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LYELL'S ELEMENTS OF GEOLOGY. 



Syenitic Granite Talcose Granite Schorl Rock Eurite .... Pegmatite. 



cally examined in hand specimens, and being fully entitled to 

 rank as a geological member of the same plutonic family as 

 granite. Syenite, however, after maintaining the granitic cha- 

 racter throughout extensive regions, is not uncommonly found 

 to lose its quartz, and to pass insensibly into syenitic-greenstone, 

 a rock of the trap family. 



Syenitic-granite. — The quadruple compound of quartz, fel- 

 spar, mica, and hornblende, may be so termed. This rock occurs 

 in Scotland and in Guernsey. 



Talcose granite, or Protogine of the French, is a mixture of 

 felspar, quartz, and talc. It abounds in the Alps, and in some 

 parts of Cornwall, producing by its decomposition the china clay, 

 more than 12,000 tons of which are annually exported from that 

 county for the potteries.* 



Schorl rock, and schorly granite. — The former of these is 

 an aggregate of schorl, or tourmaline, and quartz. When fel- 

 spar and mica are also present, it may be called schorly granite. 

 This kind of granite is comparatively rare. 



Eurite. — A rock in which all the ingredients of granite are 

 blended into a finely granular mass. Crystals of quartz and 

 mica are sometimes scattered through the base of Eurite. 



Pegmatite. — A name given by the French writers to a variety 

 of granite ; a granular mixture of quartz and felspar ; frequent 

 in granite veins ; passes into graphic granite. 



All these granites pass into certain kinds of trap, a circum- 

 stance which affords one of many arguments in favour of what 

 is now the prevailing opinion, that the granites are also of igneous 

 origin. The contrast of the most crystalline form of granite, to 

 that of the most common and earthy trap, is undoubtedly great ; 

 but each member of the volcanic class is capable of becoming 

 porphyritic, and the base of the porphyry may be more and 

 more crystalline, until the mass passes to the kind of granite 

 most nearly allied in mineral composition. 



The minerals which constitute alike the granitic and volcanic 

 rocks, consist, almost exclusively, of seven elements, namely, 

 silica, alumina, magnesia, lime, soda, potash, and iron ; and these 

 may sometimes exist in about the same proportions in a porous 

 lava, a compact trap, or a crystalline granite. It may perhaps 

 be found, on farther examination, for on this subject we have yet 

 much to learn, that the presence of these elements in certain pro- 

 portions is more favourable than in others to their assuming a 

 crystalline or true granitic structure ; but it is also ascertained 

 by experiment, that the same materials may, under different cir- 



* Boass on Primary Geology, p. 16. 



