XC PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [vol. Ixxiil, 



Menelez granites are situated on the prolongation of the axis o£ 

 Bray, the Dartmoor granite on the Purbeck axis, and the Lundy 

 granite on the axis of North Devon. Under such mechanical con- 

 ditions intrusion did not assume the most boldly transgressive 

 habit. The form of any individual mass, in so far as it is revealed, 

 j^resents a gently domed upper surface, though with steeply sloping 

 sides. 



The main Hercynian zone of plutonic intrusion, extending from 

 Britanny eastwards into Central Em-ope, presents a Avide diversity 

 of calcic rock-types, ranging, for example, in the Harz district 

 itself from peridotite to granite ; but only the latest and most acid 

 differentiate from the primitive magma penetrated into the northern 

 fringe of tlie region, and is represented by the Cornish granites, rich 

 in alkali-felspars, and containing abundant light as well as dark 

 mica. The gaseous constituents, in which this granitic magma was 

 peculiarly rich, were eventually driven farther still, and seem to 

 have permeated the country over a wide extent. To pneumatolytic 

 operations connected with this invasion we owe the mineral wealth, 

 not only of Cornwall, but probably, as Dr. A. M. Finlayson has 

 maintained, of other mining districts much farther north. 



The observed pneumatolytic effects in Cornwall were not imme- 

 diately connected with the intrusion of the granite, but belonged 

 rather to the succeeding dyke-phase. As is well known, the fissures 

 occupied by the principal lodes follow the general direction of the 

 strike, while -the 'cross-courses' intersect them at right angles, and 

 often shift them. The two sets of fractures and displacement 

 clearl}^ represent successive stages in the relief of crustal stress and 

 the establishment of equilibrium. Of the two complementar}^ 

 groups of dykes the quartz -porphyries, often themselves moditied 

 b}"^ pneumatolysis, have the same roughly east-and-west bearing as 

 the principal lodes, while the lamprophyres tend to run north and 

 south like the ' cross-courses.' 



Owing to the incompleteness of the Hercynian record in Britain, 

 the next manifestation of igneous action, coming after a long hiatus, 

 appears as an isolated episode. The evidence of it is seen in the 

 lower part of the New Red Sandstone succession, which rests with 

 discordance upon the folded Devonian and Carboniferous strata. 

 Whether Permian or Triassic, a question not yet finally resolved, 

 these younger strata belong to a time when folding had wholly 

 ceased, and a new movement had set in, which was of the nature of 

 a local subsidence. As seen in the Exeter district, the volcanic 



