214 GRANITES AND GRANITIC VEIN-STONES. [XL 



their faces are concave, and have lost their polish, retaining 

 only a somewhat greasy lustre. A thin shining green layer, 

 apparently of a silicate of copper, covers the surfaces of the ore 

 in contact with the crystals. From the mode of their arrange- 

 ment in certain specimens, it is evident that these prisms of 

 quartz, lining drusy cavities, were partially dissolved previous 

 to the deposition of the metallic sulphide. 



44. Some of the more important types of Laurentian 

 vein-stones may now be noticed. Those made up of quartz 

 with orthoclase, muscovite, and black tourmaline, often with 

 zircon, are not unfrequent in the Laurentian gneiss, though so 

 far as yet observed less abundant than in the gneisses and 

 mica-schists of the White Mountain series. It is true, as 

 already pointed out, that, from the greater softness of the en- 

 closing rocks, the veins of the latter series are often weathered 

 into relief ( 20), and are thus rendered more conspicuous 

 than those in the harder Laurentian gneisses. Among other 

 examples of this first type of granitic veins may be mentioned 

 those in Yeo's Island among the Thousand Isles of the St. 

 Lawrence, and the well-known vein in Greenfield, near Sara- 

 toga, remarkable for affording crystals of chrysoberyl. A fre- 

 quent type among the Laurentian granitic veins is characterized 

 by great cleavable masses of reddish or reddish-brown orthoclase, 

 with quartz and but little mica. With these are sometimes 

 associated equally large masses of white or pale-colored albite ; 

 these veins are sometimes of great size, one hundred feet or 

 more in breadth. The perthite of Thompson, well known as a 

 cleavable feldspar made up of alternate thin plates of reddish- 

 brown orthoclase and white albite, forms with quartz a large 

 granitic vein ; while the peristerite of the same author is an 

 opalescent or chatoyant white albite, with blue and green re- 

 flections, which occurs with quartz in another vein in the same 

 region. Some of the veins of red orthoclase include large 

 cleavable masses of dark green hornblende, occasionally with 

 magnetite. A remarkable vein about eighty feet in width, in 

 Buckingham, Quebec, is composed entirely of reddish-white 

 orthoclase and cleavable magnetite, the latter in masses some- 



