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, 



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