708 PASSAGE OF GRANITE INTO TRAP. [Ch. XXXIIL 



Syenite. — When hornblende is the substitute for mica, which is 

 yery commonly the case, the rock becomes Syenite ; so called from 

 the celebrated ancient quarries of Syene in Egypt. It has all the 

 appearance of ordinary granite, except when mineral ogically examined 

 in hand specimens, and is fully entitled to rank as a geological mem- 

 ber of the same plutonic family as granite. Syenite, however, after 

 maintaining the granitic character throughout extensive regions, is not 

 uncommonly found to lose its quartz, and to pass insensibly into 

 syenitic greenstone, a rock of the trap family. Werner considered 

 syenite as a binary compound of felspar and hornblende, and regarded 

 quartz as merely one of its occasional minerals. 



Syenitic Granite. — The quadruple compound of quartz, felspar, 

 mica, and hornblende, may be so termed. This rock occurs in Scot- 

 land and in Guernsey. 



Taleose Granite, or Protogine of the French, is a mixture of felspar, 

 quartz, and talc. It abounds in the Alps, and in some parts of Corn- 

 wall, producing by its decomposition the China clay, more than 12,000 

 tons of which are annually exported from that country for the pot- 

 teries.* 



Schorl-Rock, and Schorly Granite. — The former of these is an 

 aggregate of schorl, or tourmaline, and quartz. When felspar 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. When crystalline, it is seen to contain 

 crystals of quartz, mica, common felspar, and soda felspar. When 

 there is no mica, and when common felspar predominates, so as to 

 give it a white color, it becomes a felspathic granite, called " white- 

 stone " (Weisstein) by Werner, or Leptynite by the French, in which 

 microscopic crystals of garnet are often present. 



All these and other varieties of granite pass into certain kinds of 

 trap — a circumstance which affords one of many arguments in favor 

 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 porphy- 

 ritic, 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 min- 

 eral 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 (see Table, p. 608) ; 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 further examination — for on this subject we have yet much to 



* Boase on Primary Geology, p. 16. 



