SYNOPSIS OF GNEISS. 233 



No division is here made of those varieties which con- 

 tain occasional minerals, however abundant, or in what- 

 ever manner disposed, since these must be considered as 

 accidental not essential substances. The following mi- 

 nerals are those of most usual occurrence : 



Garnet. This is found either diffused through the mass, 

 or crystallized and disposed in a laminar manner, or irre- 

 gularly scattered. 



Actinolile. This mineral occurs under various forms, 

 sometimes, as above noticed, as a substitute for hornblende. 



Quartz. The quartz occurring in gneiss is often marked 

 by peculiar characters. The most common is a slight 

 opacity varying from a waxy to a chalcedonic and to an 

 opalescent appearance. This is particularly the case in 

 the laminar subdivision. It has sometimes but improperly 

 been termed chalcedony. The milk quartz of mineralo- 

 gists, of whatever colour, belongs to this variety, and is, 

 1 believe, always an inmate of gneiss. It also occurs very 

 finely granular, as was noticed in Harris. In the vicinity 

 of Loch Mare, fetid quartz, similar to that of Nantes, is 

 not uncommon, its smell on friction somewhat resembling 

 that of putrid sea weed. The quartz is also found of 

 various colours in the gneiss of the north-western coast 

 and in that of the Long Isle. The prevailing are the 

 various shades of ash grey, varying gradually from the 

 lightest tint to jet black, in which last state it occurs in 

 the gneiss of Ben Lair in Ross-shire; but very rarely. 

 Next to these the most common colours are reds, varying 

 from a pale claret to a dilute pink. French grey is less 

 common ; it was mentioned as found in North Rona, and 

 it also occurs in the gneiss of Glen Tilt. In one of the 

 islands of Loch Hourn I observed it of a dilute green, 



tion has been rigidly limited to the varieties actually observed. It is 

 evident that some of them would equally admit of being ranked with 

 other rocks, but such are the transitions of these compounded bodies 

 that every arrangement must be subject to this kind of imperfection. 



