HEMITHRENES AND OTHER CALClTIC ROCKS. 49 



rather, promontory (called Glassillaun), separated from the main- 

 land by a ditch -like channel, containing water only at high tide. 



The rocks of the locality are well-bedded metamorphics, 

 slightly developed as such, consisting of quartzites, micaceous 

 and hornblendic schists : they dip to the north at an angle of 

 60, and strike east and west. Here and there they possess 

 some exceptional features. 



Plate IV. represents a ground-plan, and Plate V. a natural 

 vertical section of this case. The dyke, which is 20 feet 

 wide, intersects somewhat obliquely the metamorphics at the 

 head of a small cove or inlet in a north and south direction. 

 It is seen in the face of the cliff, which is about 20 feet in 

 height ; thence it passes southward into the sea. The cove is 

 bounded on its east side by a narrow projecting mass of rock, 

 and on its west by Glassillaun and a portion of the mainland. 



The dyke, of a dark grey or nearly black colour, and finely 

 crystalline in structure, consists essentially of the mineral 

 silicates augite (or hornblende) in long crystals, a granular 

 feldspar (possibly labradorite) , and a platy one looking like ortho- 

 clase. A little magnetite is also present. Calcite often occupies 

 narrow interspaces between the siliceous minerals, causing the 

 rock, on the application of a weak acid, to effervesce slightly. 



The metamorphics of the promontory, principally of the kind 

 common on the adjacent mainland, are strongly bedded, with 

 indications of lamellation; but in certain places immediately 

 adjoining the dyke the beds become changed into bands and 

 intrusive-like masses, distinctly striped with green and pale 

 brown (due to the alternation of layers respectively distin- 

 guished by these colours) , and varying from an eighth to an 

 inch or more in thickness. The ditch-like channel was at. one 

 time entirely occupied by one of the bands; but, owing to the 

 softness of its component layers, the band has been in great 

 part washed out, and an open separation has taken its place. 

 On the east side of the cove the same striped rock is also seen, 

 but in small portions, enclosed, as it were, in the gneiss, into 

 the bedding of which the layers pass uninterruptedly : on the 

 west side it forms intrusive-like masses, which pass imper- 

 ceptibly, occasionally abruptly, into the adjacent gneiss; but 

 their striping is in some places strikingly undulated, and 

 appears to be independent of the lamellation of the gneiss. 



