 

 central area of Dartmoor had been long weakened by the extensive 

 volcanic action previously referred to. Yolcanic rocks, principally 

 eruptives, are thickly clustered together on each side of what 

 is now the granite — as at Tavistock on the west and in the 

 Teign Valley district on the east side, — running along a line of old 

 pre-granitic fissures. All these volcanic rocks are decidedly Lower 

 Culm in age, but older than the associated granite, as the latter 

 truncates and indurates them. 



The central portion of the area of Dartmoor, now occupied 

 by the granite, was clearl}'^ a previous centre of volcanic activity 



1 Vol. li (1895), p. 609. 



