Page:VCH Cornwall 1.djvu/80

 A HISTORY OF CORNWALL is more distinctly predominant, in proportion to the other constituents, in granite than in elvan. A characteristic feature of the elvans is the tendency for the white mica to assume a radiating or plumose habit, and in the matrix of the rock to combine with the felspar in forming a pegmatitic structure. The granophyric habit is not confined to this type, but pseudo-spheru- litic structures have also been noted. In width the elvans range from a few feet to many yards ; a dyke however of abnormal dimensions has been traced following an east and west course through Gwennap, and attaining a width of 150 to 200 yards. Although the elvans are usually very steeply inclined, they exhibit much variation in this respect, and sometimes lie nearly flat. They occur in greatest abundance in districts invaded by granite, although the number that pierce that rock is far inferior to those enclosed by the killas. Occasionally they are found penetrating green- stone, as at Burntcoose in the parish of Gwennap. Besides the differences we have noted these rocks present great variations in their decomposition, being often represented by a soft clay, as their felspars readily decompose into kaolin. On the other hand the durability of certain elvans is such that they provide some of the best building material which the county possesses. Their general distribution is noted on the map, from which it will be seen that the greatest departure from their usual trend, parallel to the longitudinal axis of the county, occurs in the elvan system of St. Austell, which follows a course slightly north of west. The mica traps of Cornwall consist of dyke-like intrusions, the range of which is mainly restricted to a belt of country running north and south between Newquay and Falmouth Bay. This peculiar type of intrusions as related to Cornwall was first described by Mr. J. H. Collins. Their most characteristic feature in the field is a marked readiness to decompose, so that a fresh surface of the rock is seldom visible, and consequently they have been utilized to a trifling extent as building material. They are usually of a rusty brown colour, and occur as dykes and sills within the slates, not usually continuous for more than short distances. They vary in width from a foot to 50 yards, and have effected a very limited amount of contact alteration on the slates which they traverse. They often exhibit spheroidal structure, cores of more solid rock being encircled by concentric outer layers of exfoliating material ; and flow structure is not uncommon, more especially in the finer grained marginal portions. Notwithstanding their advanced state of decomposition, the ground-mass of these rocks can generally be seen to consist of a medium-grained admixture of biotite and felspar, while inclusions of quartz and crystalline felspathic rock are character- istic. These inclusions range to as large as 9 inches and may be rounded, 32