Page:VCH Cornwall 1.djvu/79

 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