﻿300 ME. AV. B. WEIGHT ON THE [May I908, 



supposition is by far the more probable. One would expect an 

 intrusion-breccia to have, in places at least, an igneous matrix, and 

 the matrix of this breccia is always siliceous and never igneous. 

 One might also expect to find attached to boulders derived from a 

 conglomerate some fragments of the matrix in which they were 

 embedded, and none of the numerous boulders seen in the magni- 

 ficent exposures of this rock in the scars in Kiloran Bay show the 

 slightest trace of any such matrix. Whatever be the origin of this 

 peculiar breccia, it has, as wiU appear later, an important bearing 

 on the subject-matter of the present paper. 



The so-called ' Scalasaig Granite ' is the largest mass of igneous 

 rock in the island. It is a diorite, but exhibits certain characters 

 indicating an affinity to the Balnahard kentaUenite mentioned below. 

 In some respects, however, it is not unlike the Kiloran syenite, 

 which it resembles in being associated at its margins with immense 

 masses of breccia. 



The Balnahard mass is a typical kentaUenite, reproducing very 

 faithfully several of the various phases of this rock described by 

 Mr. Kynaston from Glen Orchy. 



The smaller plutonic masses are four in number, and are all 

 syenitic in character. One occurs in Laimaolean, one in Pigs' 

 Paradise, and two to the north of Balnahard. Of these last two 

 one has a small amount of breccia at its margin. 



(h). — The ' lamprophyre ' dykes and sheets are both numerous 

 and of widely different ages. The great majority of them are so 

 decomposed as to render determination impossible. Many eflfervesce 

 freely with acid.^ A few of the more recent, however, are com- 

 paratively fresh, and these, while varying in character from place to 

 place, may all be described as vogesites. Lamprophyres of all ages 

 are abundant in the northern half of the island, but are wanting 

 in the extreme south of Colonsay and in Oronsay. 



(c). — Basalt dykes of Tertiary age are fairly common in the 

 southern half of the islands, but become rare and are finally 

 wanting towards the north. They have the usual north-west and 

 south-east trend. Two dykes of monchiquite having a similar 

 trend, and probably of much the same age, have also been found. 



III. The Two Eaeth-Movements. 



Perhaps one of the most noticeable features of the Torridon 

 sediments of these islands is the presence of a well marked cleavage. 

 It is of course best developed in the more pelitic beds, being a true 

 close cleavage of the slaty type, produced by parallel arrangement 

 of the mineral particles of which the rock is composed. Owing to 

 its obvious similarity to the slaty cleavage of other districts, there 



^ While there is a considerable presumption that all these dykes are lampro- 

 phyres, yet many are in so advanced a state of alteration as to make it 

 impossible to say so positively. 



