﻿Yol. 64.] THE TWO EAETH-MOVEMENTS OF COLOKSAY. 297 



15. The Two Eaeth-Movemei^ts of Colonsay. By William Bodkke 



Weight, B.A., E.G.S. (Communicated by permission of the 



Director of H.M. Geological Survey. Bead February 19th, 



1908.) 



[Plates XXXIII & XXXIV.J 



The field-work which forms the basis of the present communication 

 was carried out by Mr. E. B. Bailey and the Author in the summer 

 of 1907, in the course of the geological survey of the island of 

 Colonsay. The systems of folding displayed here and in the sister 

 isle of Oronsay were found to possess features of more than usual 

 interest, and special attention was given to the subject as far as 

 the rapid nature of the work permitted. The investigation was 

 facilitated by the rocky character of the islands, the surface of 

 which is but little obscured by superficial deposits. A fairly con- 

 tinuous series of observations was thus rendered possible, and 

 although a thorough understanding of the subject was by no means 

 arrived at, yet certain facts came to light which may possibly have 

 an important bearing on the erogenic history of the Highlands. 

 For clearness and convenience of reference the subject will be 

 discussed under the following heads : — 



I. The Stratigraphy and Chief Tectonic Features of the Islands. 

 II. The Igneous Rucks. 



III. The Two Earth-Movements. 



IV. The Relation of the Earth-Movements to the Lamprophyres. 

 V. The Relation of the Earth-Movements to the Plutonic Masses. 



VI. The Varied Age of the Lamprophyres. 

 VII. The Nature of the Second Clearage. 



I. The Steatigeaphy and Chief Tectonic Feattjees 

 OF THE Islands. 



The sedimentary rocks of supposed Lower Torridon age which 

 form the greater part of Colonsay and Oronsay consist of an alter- 

 nating series of grits, flags, and mudstones, with a well marked bed 

 of sandy limestone near the top. A description of the various 

 members -of this series is not essential to the present discussion, but 

 a general idea of the relation of the beds may be obtained from fig. 1 

 (p. 298). The Colonsay Limestone, which, with the beds above and 

 below it, constitutes an easily recognizable horizon, occurs on the 

 eastern coast of the island, dipping out to sea at a low angle. If a 

 traverse be made in a westerly or southerly direction across the 

 islands from this outcrop, successively lower beds are passed over, 

 all dipping east or north-east at gentle angles ; and on finally 

 reaching the extreme outlying parts of Oronsay and Ardskinish, 

 there is still no indication of any base to this enormously thick 

 series of sediments. Throughout this whole traverse one has 

 passed continuously from higher to lower beds,and there has been 



