FKOM THE NOETH-WEST HIGHLANDS. 415 



ally, apatite. Some granules of magnetite, haematite, or, perhaps, 

 ilmenite are usually present. 



The quartz occurs as rounded to rather elongated grains of 

 variable size, often grouped. Commonly, as seems to be usual in 

 these very ancient rocks, they contain so many minute enclosures as 

 to have a rather " dusty " aspect. Some of these may be microliths 

 of various minerals (a micaceous one among them) ; but others are 

 certainly cavities, not seldom very irregular in form. These fre- 

 quently contain bubbles relatively as well as absolutely of very 

 small size, which move freely. 



The felspar occurs in grains, with outlines either rounded or some- 

 what irregular. A portion, no doubt, is orthoclase, but Carlsbad 

 twinning does not appear to be common. Very characteristic micro- 

 cline is almost always present in the slide, and is frequently com- 

 mon. Another triclinic felspar is also frequent, this exhibits re- 

 peated oscillatory twinning, and the laminae generally extinguish at 

 small angles (commonly less than 10°) with the twin plane, when 

 it is coincident with a vibration-plane of one of the crossed Nicols. 

 Hence most of it is probably albite or oligoclase. 



The hornblende is usually green, dichroic, and shows characteristic 

 cleavage. The black mica is frequently more or less replaced by a 

 green secondary product allied to chlorite, as is common in ancient 

 rocks. The white mica occurs in moderate-sized plates, exhibits a 

 rather wavy cleavage, and gives bright colours with the two Nicols. 



The sphene (or what I take for it, as the identification is not 

 always certain) is not very clear or characteristic. The other 

 minerals do not call for special remark. 



Hebridean Gneisses. 



In this Hebridean group I place the following specimens of Dr. 

 Callaway's collection : — 



62 (Durness). In this specimen some of the plagioclase extin- 

 guishes at angles too large for oligoclase or albite ; there is a good 

 deal of apatite ; and we find, with a considerable quantity of most 

 characteristic hornblende, a pale green mineral in rather irregular 

 clusters, associated with earthy-locking granules. The cleavage in 

 the cross sections is not very distinct, but the extinction -angle in 

 longitudinal sections is often much too large for hornblende, so that 

 I am induced to regard it as an abnormal form of augite — a 

 mineral, so far as my experience goes, extremely rare in all rocks 

 of this description. 



64 (Ben Arnaboll, Loch Emboli, p. 396) contains a considerable 

 quantity of black mica. The structure of this rock is a little peculiar, 

 and I could conceive it possible that it was an igneous rock crystallized 

 under a definite pressure. I think, however, it is a true gneiss. 



75 (Glen Coul, p. 384), a hornblendic gneiss, contains a good deal of 

 a pale green mineral, which appears to be an alteration product after 

 black mica. 



76 (Eagle Eock, south-east spur, pp. 374, 381). This specimen con- 

 tains very little quartz. The structure is rather obscured by crushing, 



