128 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [May 1 894, 



porphyries into the ' felspathic traps.' The presence of quartz- 

 inclusions misled De la Beche into terming these rocks ' quartziferous 

 porphyries.' 



It would hardly be possible to broach the subject of Devonian 

 geology without some allusion to the Dartmoor granite. Why the 

 igneous origin of this particular granite should have been attacked 

 any more than that of any other is, perhaps, difficult to explain. 

 It is bigger and more accessible than most of our English granite- 

 masses, and having become, as it were, a stock subject, is exhibited 

 from time to time by the demonstrators in different attitudes to 

 please the fancy of various audiences. The latest difficulty, it would 

 seem, arises from the supposed want of a satisfactory explanation 

 of the structural relations of the granite to the surrounding rocks. 

 Thus Mr. Ussher has recently announced that " from the relations of 

 the stratified rocks to the granites of Devon and Cornwall there is 

 no obtainable evidence as to the upheaval of the latter." * Con- 

 sequently the view is advanced that the granite of Dartmoor 

 resulted from the metamorphism of pre-existing rocks which had, 

 in a rigid state, exercised an obstructive influence on the north-and- 

 south movements, and had thereby produced great mechanical 

 effects on the surrounding strata. 



These views of Mr. Ussher were criticized by General M c Mahon, 

 in a paper entitled ' Notes on Dartmoor,' read before the Society 

 towards the close of last session. The Author gave some of the 

 results of a visit to the western borders of Dartmoor, detailing 

 certain cases of eruptive granite-veins intruding into the Culm- 

 measures in the immediate vicinity of the main mass of granite. 

 The latter, in the locality described, is porphyritic down to its 

 boundary, and the veins are also porphyritic. All the circumstances 

 led the Author to believe that these veins are real apophyses from 

 the main mass, and that the view adopted by De la Beche regarding 

 the origin of the Dartmoor granite is the true one. He further 

 commented on the improbability that a tremendous squeeze, 

 sufficient to fuse such a mass of pre-Devonian rock into granite, 

 should have left the Culm-measures, outside the zone of marginal 

 contact-metamorphism, almost untouched. It may be that Mr. 

 Ussher's views on this subject have been somewhat misunderstood, 

 and an attempt was made to show that this was so. But whatever 

 they may be, there is such a touch of vagueness about 'pre-Devonian' 

 rock that we may hope for a further revelation on this score. 



1 Geol. Mag. 1892, p. 467. 



