66 COLL. MINERALS. 



cases a constituent part of the gneiss, while at the same 

 time it may be observed in the state of distinct concretions 

 dispersed throughout the beds. In such instances it 

 sometimes occupies cavities, and is then regularly crystal- 

 lized. It occurs in the same manner in a kind of quartz 

 rock which appears to alternate with the gneiss, but 

 which was not described in speaking of the other alter- 

 nating substances, because from its inaccessible position I 

 could not pronounce decisively respecting it. Both these 

 rocks are remarkable as well for their peculiarity as for 

 their beauty, when considered as mineral specimens. 



A bed occurs among the gneiss at the foot of Ben 

 Feoul, of a singular aspect, but appearing to be prin- 

 cipally composed of actinolite under some modifica- 

 tion. It is of a confused lamellar structure, the 

 lamellae having a tendency to the fibrous fracture, and 

 being of a dull purplish brown colour. On a nearer exa- 

 mination it is found to contain interspersed crystals of the 

 green lamellar actinolite which occurs in the gneiss 

 above described. The brown part which constitutes the 

 main body of the rock, is opake and dull, but possesses in 

 every other respect the characters of the green part ; and 

 a perfect idea of it may perhaps be formed by imagin- 

 ing it to be the same substance overcharged with an oxide 

 of iron, just as quartz sometimes is with chlorite or calca- 

 reous spar with sand. 



Garnets are found in the gneiss, here as in many other 

 places, but they are neither abundant nor well denned, 

 being at the same time confined to those laminae which 

 possess the most granitic aspect and structure. Massive 

 brown garnet is also found in large irregular lumps, dif- 

 fused, as it were, through the rock. 



Independently of the more common forms in which 

 quartz here appears, it occurs also under some of its 

 less frequent modifications ; these, as far as I have ob- 

 served in Scotland, seeming to be always limited to 

 gneiss. The most remarkable example is to be seen 



