GEOLOGY 



renewed igneous action on a colossal scale broke out in the Tertiary 

 period, producing a new set of fissures of N.N.W. trend. It has left 

 imposing relics in the British Isles, along the line of the Inner Hebrides 

 (Skye, Mull, Rum, etc.) and in the north-east of Ireland, while along 

 its Atlantic prolongation remnants of volcanic activity are still manifested. 

 In Cornwall no igneous rocks connected with this epoch are known to 

 occur, unless we except the phonolite of the Wolf Rock, which may 

 probably be referred to it. But the fissures in connection with the line of 

 disruption extend to regions far removed from the volcanic centres, and 

 we have little hesitation in assigning to that Tertiary epoch the extensive 

 system of fractures known in Cornwall as cross-courses. 



The elvan dykes which have been already incidentally alluded to 

 make a conspicuous feature in Cornish geology. Related to the granite 

 by the closest ties of chemical affinity, their difference in mineral com- 

 position is mainly one of degree of crystallization. Instead of being 

 restricted to the plutonic phase, involving such slow cooling that the mass 

 is entirely built up of visible crystals, the elvans represent a type of intru- 

 sion in which the contents of the subterranean reservoirs have been 

 injected into a series of fissures, forming wall-like masses, the cooling of 

 which has not been sufficiently protracted to admit of so coarse a type of 

 crystallization. As some of them pierce the granite their intrusion must 

 in part at least be referred to a period subsequent to its consolidation, 

 and from their intimate petrological relationship we may confidently 

 infer that both types of intrusion have been derived from a common 

 source of supply. 



After their mode of occurrence the most essential particular in 

 which the elvans differ from the granite is the occurrence of a base or 

 matrix in which individual minerals are porphyritically embedded. 

 These minerals are precisely similar to those of the granite. They 

 exhibit however more perfect crystalline shape, and this is particularly 

 illustrated by the quartz crystals. This mineral, instead of forming as 

 in granite irregular shapes among earlier developed minerals, has been 

 free to build up its own type of crystal, viz. a short prism bounded by 

 terminal pyramids. These quartz crystals have often rounded angles 

 which in extreme cases result in their appearance as bleb-like patches 

 without crystalline form. 



Not only are there all degrees in the texture of the matrix, but the 

 variation is equally wide in the porphyritic constituents both as regards 

 individual size, number, and the nature of the mineral which is porphy- 

 ritic. In some cases the porphyritic felspars are of large size and have 

 been formed in the plutonic magmas from which the elvans have been 

 derived ; and there is necessarily every gradation between crystallization 

 of the dyke and plutonic phase, the growth of many crystals having 

 been continuous in both of these conditions. 



Although identical minerals occur in both elvan and granite, mica 



