Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/433

 transmitted light, it is found to consist of a base of plagioclase felspar, in minute, somewhat decomposed crystals, with abundant black grains of titaniferous iron, and a brown, much decayed mineral, which may be augite. The higher part of the bed, at Dunan Thalasgair is darker in colour, and, when examined microscopically, has much the character of an anamesite. Indeed the whole rock might be regarded as a highly felspathic basalt-rock in which the ferruginous silicates are poorly developed.

This rock is, as a whole, strongly amygdaloidal, the cavities in the upper part of the bed being sometimes so flattened and elongated as to impart a kind of fissile texture to the mass. This is more particularly to be noted at the precipice of Dunan Thalasgair. Throughout a considerable part of the bed, the calc-spar and zeolites of the kernels have disappeared, and the rock has resumed its original vesicular aspect.

y. Tuffs, Breccias, &c.

One feature which distinguishes the Tertiary volcanic series of Britain from those of earlier geological periods is the comparative paucity and thinness of the intercalated beds of fragmentary materials. Among the contemporaneous igneous masses of the Silurian, Old Red Sandstone, Carboniferous, and Permian periods we find within our own borders enormous beds of tuff and volcanic breccia or conglomerate ; but among the great basalt-plateaux of our northwestern tracts such intercalations are represented by mere thin infrequent layers. This appears to be the case at least from the south of Antrim to the north of Skye — the most important tuffs in that extended area, so far as I am aware, being those of the cliffs at the Giant's Causeway. In Eigg this comparative insignificance of the fragmental as contrasted with the crystalline or lava-form rocks is characteristically maintained. Throughout the greater part of the cliff-sections one bed of dolerite or basalt follows another without the intervention of any dividing layer of tuff or other deposit. Here and there, indeed, between the beds, we not unfrequently meet with a thin irregular seam of red earth, which, when fine, might be called bole. In the cliff below Dunan Thalasgair, for example, several of the dolerite-beds are not only covered by this substance, but seem to pass into it. This may be observed also throughout the Inner Hebrides, and conspicuously along many parts of the Antrim coast-line. I have recently observed a precisely similar red parting between several of the lava-streams which have been laid open by the sea, and by artificial excavations, between Naples and Pompeii ; and I may add that it is likewise to be observed between the sheets of melaphyre interbedded with the lower carboniferous rocks of Kinghorn, in Fife. In all these cases I regard this red layer as marking a surface of the igneous rock, decomposed into clay or soil by exposure, and subsequently heated and altered by the overflow upon it of the next sheet of molten material.

At the north end of Eigg, along the cliff's of Beinn Bhuidh, a bed of coarse doleritic or basaltic breccia is interstratified with the other