230 B- f^^^*™ 



conipju'ison with orthoclase, and is nsnally enclosed by the letter. 

 The twiniiiiig Ininellte extingnisli liu'ht :it small ntigles, consetinently 

 it iiia\' he fairly supposed th;it the miner:il belongs to the oligoclase- 

 andesine series. The broad zoning is distinct, with differentiated 

 exti notion-angles. The orthoclase is anguLir and :dlot)-ioinorphic, its 

 apparent regidnr outlines having resulted from the (hsposition of the 

 adjnee)it ervstals, especially of plagioclase. (^a;n'tz fills up tlie in- 

 terstitial sj^aces left by other minerals, contains crystalloids of feldspars, 

 and a])])ears somewhat fihrillaled, especially when viewed l)y po]arizc(l 

 light lietween crossed Xicols. The quartz is comparati\ely free from 

 hc[uid irjcl usions. 



As to the structure I have to say only a few words. One 

 of the most interesting features of our granite is the presence 

 of the allotriomorphic feldspar, playing just the same role as the 

 granite-(piartz; that is to say, the interstitial masses of a non-striated 

 feldspar Ijetween a more or less well-finished ]^lagioclase. These 

 masses show a homogeneous extinction over a Avide area, like the 

 vitreous base of many neoA'olcanic rocks ; and though occurring in 

 detached spaces, they ]iossess surely an optical continuity. Where 

 this kind of leidsjiar comes in direct contact with e([ually allotriomor- 

 phic (quartz, the boundary is an irregular one, and tlie two must be 

 supposed to be sviK^hrc^ious productions in the act of crystallization 

 of the i-ock. To wl^at class of feldspar it may belong is very difficult 

 to decide. It looks like adularia and fresh, while the enclosed 

 plagioclase exhibits more or less an advanced stage of decomposition. 

 Notwithstanding the fresh a])})earance of the feldspar, it displays only 

 weak j)olarization-colours, by which the other feldspars and quartz 

 are readily distinguished. Througli the want of crystal lographic 

 outliiies there is no means of getting any optical proof. Lately, 



1 Zeitschr. f. Krystallographie. 1890, XVHI, p. 192 



