Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/430

 1. The Basalt-Plateau.

The cake of volcanic rocks which has been referred to as overlying the Oolitic strata consists of a succession of beds, varying in thickness individually from a few feet to at least 50 or 60 feet, and having a united depth of not less than 1100 feet. They consist of dolerites, anamesites, and basalts, porphyrites, and tuffs or breccias. That they are the result of the outpouring of volcanic material at the surface, and not of its intrusion among the other rocks beneath the surface in other words, that they are interbedded or contemporaneous and not intrusive or subsequent masses, is shown by the internal texture of the crystalline rocks, and by the associated tuffs. Admirable sections are everywhere obtainable along the line of cliffs by which the island is almost continuously girdled.

a. Dolerites, Anamesites, and Basalts.

By much the larger part of the beds of the basalt-plateau consists of basaltic rocks (dolerite, anamesite, or basalt). These varieties of the same great family of volcanic rocks possess the same characters in Eigg which they retain throughout the Inner Hebrides and Antrim. The dolerite usually appears as a crystalline granular mass, passing on the one hand through anamesite into basalt, and on the other into a coarse aggregate, which shows on its weathered surfaces large crystals of augite. It is seldom that the rock becomes so black and compact as to deserve the name of basalt, except in the dykes to be afterwards described. Examined microscopically, these rocks fully bear out the observations of Zirkel on the presence of a non- crystallized matrix in basalt-rocks*. They occasionally abound in minute needles of apatite, which, along with the beautifully striated felspar, form a matted network of crystals, through which olivine, augite, and titaniferous iron are scattered. In some specimens the decomposition of the minerals is well illustrated. It may be added that these rocks very closely resemble, in composition and texture, the crystalline intrusive augitic rocks in the Scottish Carboniferous series — so closely, indeed, that no line of separation, so far as I have yet seen, can be drawn between them.

The bedded arrangement of the basalt-rocks, so characteristic of the vast miocene volcanic region from Antrim to Iceland, is well seen in Eigg. Along the cliffs at the north end of Beinn Bhuidh, and again along the south-western shore, the succession of beds is shown in noble vertical sections, while all over the southern half of the island the terraced or step-like hill-sides, formed by the outcrop of the beds, are everywhere visible. Even from a distance, therefore, the interbedded nature of these volcanic rocks can be readily determined. The beds range in thickness from perhaps 20 to 50 or 60 feet. They seem quite continuous when looked at from the they band the precipices with parallel stripes of darker and lighter

1869.
 * See his Mikroskopische Untersuchungen uber die Basaltgesteine. Bonn,