70 A DESCRIPTION OF THE [Ch. V. 



less than a quarter of an inch in diameter, and attain to that 

 of three inches. These concretions are often thickly dis- 

 seminated, and disposed in a laminar direction, corresponding 

 with the general structure of the rock ; the laminae of which, 

 however, are sometimes bent and accommodated to the shape 

 of the felspar.* These imbedded concretions of felspar are 

 not always crystalline, but are found gradually passing into 

 the common foliated and compact varieties of this mineral. 



In the latter state, the felspar occurs often in the form of 

 veins, laminae, and even beds parallel with the strata of the 

 gneiss : it exhibits various tints of yellow and green, and is 

 often combined with other minerals ; thus becoming, in some 

 instances, porphyritic : of this nature is the porphyry vein 

 traversing the gneiss, in the island of lona.f The gneiss of 

 these islands frequently passes, by the most imperceptible 

 gradations, into a fine fissile slate : and this is always the 

 case when the gneiss contains compact felspar. 



The gneiss sometimes becomes talcose, and it is then 

 associated with magnesian rocks. This occurrence is well 

 displayed in the island of Scalpa ; where, in consequence of 

 extensive excavations for the lighthouse, a bed of serpentine 

 has been exposed, traversing the promontory, and disposed 

 in an irregular manner, like the gneiss in which it lies. The 

 gneiss, in the vicinity of the serpentine, is in some parts 

 almost an entire mass of compact felspar, mottled with red 

 and white, and its laminar disposition being sometimes 

 marked by alternations of these colours : it is occasionally 

 interlaminated with clay-slate, and more rarely with talc; 

 forming a talcaceous gneiss, which here marks the boundary 

 of the serpentine. In other places, the transition between 

 these rocks is effected by a serpentine, so full of hornblende, 

 as to be scarcely distinguishable from a true hornblende-rock. 

 Some parts of this rock are schistose ; their colours being 

 likewise disposed in a laminar manner, so as to present a 

 dark greenish basis, striped, and in some directions speckled, 



* Western Islands of Scotland, vol. i. p. 193. f Idem, p. 21. 



