354 Proceedings of the British Association. 



exceptions there must be ; still it must be allowed, that organic life 

 must have a constant relation to the state of the actual surface. He 

 came to the conclusion, that the Devon district would not offer any 

 anomaly in geological arrangement, but that it would correspond in 

 arrangement with the other parts of the country, and that a fruitful 

 source of error is the hitherto vague term Greywacke, which has 

 been applied indiscriminately to a great variety of rocks, so as to 

 include many of different ages throughout this county. — Dr. Buck- 

 land congratulated the meeting on the difference of opinion among 

 the geologists present, such a difference producing discussion, which 

 was the sure means of arriving at truth. He considered, that the 

 true solution of the question at issue would be in the middle course ; 

 that, no doubt, it could not be easily granted, that the series under 

 consideration was carboniferous, when no true coal was contained 

 in it ; but, were we to adopt the new terra, culmiferous, we should 

 get rid of the difficulty. This culmiferous series he regarded as the 

 lowest portion of the coal formation, and as resting upon the Siluri- 

 an rocks. He alluded to the difficulty of making geological maps ; 

 these must be constantly modified, according to the extent of inves- 

 tigation : errors of omission must be committed by every pioneer in 

 geology, which can be corrected only by the researches of succeed- 

 ing observers. 



After the discussion was closed, Mr. De la Beche exhibited a 

 part of the Ordnance Geological Map of Devon, and such parts of 

 that of Cornwall as have been finished ; and pointed out the gene- 

 ral parallelism of certain great lines of dislocation both in the me- 

 talHferous and non-metalliferous districts. He stated that he con- 

 sidered such lines to have been produced at the same geological 

 epoch, and attributed the fact of the occurrence of the ores of use- 

 ful metals in some situations and not in others, to conditions which 

 were to be found in the one and not in the other. The conditions 

 most favorable to the occurrence of the tin and copper ores of 

 Cornwall and Devon, are the proximity to the junction lines of the 

 granitic and slate systems of those counties ; the intermixture of 

 granitic and porphyritic dykes with the slates, or with the masses 

 of granite ; the occurrence of great lines of dislocation traversing 

 the lodes or mineral veins, and termed cross courses, &c. The au- 

 thor pointed out numerous other conditions, and then noticed the 

 beneficial effects of the proximity of the granitic or porphyritic 

 dykes provincially termed elvans, and which alike traverse the 



