Vol. 59.] TORRIDONIAN, ETC. OF THE ISLE OF RUM. 213 



Loch Gainmhich. The basic rock here forms a mere seam a few 

 inches thick, and has presumably been more vulnerable to the 

 invading acid magma. It is of rather coarse texture, and has a 

 pronounced fissile structure, due to the parallel disposition of the 

 crystals of hornblende, which is again the chief constituent [10495]. 

 These crystals, ranging up to a quarter of an inch in length, are of 

 ragged shape and of green colour. Orthoclase becomes here a more 

 abundant element, while the accompanying striated felspar seems 

 to be an oligoclase. Some quartz occurs, but only in the interior 

 of the hornblende, where also strings of magnetite-granules and a 

 little brown mica seem to have resulted from a certain corrosive 

 action. 



It is doubtful whether the sedimentary rocks have contributed 

 in any sensible degree to the composition of the gneisses. At a 

 junction of gneiss with pebbly grits to the east of Priomh-loch 

 Mor there is evidence of a certain amount of incorporation of the 

 latter rock in the former, pebbles of quartz being recognizable in 

 the gneiss for a short distance from the actual contact. The 

 felspathic sandstone, which is the common type in the Torridonian, 

 would probably be more easily attacked than a purely quartzose 

 rock, and there is evidence at several localities in Hum that the 

 sandstone has been partly fused in contact with an intrusion. 

 Mr. Clough has noticed a like effect on the edge of peridotite- 

 intrusions on the Isle of Soay, where a certain mingling of the fused 

 sandstone with the ultrabasic magma can be verified. All my 

 observations go to show, however, that any such action is of 

 exceedingly limited extent. Again, at certain junctions of the 

 gneisses with Torridonian shales it may be seen that the igneous 

 magma has penetrated in thin leaves for a short distance along the 

 laminae of the shale, the latter being highly metamorphosed. I 

 have found nothing to suggest that these local effects have any 

 significance in respect of the origin of the gneissic rocks. It is at 

 least certain that merely metamorphosed sediments form no part of 

 the complex, which consists wholly of rocks crystallized from igneous 

 fusion. 



VI. Summary of Conclusions. 



In conclusion I will indicate summarily the chief results to be 

 deduced from the observations recorded above. It has been shown, 

 firstly, that the highly- disturbed region of the North- Western 

 Highlands, extending into the south-eastern part of Skye, is further 

 prolonged into the Isle of Rum, where a belt of overthrust strata 

 borders the principal high ground. It is perhaps not impossible 

 that the main surface of movement here corresponds with the great 

 ; Moine Thrust,'* which Mr. Clough has traced through the Sleat 

 district of Skye to within about 10 miles of our ground. It is 

 however, more probable that we have to do here with one of .the 

 less extensive displacements beneath, and in advance of, the great 

 one. Its only effect, technically, in the mountain-border is to 



