PRAGMEK'TS OF OTHEE EOCKS CONTAINED IN GEANITE. 19 



grey colour, is finely granular and contains numerous dark in- 

 clusions. 



When thin sections of this rock are examined under the micro- 

 scope they are found to be composed of quartz, orthoclase, a triclinic 

 felspar, which Prof. Haughton has determined to be albite*, a dark 

 greenish mica, green hornblende, some needles of apatite, a little 

 sphene, and occasionally magnetite or pyrites. The quartz of this 

 granite is often traversed by minute hair-like crystals, which are 

 perhaps rutile ; and the hornblende and mica are so mixed as almost 

 to suggest the possibility of one being sometimes an alteration- 

 product of the other. 



The inclusions in this rock are, as is usually the case in granites, 

 of two kinds. The first variety is commonly more or less ovoid in 

 form, is finer in grain than the surrounding granite, considerably 

 darker in colour, and sometimes encloses crystals of orthoclase of 

 considerable size. Such bodies contain all the constituents of the 

 enclosing rock and, with the exception that dark mica is in con- 

 siderable excess, they are present in nearly the same proportions. 



Those belonging to the second category are angular or subangular 

 in form, very dark in colour, and often exhibit a distinctly foliated 

 structure. These are composed of granular quartz, black and greenish- 

 black micas, green hornblende, a few imperfect crystals of felspar, 

 specks of magnetite, with pyrites, and occasionally a little apatite. 

 A few minute crystals are also sometimes present of a mineral which 

 may perhaps be epidote. 



General Conclusions. 



The inclusions contained in granites are of two distinct kinds. 

 Those of the first class are the result of an abnormal arrangement of 

 the minerals constituting the granite itself; while those belonging to 

 the second represent fragments of other rocks enclosed within its 

 mass. 



Inclusions of the first class are frequently, but not always, raore 

 or less ovoid in form, and are, essentially, composed of a fine-grained 

 variety of the granite in which they occur ; the proportion of tri- 

 clinic felspar present in such granitic inclusions is often greater than 

 in the rock which encloses them. 



They are sometimes porphyritic in structure, and contain crystals 

 of the characteristic felspar of the enclosing rock. Thus, when the 

 felspar of the surrounding granite is either red or pink in colour, 

 that which is porphyritically distributed in the inclusion will, as a 

 rule, be correspondingly red or pink ; whereas if white felspar be a 

 characteristic of the ordinary granite, the porphyritic crystals of the 

 inclusions will be likewise white. 



The angles of felspar crystals so contained in inclusions are 

 frequently much rounded, and the presence of a large proportion of 



* Quart. Journ. Geol. Soo. toI. xii. p. 189. 



c 2 



