98 GEOLOGY OF ARRAX. 



distinctly the three ingredients, quartz, felspar, and mica. 

 The felspar is either white or red, more usually white in Arran, 

 being always in such quantity as to give its colour to the 

 rock, and is easily known by its laminated structure and 

 rhombic form, which presents, in a fractured specimen, 

 almost a square face. The quartz is harder, gray, or vitreous, 

 and, if in crystals, the form is a six-sided prism, terminated 

 by a six-sided pyramid; the sides of the prism being 

 finely striated across, those of the pyramid unstriated. 

 No specimens are met with containing the smoke quartz 

 or cairngorm crystals, which ate found on the back of 

 Suidhe-Fergus and the Castles, and are of the same 

 form as the common rock-ciystal, or " Arran diamond." 

 The mica is dark-gray, dark-brown, or almost black, in 

 thin, flat, four-sided prisms, bevelled at each end with 

 two surfaces; so that when these are equal in length to the 

 others, the crystal becomes a thin six-sided prism. When 

 hornblende takes the place of mica, and the rock consists of 

 quartz, felspar, and hornblende, it is called syenite. Horn- 

 blende is easily known from mica ; the crystal is a long 

 narrow prism, not divisible, like mica, by the point of a pen- 

 knife, into thin light flakes, and the colour is darker. Syenite 

 has a mottled, dark-green, red, or white aspect, accord- 

 ing to the colour of the felspar, and is much tougher than 

 common granite. We have not met with it among the 

 northern mountains, and it may have found its way here 

 from the Ploverfield district. Some syenites contain also a 

 little mica; but such structure has not been noticed by us 

 in Arran. Another variety found here consists of quartz 

 and felspar, and has either no mica or extremely little; it is 

 called Eurite, and is usually tougher than other kinds of 

 granite. It is the Weiss-stein of the Germans, and the 

 Elvan of Cornish miners. 



We do not find gneiss in our Wayside Collection it does 

 not exist in Arran, and is only found in situ far to the north- 

 west. Mica slate, consisting of quartz and mica, which occurs 



