Geology and Geography. 341 



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 term, 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 

 Silurian rocks. He alluded to the difficulty of making geological 

 maps ; these must be constantly modified, according to the extent 

 of investigation : errors of omission must be committed by every 

 pioneer in geology, which can only be corrected by the researches 

 of succeeding 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- 

 talliferous 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 favourable 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 

 granitic and the slate systems. In support of this view, he in- 

 stanced more particularly the mines in the vicinity of Marazion, 

 where the lodes or mineral veins traverse lines of elvans obliquely, 

 and where very rich bunches of ore have been obtained at such junc- 

 tions. Indeed the miners of that part of the country are perfectly 

 aware of the value of these junctions, and carry their work on as 

 much as possible within their favourable influence. The author direct- 

 ed the attention of the Section to the fact, that all the great mines of 

 Cornwall are situated amid the above conditions, and to the ad- 

 vantages which geology could thus confer upon the community, by 



