333 



We will now give an account of the British trachytic rocks of 

 acid composition. 



West of England. The " elvans " of Cornwall have been described 

 in general terms by Mr. J. A. PHILLIPS.^ We copy his description 

 verbatim. It will apply equally to the elvans of Devonshire. " The 

 elvans of Cornwall are rocks occurring in veins or dykes, which have 

 almost identically the same ultimate chemical and mineralogical composi- 

 tion as the granites of the district ; the aggregation of their constituents, 

 however, is often very different. 



" In elvans, the quartz, instead of forming, as in granite, a kind of 

 crystalline residual base, is usually, together with felspar, porphyriti- 

 cally enclosed, in the form of crystals, in a felspathic or quartzo- 

 felspathic base ; mica, schorl and chlorite are often present to some 

 extent, while pinite is by no means an unfrequent accessory. Graphite 

 in the form of small nodular masses is sometimes found in Cornish 

 elvans. The quartz-crystals of elvans are often double hexagonal 

 pyramids connected at the bases by a short prism. These, which are 

 either glassy and transparent, white and opaque, or somewhat smoky, 

 have often rounded angles. This removal of the edges is sometimes 

 so complete that the patches of quartz in an elvan present the appear- 

 ance of mere gumlike blebs 



" The felspar in elvans is often in the form of large, well-defined 

 crystals, which may be either transparent and colourless, or white, pink, 

 red, or grey; in other varieties the crystals are very minute, and can 

 ' only be discovered by the aid of a lens. They are readily decomposed by 

 weathering into kaolin ; and the cavities resulting from its subsequent 

 removal are sometimes lined with gothite. More frequently they have 

 been re-filled with schorl or chlorite ; while, in the well-known pseudo- 

 morphs of Huel Coates, felspar has been replaced by cassiterite. 



" Schorl occurs either as isolated crystals or in stellate groups. Mica 

 is often disseminated through the mass ; but in some cases, particularly 

 in the coarse-grained elvans, it is found in crystalline aggregations. 



" Elvan courses vary in width from a few feet to several fathoms ; they 

 are more numerous in the vicinity of granite than elsewhere, and 

 traverse alike both granites and slates. They frequently conform, both 

 in direction and in dip, to one series of joints in the rocks which 

 they traverse ; but they seldom penetrate between the cleavage planes 

 of slates. 



" In slate they generally consist of a compact felspathic or quartzo- 

 felspathic base containing crystals of felspar and crystalline or gum- 

 like patches of quartz. When enclosed in granite, a similar base 

 prevails, mica and schorl are frequently present, and porphyritically- 

 embedded crystals are numerous, but the rock is generally finer-grained 

 than when it is in the slate. In both rocks, however, it is usually coarser 

 and more porphyritic near the middle of the dyke than towards its sides. 



(1) Q.J.G.S. Vol. XXXT. (1875), p. 334. 



