Page:VCH Cornwall 1.djvu/78

 A HISTORY OF CORNWALL The set of major joints which trends approximately E.N.E. corre- sponds to an extensive system of fissures which have been formed in the granites and Paleozoic rocks of Cornwall by the subterranean dis- turbances to which the former owe their origin and the latter their deformation. This system of fissures is parallel with the axis of granitic intrusion, and also with the general trend of the elvan dykes and mineral lodes, both of which have taken advantage of the fissures for the intro- duction of the material that enters into their composition. These phenomena are well illustrated in the Carn Menelez mass. Within that granite of the Mabe and Constantine districts mineral lodes and elvan dykes are practically absent, while the E.N.E. system of fissuring is almost entirely confined to the visible joints. These joint- faces however frequently display slickensided surfaces which indicate rela- tive displacement of the walls, and are sometimes seen to be small faults that have displaced the horizontal beds of granite to the extent of a few feet. Moreover the finer grained aplite veins tend to take a course parallel to these same joints. In the parish of Wendron, which forms the central belt of that granite, this E.N.E. fissuring has been so sensibly increased that the stone which is quarried on a large scale in the two former parishes previously mentioned is no longer wrought. With this change elvan dykes make their appearance, also an extremely attenuated set of mineral veins which have been worked for tin ; moreover the granite, which is of coarse texture, is extensively veined by finer grained material. The E.N.E. fissuring is no longer confined to the major joints, but the interspaces between the fissures are very closely fractured along parallel planes. Frequently they consist of tiny cracks which sever the various crystals along their course. In the next stage the cracks have been infilled with quartz, which has been followed by the introduction of schorl as an accompaniment of that mineral, and finally to almost its total exclusion. With the oncoming of wider veins the rock takes on a banded appearance, the black schorlaceous veins standing out in striking contrast to the granite. The schorlaceous veins carry tin, and in some instances it is quite clear that the lode follows the E.N.E. joint. Over the whole district the schorlaceous bands rarely exceed a few inches in width. When this granite however is followed into the districts of Redruth and Camborne, the magnitude of the fissures and lodes and the extent of the elvans have reached their maximum at that marginal zone of the mass which constitutes the heart of the mineral area of Cornwall. These E.N.E. fissures, into many of which elvans have been in- jected, while others have formed the repositories of the mineral ores, especially copper and tin, have been brought about by the volcanic dis- turbances of the newer Paleozoic periods, whereas the set of transverse fissures, locally known as cross-courses, which have effected vertical dis- placements of considerable magnitude, are of later date. After a pro- longed cessation of volcanic activity, extending over the Mesozoic ages, 30