Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/432

 del Annunziata. In some cases the slaggy upper part of one bed and the corresponding lower part of the bed above it seem to pass into each other, although the general bedded structure of the whole remains very marked at a little distance. Here and there, as at the north end of Beinn Bhuidh, illustrations are afforded of the elongation of the cavities along the upper surface, showing the direction in which the lava was moving before it finally cooled and consolidated.

2nd. Some of the beds are coarsely amygdaloidal throughout. In the kernels are found the usual minerals which result from the decomposition of basalt-rocks — mesotype, stilbite, calc-spar, amethyst, chalcedony, quartz-crystals, &c. And it is to be remarked, in Eigg as elsewhere throughout the "Western Islands, that the abundance of the amygdaloidal minerals is proportioned to the amount of alteration which has been undergone by the general matrix of the rock in which they lie.

3rd. Although the interbedded sheets are sometimes seen to die out along the line of cliff, they never penetrate or otherwise disturb each other. This feature is one which has not been recognized by previous writers on the igneous rocks of the Inner Hebrides. It has been lost sight of among the proofs of intrusion furnished by so many of the basaltic sheets ; and thus the " trap " or " overlying rocks " of Skye and the other islands have come to be regarded as typical examples of intrusive igneous masses, and described and figured as such in innumerable text-books. Yet no fact is more absolutely certain than that the vast mass of the basaltic rocks of these regions consists of interbedded sheets, which flowed out, one over another, at the surface, and have no intrusive characters. They are traversed, however, by intrusive sheets and dykes, as will be pointed out in the sequel.

4th. The occurrence of intercalated tuffs, volcanic breccias, and layers of burnt soil in Eigg, and of shales with remains of land- plants and seams of coal in the other islands, completes the proof that the basaltic beds forming the great plateaux, must be regarded as of interbedded or contemporaneous origin — that is, sheets which were poured out as lava above ground, and not injected among older rocks below.

β. Poryphyrite.

Under this term I include a well-marked bed, forming a conspicuous band along the range of cliffs which flank the plateau of Beinn Bhuidh (see figs. 2 and 3). It lies near the base of the volcanic series. Owing to the flatness of the beds and to denudation, it has been uncovered, so as to stretch over most of the bottom of the hollow between Kildonan and the Bay of Laig. But I did not find it in the southern half of the island. This rock is of a pale grey colour. It consists of a finely crystalline felspathic base, through which a few small plagioclase crystals and grains of titaniferous iron can be seen with the lens. Examined with the microscope by