﻿Vol. 64.] THE TWO EAETH-MOVEMEjSTTS OP COLOISrSAT. 29^ 



no repetition of any importance. In the northern part of Colonsay 

 the relations are slightly more complex. The outcrop of the lime- 

 stone mentioned above skirts the east coast of the island for some 

 distance, and then passes out to sea north of Scalasaig. A traverse 

 made thence to Kiloran Bay passes at first over successively lower 

 beds dipping south-eastwards, and then the dip is reversed and the 

 same series is repeated in ascending order until the limestone is 

 once more reached. The anticline thus crossed has a north-easterly 

 trend and brings to the surface along its axis the rocks of the 

 Kiloran and Millbuie Groups which underlie the limestone. From 

 the manner in which the limestone circles round Kiloran Bay, it is 

 clear that the latter here occupies the centre of a synclinal basin. 

 Finally the north end of the island has an anticlinal structure and 

 a mass of gneiss, presumably of Lewisian age, occupies the centre 

 of the fold which has a north-east trend. It has proved impossible 

 to determine whether this is part of the old irregular floor on which 

 the Torridon sediments have been laid down, or whether its relations 

 to these sediments is due to thrusting. 



It will thus be seen that the structure is, broadly speaking, not 

 at all complex. Over the greater part of the islands, we find a 

 single conformable sequence of sediments dipping east and north- 

 east at gentle angles, and in the extreme north the upper portion 

 of the same series forming a marked synclinal basin flanked by two 

 anticlines. 



II. The Ign-eous Eocks. 



The igneous intrusions of Colonsay and Oronsay are best described 

 under three heads : — 



(a) Three main plutonic masses and several smaller ones. 

 {b) An extensive series of lamprophyre dykes and sheets, 

 (c) A number of Tertiary dykes of basalt. 



(a). — Of the plutonic masses, that with which we are most con- 

 cerned in the present paper lies on the northern side of Kiloran Bay. 

 It is in its central portions a hornblende-syenite, but exhibits 

 towards its margins a basic phase, which is perhaps best described 

 under the name of hornblendite. It is intruded in the centre of 

 a remarkable siliceous breccia, the material of which it has caught 

 up and incorporated in its mass. The breccia contains, besides 

 fragments of the neighbouring Torridonian sediments, both angular 

 blocks and well rounded water worn boulders of pebbly quartzite. 

 There is not space for entering here into a discussion of the rather 

 difficult question of its origin. It will be sufficient to say that 

 there are two possibilities with regard to it. It may be of the 

 nature of a plutonic breccia formed during the intrusion of the 

 syenite, the waterworn boulders being derived from some con- 

 glomeratic bed lying below; or it may be a subaerial breccia formed 

 m a fissure or pipe, into which the waterworn boulders and other 

 material dropped from above. It must be admitted that the lattei' 



