Yol. 59.] TOKRIDONIAN, ETC. OF THE ISLE OF RUM. 19£ 



direction. In parts of its course it is merely a band of bleached and 

 metamorphosed sandstone with more or less evident brecciation ; but 

 in other places part or the whole of its width has been injected with 

 the basalt-magma, and reactions have ensued between the two 

 rocks. Sometimes the igneous rock forms a matrix exceeding in 

 amount the sandstone-fragments which it encloses. This band or 

 dyke ranges up to 6 feet in width, and in one place it bifurcates. A 

 snorter band a little farther north reaches locally a width of 50 feet, 

 but only a small portion of this width, on the western border, is 

 injected with basalt, although the sandstone is conspicuously meta- 

 morphosed throughout. Here, as in other cases, the effects are 

 narrowly localized, the sandstone immediately bordering the crush- 

 band showing no perceptible alteration. 



Whatever be the date of the crushing, it is reasonable to assume 

 that the igneous injection is of Tertiary age, as are the numerous 

 basalt-dykes in the same district. The reactions noticed are pre- 

 cisely like those observed by the writer in many Tertiary dykes in 

 Skye, where a basaltic magma has enclosed quartz of extraneous 

 origin. The special interest of the occurrences here described lies 

 in the very clear evidence of metamorphism, unmistakably of 

 thermal type, in many parts of these crush-bands where no igneous 

 rock is visible. To suppose that the present surface of the ground 

 happens to coincide almost exactly with the upward limit of the 

 basaltic injections would be a highly artificial hypothesis ; for the 

 instances are numerous, and occur at altitudes varying from 100 to 

 800 feet within a distance of three-quarters of a mile. The pheno- 

 mena described rather suggest that, under certain conditions, notable 

 metamorphism may be effected by some kind of solfataric agency, 

 operating along vertical bands of rock disintegrated by crushing. 



II. The Monadh-Dubh Overthrust. 



Passing now to the more highly disturbed districts, we turn, first, 

 to the small area in the north-western part of the island, where the 

 hilly moorland named Monadh Dubh rises to an altitude of 

 about 800 feet. A part of this is made by a cake of overthrust 

 rocks, measuring about 1 mile by two-thirds of a mile, resting on 

 the ordinary red sandstones of the district and cut off to the north- 

 west by the sea. The present limits and general disposition of the 

 overthrust mass are well displayed, the boundary being marked by 

 an escarpment, which runs round from coast to coast, but is most 

 prominent on the southern and south-eastern sides. The eastern 

 boundary is highly irregular in ground-plan, depending on the 

 details of the surface-relief, while the general inclination of the 

 overthrust-surface does not here differ greatly from the slope of the 

 hill. The base of the overthrust cake of rocks, marking the main 

 surface of movement, inclines to the north-west, the dip being gentle 

 in the higher part, but increasing seaward to about 20° (see section, 

 fig. 2, p. 194). 



