beds are to be seen at that place to a considerable extent reposing upon granite; and the line of junction, which begins here at the sea-side, may be traced by the eye for some miles across the country. The regularity of this junction is remarkable on the top of Rochestown hill, adjoining that of Killiney; where ledges of granite, against the foot of which the incumbent rocks incline, present in several places, a rectilinear course for many fathoms together. On the shore at the base of Killiney-hill, the granite is traversed by numerous veins, many of which themselves consist of granite; and in some instances, two granite veins, differing from each other and from the mass, in fineness of grain and in proportion of their ingredients, are seen to intersect; one vein often deranging the continuity of the other's direction. The substance of these veins is perfectly continuous with that of the mass through which they run, and the surface of the fracture passes through both without interruption.
The conical masses of the Sugar-loaf mountains, with the summits of Braybead, and Shankill, resembling them in structure, are composed of quartz; and it may be remarked, that the conical form appears to be in some measure characteristic of mountains composed of that substance; for Mr. Jameson informs me, that he has seen in Lusatia detached conical summits composed of it; and that the well-known Paps of Jura, and the conical summits in the mountains separating Caithness from Sutherland, are of the same material; as also is, according to Dr. Berger, the mountain Durnshill near the town of Portsoy.[1]
The actual contact of granite with incumbent rocks, has been observed
- ↑ Humboldt states, that in South America, quartz constitutes, exclusively, a mass of more than nine thousand five hundred feet in thickness, which he considers as of a “ formation” peculiar to the Andes. He has not mentioned the form of the summits. Tableau Phys. p. 128.
