﻿174 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Feb. 6, 



In numerous places the strata are strikingly interfered with by 

 intrusions of granite, as well as by hornblende-rock and greenstone. 

 In one tract only, however, that fell under our observation does gra- 

 nite occupy so considerable a surface as to be entitled to a place in 

 our forthcoming sketch-map of Scotland, — a feature unnoticed by 

 Macculloch and other geologists. This mass of granite comes out 

 at Dalbeg, on the west coast of Lewis. There, whilst the hills of 

 Baravas range in geographical outline from N.E. to S.W., the granitic 

 ridge is protruded from them to the coast on the north-west, and is 

 thus shown to conform in its direction to that of the main masses 

 of the ancient gneiss. This granite is coarse-grained and of various 

 colours (though the prevailing tint is red), and, containing felspar, 

 quartz, and mica, is entirely free from hornblende. As veins of this 

 granite traverse the gneissose rocks, it is evidently of later origin 

 than the deposition of the gneiss. 



Besides the usual varieties of gneiss, whether consisting of layers, 

 of quartz and felspar with mica or with hornblende, white hyaline 

 or flinty quartz is occasionally seen in the form of layers of both pink 

 and white colours, sometimes presenting the appearance of having 

 been injected amid the layers of gneiss, in the same manner as 

 the numerous granitic veins. In some places the laminated and 

 bedded gneiss has undergone great decomposition, as at Garrabost, 

 in the promontory of Eye. There, near the entrance of the Chemical 

 Works * under the direction of Mr. Paul, the gneiss, consisting of 

 fine layers of quartz, felspar, and mica, has been so affected by 

 atmospheric influences as to exhibit the following appearances. 

 The felspar having decomposed and passed into heaps of clay, the 

 quartz-grains and the flakes of mica remain in the form of a frame- 

 work which might pass for an incipient band of soft ordinary mica- 

 ceous sandstone formed on the shore of the present sea. Again, the 

 felspar of the old gneiss contains a considerable admixture of lime, 

 and hence it affords, on decomposing, not merely good clay, but 

 much carbonate of lime in solution. Thus detached fragments of the 

 overlying conglomerate, of which we are about to speak as seen on 

 either side of the bay of Loch Tua, over which the water trickles 

 down from the decomposing gneiss to the shore, as well as the 

 pebbles and shells of the present bay, are bound together on the 

 slopes of the cliffs and on the sea-shore by the cementing carbonate 

 of lime, and form a hard calcareous grit and conglomerate. Before 

 we quit the consideration of this fundamental British rock, as seen in 

 the outer Hebrides, we would beg our readers to consult the various 

 chapters of Dr. Macculloch in which he dwells upon the dull and 

 monotonous character of the gneiss in all this range of islands, as con- 

 trasted with the descriptions of the same author of his so-called gneiss 

 of interior portions of the mainland. For, although he grouped various 

 and dissimilar rocks in the family of gneiss, and gave no proofs of 



stupendous erratic blocks. The lulls of the Lewis are too low to hare been the 

 seats of glaciers : and on that northern portion of the island erratics are scarcely 

 to be discovered. 



* For the distillation of bitumen from peat. 



