564 PASSAGE OF [On. XXXIII 



The uniform mineral character of large masses of granite seems tc 

 indicate that large quantities of the component elements were thoroughly 

 mixed up together, and then crystallized under precisely similar condi- 

 tions. There are, however, many accidental, or " occasional," minerals, 

 as they are termed, which belong to granite. Among these black schorl 

 or tourmaline, actinolite, zircon, garnet, and fluor spar, are not uncom- 

 mon ; but they are too sparingly dispersed to modify the general aspect 

 of the rock. They show, nevertheless, that the ingredients were not 

 everywhere exactly the same ; and a still greater variation may be traced 

 in the ever-varying proportions of the felspar, quartz, and mica. 



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

 commonly the case, the rock becomes Syenite : so called from the cele- 

 brated ancient quarries of Syene in Egypt. It has all the appearance of 

 ordinary granite, except where mineralogically examined in hand specimens, 

 and is fully entitled to rank as a geological member of the same plutonic 

 family as granite. Syenite, however, after maintaining the granitic char- 

 acter 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 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 country for the potteries.* 



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 com- 

 paratively 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 " whitestone" (Weis- 

 stein) 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 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. 

 * Boase on Primary Geology, p. 16. 



