334 J. ARTHUR PHILLIPS ON THE ROCKS OF 



Zirkel has apparently satisfied himself on this point, since he 

 observes that the calculation by which the temperature at which a 

 rock was formed is deduced from the proportion of the bubble to 

 the liquid, is so very doubtful as to afford no certain data, although 

 it would be otherwise of great value to geologists. Fluid-cavities 

 are constantly met with in the same crystals, in which bubbles vary 

 greatly in relative size ; large cavities occur containing small bub- 

 bles by the side of small cavities with large ones, &c* 



Elvans. 



The elvans of Cornwall are rocks occurring in veins or dykes, 

 which have almost identically the same ultimate chemical and 

 mineralogical composition as the granites of the district ; the ag- 

 gregation 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 the felspar, por- 

 phyritically 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 some- 

 what smoky, have often rounded angles. This removal of the edges 

 is sometimes so complete that the patches of quartz in an elvan 

 present the appearance of mere gum-like blebsf . Some of the 

 phenomena connected with the formation of such crystallized bodies 

 will be noticed when describing the microscopic structure of these 

 rocks. 



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

 tals, 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 decom- 

 posed by weathering into kaolin ; and the cavities resulting from its 

 subsequent removal are in some cases lined with gothite. More fre- 

 quently they have been re-filled with schorl or chlorite ; while in the 

 well-known pseudomorphs of Huel Coates, felspar has been re- 

 placed 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 crystal- 

 line 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 con- 

 form, both in direction and in dip, to one series of joints in the 



* Mikroskopische Beschaffenheit der Mineralien und Gesteine, pp. 45-46. 

 t The crystals of felspar are sometimes similarly rounded, but less fre- 

 quently and to a much less extent. 



