206 ME. A. HAEKEE ON THE OVEETHEUST [May 1903, 



banding is truncated by later intrusions, for the same thing is seen 

 in other places where the gneiss is in contact with sandstones or 

 shales. Gneissic banding and foliation, though of the nature of 

 fluxion-structures, are not necessarily parallel to the boundaries of 

 the mass. 



Prom the Dibidil district we obtain evidence of a more positive 

 nature. Here the main body of gabbro has retired somewhat 

 inland ; but, as elsewhere, there are numerous small intrusions of 

 that rock in the country fringing the mountains. These intrusions 

 take the form of lenticles and dyke-like strips, some of which are 

 closely associated with the gneiss, in several places forming an 

 inconstant border to it. The strip of gneiss which occupies the 

 lower part of the Dibidil River is partly bordered by gabbro on 

 both sides, and again at its southerly termination. The relation 

 suggested is that the gneiss has been intruded along the side of, 

 and partly through, the gabbro, just as granites have been intruded 

 beside and partly into gabbros at many other places in the Inner 

 Hebrides. Where the junction is clearly exposed, direct proof of 

 this sequence of intrusions is found. At a place on the left bank of 

 the river, a little way below the ford, the gabbro is seen to be pene- 

 trated by tongues of gneiss running out from the main body, as well 

 as by narrower reticulating veins consisting chiefly of the pegmatoid 

 rock which here, as elsewhere, is a component element of the gneiss. 

 At the same place, detached blocks of unmistakable gabbro are 

 enveloped in the gneiss itself. This gneiss is quite typical, and the 

 gabbro is indistinguishable from that of other intrusions in the 

 district. Here, then, we seem to have ocular proof of the conclusion, 

 already foreshadowed by other considerations, that the gneissic 

 rocks of Rum belong to the great Tertiary suite of eruptions. The 

 only alternative, with the evidence before us, is to assume the 

 existence of an older group of gabbros identical in appearance 

 with the Tertiary gabbro in the immediate vicinity — an artificial 

 hypothesis not supported by any known facts. 



Another enquiry, not yet touched, relates to the very significant 

 distribution of the gneissic intrusions. The fact that all the 

 occurrences noted are associated with the highly-disturbed belt of 

 country suggests some connection between the gneissic structure 

 and the crust-movements ; of course, those of Tertiary date, which 

 recurred, as has been noted, in places already affected by the much 

 more intense Palaeozoic disturbances. A like conclusion is enforced 

 by an examination of the strike of the gneissic banding and foliation, 

 which is not concordant with the boundaries of the several intrusive 

 bodies, but seems to obey some larger law. The dips, almost 

 always at high angles, aie northerly in the central part of the 

 island, inclining rather towards north-north-east on Beinn a' 

 Bharr-shaibh. In the small intrusion of gneiss on the lower slope 

 of Beinn nan Stac the dip is west-south-westerly. Rapid changes of 

 strike and dip are found only in the Dibidil neighbourhood, where 

 the older system of crust-movements had been especially vigorous, 



