Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/431

 brown ; and their continuity is still further indicated by the slender lines of bright herbage which have taken root along the decaying upper or under surfaces of the flows. Yet, on closer examination, we find them not un frequently to die out, the place of one bed being taken by another, or even by more than one, in continuation of the same horizon. This is particularly noticeable along the cliff-line on the east side of Beinn Bhuidh. There is considerable diversity in the colour and texture, as well as the structure, of the different beds. Some of them, in which the rock is more compact and weathered, are divided by vertical joints, which in some cases increase in number till the rock acquires a rudely columnar structure. This may be admirably seen along the coast north of the harbour, where a long line of columnar cliff shows in some places curved and radiating columns. Other beds are formed of a dark compact amorphous mass, usually amygdaloidal, and occasionally very markedly so. A not infrequent variety occurs in the form of a dull green amygdaloidal and scoriaceous rock, in which balls of more compact material are wrapped, as it were, in a softer decomposed base. At the south end of the island, a peculiar band of rock occurs, in which the process of weathering reveals a succession of layers, a few inches thick, formed of nodular pieces of compact blue anamesite or basalt, with a bright red crust. These layers lie a few inches apart, in a soft, dirty-green, crumbling, and often highly amygdaloidal rock. The band in which these features are seen runs as an intercalation, about 3 or 4 yards thick, among the sheets of hard crystalline anamesite.

As an illustration of the bedded arrangement of these rocks, and of the way in which they succeed each other along the same horizontal plane, reference may be made to the accompanying diagram (fig. 3) of part of the cliff-section north of Kildonan, on the east side of the island.

Fig. 3. Diagram of interbedded Volcanic Rocks on the east side of Island of Eigg.

g. Compact jointed dolerite. f. Dull dirty-green decomposing amydaloidal dolerite. e. Compact crystalline dolerite, more finely jointed than bed g. d. Pale grey porphyrite. c. Dolerite, which a little further north is formed of several beds. b. Columnar dolerite. a. Oolitic strata.

The tests by which the true interbedded or contemporaneous character of the flows of the doleritic plateau can be determined are well exposed in Eigg. 1st. The upper and under surfaces of the successive flows have very commonly a rough slaggy character, even when the central portion is compact and crystalline. In this respect they perfectly resemble sections of recent lava-streams, such, for example, as those exposed along the Bay of Naples, around Torre