214 ME. A. HARKER ON THE OVERTHRUST [May I903, 



cause lower parts of the Torridonian to rest on higher parts of the 

 same series. On a smaller scale, too, the mechanical re-arrangement 

 of the rock-masses affected is not of the most extreme type, being- 

 limited in general to contortion and brecciation. The mechanical 

 conditions controlling these several processes form a subject of 

 enquiry outside the scope of the present communication. I have, 

 however, pointed out that the Torridonian strata forming the 

 northern half of Rum, which I regard as, in a general sense, in 

 their natural position, evince nevertheless abundant indications of 

 disturbance on a small scale ; and from this we may not improbably 

 infer the existence of an important surface of displacement at no 

 great distance above the present surface of the ground. 



This inference is in some measure strengthened by the occurrence 

 of an isolated patch of overthrust and highly disturbed rocks on 

 Monadh Dubh, in the north-western part of the island. We cannot, 

 however, assume that the surface of displacement in this place 

 is identical with that of the mountain-border. Both the tectonic 

 arrangement and the induced rock-structures seem to point to a 

 higher order of disturbance ; for the Cambrian limestones have here 

 been involved with the Torridonian rocks, and, in addition to 

 brecciation of a more advanced type than before, there has been a 

 shearing of the sandstone to produce a schist or mylonite. The 

 actual disposition of the rocks, as described above, has suggested 

 that the overthrust seen on Monadh Dubh may be subordinate to one 

 of more imposing magnitude at a somewhat higher horizon. 



In Rum, as in Skye, the mapping proves that the ' thrust-planes ' 

 have in some places been bent into bold curves ; and this folding is 

 most probably referable, at least in great part, to a Tertiary epoch. 

 It is certain that in both islands there was a belated and relatively 

 feeble revival of crust-movements at a late date ; and, though we 

 cannot point to Tertiary overthrusts, we find locally evidence of 

 considerable brecciation and comminution of rock-masses at a time 

 posterior to the great plutonic intrusions. In this way must be 

 explained the local brecciation of the gneiss near Dibidil. 



The second principal object of the present communication is to 

 draw attention to a group of igneous gneisses in the central and 

 south-eastern districts of Rum, to establish their Tertiary age, and 

 to point out how the strong gneissic banding which they so fre- 

 quently display has originated. As regards the question of age, we 

 have seen that the rocks are intrusive in the disturbed Torridonian 

 strata, that they are newer than the great system of crust-move- 

 ments, and that they are found to send veins into the Tertiary 

 gabbro. The evidence upon which this last statement rests has been 

 observed only at a single locality, but it does not seem seriously 

 open to question. There is further the consideration that where 

 the gneiss is contiguous with the ordinary granite (or granophyre) 

 of the western district, there appears to be a passage from the 

 one rock into the other. Petrographically the purely-acid portion of 

 the gneissic complex, which itself is not banded, is identical with 

 the prevalent type of the admittedly-Tertiary acid intrusions; while 



