£00 jvir. a. harker on the overthrust [Ma)- 1903, 



cut off to the west and south by intrusive rocks, it has a length of 

 a mile and a width varying up to a quarter of a mile. Its actual 

 thickness is not easily estimated, but is probably about 400 or 500 

 feet. This occurrence was noticed by Sir Archibald Geikie. 1 



A description of this crush-breccia will apply also to the smaller 

 patches mentioned above. It is a rock of striking and characteristic 

 aspect. In most places both sandstone and shale enter into its 

 composition, fragments of grey sandstone being embedded in a 

 darker and nearly black matrix which consists largely of crushed 

 shale. There are, however, fragments of shale also, though they 

 are less abundant, while, on the other hand, the sandstone has 

 contributed in varying amount to the comminuted matrix. The 

 fragments usually range in diameter from 2 or 3 inches down- 

 ward, though blocks of larger dimensions are also found. The 

 angles are in general more or less rounded, but the sandstone- 

 fragments have not undergone so advanced a degree of attrition 

 here as at Monadh Dubh : a fact attributable perhaps to the inter- 

 position of the softer shale. It is noticeable that the sandstone in 

 the breccia is mainly of the fine-grained grey variety found in the 

 passage-beds to which I have alluded, confirming the supposition 

 that the breccia is formed in great part by the breaking up of 

 those beds. Immediately overlain by the massive plutonic rocks 

 of the mountain-tract, and invaded, moreover, by two considerable 

 masses of a peculiar porphyritic felsite, the breccia is in most parts 

 more or less metamorphosed ; the sandstone in it being sometimes 

 almost converted into a quartzite, while the shale is much indurated 

 and otherwise altered. It is clear that this metamorphism is 

 posterior to the brecciation, even apart from any evidence as to the 

 Tertiary age of the intrusions. The porphyritic felsite is in some 

 places crowded with fragments picked up from the breccia, and it 

 also encloses a large amount of gabbro- debris in a partly-digested 

 state. 



The limestone which was so noteworthy a feature of the Monadh- 

 Dubh breccia is wanting here, nor have I detected fragments of 

 that rock in any of the breccias in this belt of country ; but Sir 

 Archibald Geikie noted a patch of limestone in Glen Dibidil. 



Following the main surface of overthrust south-eastward from 

 Coire Dubh, we find that a little before reaching Allt M6r na h-Uamha 

 it is cut out by the encroachment of the gabbro, and is lost for 

 about 900 yards. It reappears at the point where the Dibidil 

 footpath crosses the next burn, Allt na h-Uamha, and is there 

 thrown down a little by a normal fault. Its course is now nearly 

 north and south. The shales above are, as usual, highly inclined 

 and contorted, and they are also indurated owing to the proximity 

 of the gabbro. The sandstones below have only a small thickness, 

 being underlain by the shale-group in its natural position, and the 

 dips are quite normal. Indeed, from here to near Dibidil, a distance 

 of 2| miles, the strata below the overthrust show, in general, no 



' ' 1 < Ancient Volcanoes of Great Britain ' vol. ii (1897) pp. 351, 352. 



