AKD GRAVELS OF ARDTUN, ETC., IN" MULL. 



287 



contours about the river-gravels shows high ground to the south 

 and east, coinciding with the boundaries of the Traps, with the river- 

 channel roughly following the present outcrop of Palaeozoic rocks in 

 the Boss *. A spur of gneiss from Benmore, represented by the 

 gneiss of Gribun, Erisgeir, and Inch Kenneth, directed its course 

 westward, and we have seen that there is but a slight trace of it along 

 the shore of Torosay. The actual river-bed is traceable for a little 

 over 9 miles, and it ran in a north-west direction. There is but little 

 hope of tracing it further in this area, unless fragments of its bed 

 should by chance be preserved in the Isles of Bac Mor, the Dutch- 

 man's Cap, or in Lunga. I succeeded in getting near enough to the 

 latter to see that there, at least, this is improbable, but continued high 

 and adverse winds frustrated my efforts to reach the former. When 

 we consider, however, that the preservation of Ardtun itself is mainly 

 due to the accident that an intrusive sheet of great hardness runs 

 through it and buttresses it at both ends, and that the presence of 

 gravels would be a source of weakness and facilitate disintegration, 

 we see that it is far more likely that its course is now under the 

 sea than in the few islets that still remain above water. Indeed the 



different quality, one in the centre and one at either end, in a direction almost 

 parallel to the Sound of Mull. [That at the southern end, though provisionally 

 called by the field-term of " felstone," is a hard, compact, black rock, weathering 

 with a smooth white crust ; under the microscope it shows a fine-grained matrix 

 with porphyritic augites, and what appear to be numerous inclusions of another 

 rock. Its specific gravity is as high as 2 - 90. — Gr. C] It is probable that the 

 syenite would long since have crumbled away but for the protection of the 

 dykes, which weather slowly and act as supports. The mountain on the opposite 

 side of Loch Ba is of precisely similar structure ; but I omitted to state that 

 there is in the first also a small dyke of gabbro and one of dolerite traversing 

 the syenite. It seems likely that the crater of which Benmore is a fragment 

 was formed at a later date, and after the Trap eruptions had ceased. So far as 

 I could make out from this particular section, the syenite or granite region 

 may have been a cluster of hills altogether outside any crater, and the felstone 

 dykes may belong to any period antecedent to the Tertiary. It is impossible to 

 say whether the Boss-of-Mull granite, which is red, is in any way connected 

 with the syenite of Benmore, but there are boulders of a grey granite lying 

 about, which seems, in some respects, intermediate between them. The Boss-of 

 3Iull granite is perhaps not far off at a low level and may be concealed by talus 

 or vegetation; for the whole district as well as the shores of all the Lochs and 

 of TJlva and Morvern are strewn with fragments of it. To suppose that this 

 was carried by ice from the low ground of the Boss to the south-west during the 

 Glacial period involves physical difficulty, while in that case it must have been 

 accompanied by blocks of the intervening gneiss, which does not appear to me 

 to be the case. 



* It is a question whether the line of junction between the Traps and gneiss, 

 west of Bunessan, is marked by a fault. At Ardtun we are close to the base of 

 the Traps. On the coast, 6 miles due west of Carsaig, the actual junction shows 

 the basalt with slightly upturned edge against the gneiss, which is perfectly 

 riddled by small veins There are no intervening secondary beds as shown in 

 Macculloch's section. The manner in which the edge of the gneiss is penetrated 

 by the Trap appears an argument against a fault, and we everywhere see 

 evidence that the secondary strata were not only very locally and unequallv 

 deposited in this area, but that only patches of them remained at the time of 

 the first Trappean eruptions. A downthrow of from 100 to 200 feet would in 

 any case bury their entire thickness, and a fault of 1000 feet is unnecessary for 

 the purpose. 



