Ch. V.] PRIMARY SCHISTOSE ROCKS. 69 



that it is sometimes predominant. In the island of Coll, it 

 gives rise to distinct beds of mica-slate, alternating with the 

 gneiss. This mineral is also, in the same place, so developed, 

 as to form large concretions. The mica, in this state, is 

 always black ; in some cases crystallized, and in others, dis- 

 posed in large plates without any definite form: and it is 

 often so intimately mixed with hornblende, that it is difficult 

 to distinguish one ingredient from the other.* 



It has been already stated, that this gneiss is characterised 

 by the presence of hornblende; which, with felspar and 

 quartz, often constitutes the gneiss, to the entire exclusion of 

 mica. The quartz, also, occasionally disappears; in which 

 case, the rock puts on an appearance intermediate between 

 gneiss and granite : again, by becoming of a finer grain, or by 

 the exclusion of some of the felspar, it gradually passes into a 

 simple hornblende-schist, or into that mixture of hornblende 

 and felspar called primitive greenstone ; and in some cases, 

 into an unfoliated hornblende-rock. It is also not uncommon 

 to find in this hornblendic gneiss larger or smaller laminae, 

 lumps, and even beds of hornblende-rock. These appear- 

 ances are well displayed in Tireyt ; and in Lewis, where 

 they also occur, the gneiss exhibits an intermixture of a dark 

 and pale colour, in pretty equal and parallel laminae ; the 

 darker parts abounding in hornblende, the lighter in felspar. :f 

 But the most curious disposition of the hornblende is to be 

 found in the gneiss of Coll, where nodules of hornblende- 

 rock, of considerable size, are completely enveloped by con- 

 centric layers of gneiss. 



The felspar in the gneiss of the Western Islands is gener- 

 ally uniformly intermixed with the other ingredients ; but 

 sometimes it is distinctly developed, imparting to the rock 

 an imperfect porphyritic character. This felspar is splendent, 

 or nearly glassy ; and the concretions, though highly crystal- 

 line within, have no external configuration : they are seldom 



* Western Islands of Scotland, vol. i. p. 63. * Idem, p. 46. 



$ Idem, p, 193. Idem, p. 64. 



F 3 



