COLL. MINERALS. 67 



on the face of a rock not far from the house of Coll, where 

 it appears to form part of a vein of which the side has been 

 exposed by the falling down of a bed of the gneiss by 

 which it was bounded. Its aspect is often waxy, an appear- 

 ance very common in the quartz veins that traverse gneiss, 

 while at times it assumes a character resembling that of 

 chalcedony. The structure is generally lamellar, and it is 

 traversed by fibres or strings of a whiter colour and of 

 greater opacity. On exposure to air it becomes dry and 

 harsh, losing much of its transparency. Where it possesses 

 the greatest degree of transparency it is opalescent, being 

 the milk quartz of mineralogists, and either exhibiting a 

 whitish and diluted milky hue, or a tinge of blue, purple, 

 or pink. These latter specimens resemble the well known 

 rose quartz, but are rarely of sufficient magnitude, purity, 

 or intensity of colour, to possess much value in the esteem 

 of a collector. 



Felspar also occurs in great variety and beauty, appear- 

 ing, like quartz, to be subject to more numerous modifica- 

 tions as a constituent of gneiss, than when it enters into 

 the composition of granite. It is found in large concre- 

 tions, sometimes occupying the granitic laminae of the 

 gneiss, and at others the veins of graphic granite. 

 Its colours are various, being white, red, brown, and 

 grey, the specimens having often a pearly aspect and 

 sometimes a glassy and splendent fracture. The gra- 

 phic granite which contains these glassy varieties is 

 often exceedingly beautiful, and as lichens rarely find 

 a lodgment on its surface, it dazzles the eyes when in 

 the sunshine by the play of reflected lights. It is 

 worthy of remark to mineralogists, that in these cases the 

 incident rays are reflected to the spectator's eye from a 

 great extent of surface, although that is often very irregular. 

 The reason is obvious, this effect resulting from the dis- 

 position of all the reflecting faces being parallel, how- 

 ever distant and minute they may be, and however sepa- 

 rated from each other by the intervening quartz which 



