106 MINERALS AND GEOLOGY 



(5) GROUP OF FELDSPATHIC SILICATES. 



[This group is composed essentially of alkaline and non-magnesian. 

 silicates, containing a high per-centage of silica (62 to 69), and 

 about 20 per cent, of alumina. H=6 to nearly 7 ; sp. gr. 2.4 2.7. 

 Crystallization, Clinorhombic or Triclinic. Fusible in thin splinters 

 only. Insoluble in acids.] 



57. Orthoclase or Potash Feldspar : White, red, flesh-red, apple- 

 green, grey, &c. Lustre more or less pearly on cleavage planes, 

 otherwise vitreous or stony. Crystallization Clinorhombic, but with 

 two well-maiked cleavage directions (parallel with base and side 

 vertical) meeting at right angles. Crystals often compound, as in 

 the more common twin combination shewn in Fig 70. Found 

 usually, however, in lamellar and granular masses. H = 6.0 ; sp. 

 gr. 2.5 2.6. BB, fusible with difficulty, unless in the form of a 

 thin pointed splinter, in which case the edge and point become 

 quickly rounded. Practically, unattacked by acids. Average com- 

 position : silica 64.8, alumina 18.4, potash 16.8 ; but many varieties 

 contain a small percentage of soda, replacing a portion of the potash. 

 Orthoclase is one of the com- 

 ponent minerals of many 

 crystalline rocks, granite, sy- 

 enite, gneiss, <fec. ; it occurs 

 also in many trappean rocks, 

 and forms the essential com- FlQ . 68 . FIG. 69. FIG. 70. 



ponent of trachytes and ordinary lavas. In the Laurentian strata 

 so widely developed throughout the more northern portions of 

 Canada (see Part Y), this mineral is consequently largely present \ 

 and well denned cleavable masses, mostly of a flesh-red or greyish- 

 white colour, may be obtained in almost every district in which 

 gneissoid rocks occur, more especially from the coarser granitic veins 

 by which these rocks are so commonly traversed. Some of the more 

 remarkable Laurentian localities comprise : the townships of North 

 Burgess, Elmsley, Grenville, Chatham, &c. : also the township of 

 Ross, and other places in the neighbourhood of Calumet Falls ; and 

 several spots on the north shore of Lake Huron. In Burgess (Lot 

 3, Con. 6), among other varieties, a striped red and brownish ortho- 

 clase occurs. This presents iridescent reflections, and is the variety 

 known as Perthite. It contains soda as well as potash. In Ross 

 (Renfrew County) large white crystals occur with apatite and spinel 



