290 PSOCEEDINGS OF THE GEOLOGICAL SOCIETY. [^Pl*- 5, 



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 thick- 

 ness 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 tuifs 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 contempo- 

 raneous 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 An- 

 trim. 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- 

 crystaUized 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 weU 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 deter- 

 mined. The beds range in thickness from perhaps 20 to 50 or 60 

 feet. They seem quite continuous when looked at from the sea, as 

 they band the precipices with parallel stripes of darker and lighter 



* See his Mikroskopische Untersuchungen iiber die Basaltgesteine. Bonn, 

 }8G9. 



