E0CXS OF THE NOETHEEX ETIGHLAJIDS. 4-13 



dolomite, which is. covered by overthrown gneiss, is, in the former, 

 less than 30 feet, and in the latter not more than 10 feet in thick- 

 ness. I cannot, however, positively assert that this thinning of the 

 dolomite is not dne to faulting. 



(2) The denudation along the line of the great dislocation has been 

 very great. The outliers of quartzite which lie upon the Arnaboll gneiss 

 are the base of the series, and not over 30 feet thick ; but within a 

 mile to the west the Quartzite and Dolomite are in full force. The 

 Dolomite and Flags have been preserved within the synclinal fold ; 

 but as we ascend the ridge we find the different groups cleared away 

 one after the other till, at the summit plateau, both in the outliers 

 and the main mass we have only the basement beds. In Assynt, 

 also, strata higher than the Quartzite are rarely found on the pro- 

 minent ground ; and their occurrence at lower levels is due to favour- 

 able circumstances, such as being enclosed in synclines, surrounded 

 by durable strata, as in the Stronchrubie basin. Towards the Eastern 

 Gneiss the Quartzite itself is usually partially thinned away, the 

 lowest beds only remaining. Near TJllapool the evidence of exces- 

 sive denudation is very clear. There are, near Loch Auchall, broad 

 bands of faulted Torridon Sandstone intervening between the Cale- 

 donian and the upper members of the Assynt series. We know 

 that this Torridon was once covered by 500 or 600 feet of quartzite, 

 flags, and dolomite, which have been clean swept away. 



That areas of dislocation should be subject to great denudation is 

 only natural when we reflect that upheaval and depression tend to 

 repeat themselves at successive epochs along lines of weakness. The 

 chain of the Wrekin, for example, a mere wedge of the Archaean 

 floor, thrust up at a fault, has been alternately rising and sinking 

 since Cambrian times. In like manner the Caledonian at the faulted 

 junction with the Assynt series has probably been elevated and de- 

 pressed more than once since the Ordovician epoch. The alterna- 

 tion, then, of marine with subaerial action would be so effectual 

 that the presence rather than the absence of fragments of a forma- 

 tion about 100 yards thick would be matter for surprise. This view 

 is greatly strengthened by the present elevation of the Caledonian. 

 All along its junction with the newer rocks on Emboli, in Assynt, 

 and near Ullapool, it forms high ground, frequently rising into lofty 

 mountains. Either, therefore, the quartzite which once rested on 

 it must have been raised up some 300 or 400 feet at least once, 

 during which time the quartzite constituted the capping of the 

 escarpment formed by the upthrow side of the fault, and would 

 suffer the usual degradation of escarpments ; or the gneiss had been 

 carved into something like its present features before the Assynt 

 epoch, in which case its superior elevation would account for the 

 attenuation of the quartzite. 



If still it should be contended that outliers of the quartzite upon 

 the Caledonian ought on my hypothesis to be more extensive, I may 

 call attention to the parallel case of the Penine fault. The great 

 escarpment which overlooks the valley of the Eden along a line of 

 20 or 30 miles, is mainly composed of Carboniferous rocks, the 



