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PKOCEEDINGS OP THE GEOLOGICAL SOCIETY. 



[Dee. 19, 



subordinate to the grits, which, with the greenstones, form the 

 predominating features in the geology of this tract. From this to 

 the head of Loch Swen, the dip of the strata becomes nearer and 

 nearer to vertical ; and in the neighbourhood of that loch the general 

 position of the strata is almost upright, leaning occasionally to S.E., 

 and occasionally to NW., but always striking from SW. to N.E. ; 

 and to the west of Loch Swen the character seems to be a mere 

 repetition of those I have been describing. We find the same thick 

 beds of grit and greenstone, accompanied by subordinate seams of 

 slate and indications of limestone, still dipping at high angles ; but 

 the inclination is now found to set in to S.E. I therefore think that 

 here we have a synclinal trough whose axis lies in the direction of 

 Loch Swen ; and, from the great apparent thickness of the strata, 

 even allowing for many repetitions, I conceive that we have in this 

 district a quantity of beds belonging to the upper grits beyond what 

 is found in Bute, where they terminate abruptly at the border of the 

 Old Red Sandstone. 



§ 5. Erom the descriptions and maps of Macculloch, in his capital 

 work on the geology of the Western Isles, it appears that the 

 easterly dip is maintained all through the islands of Shuna, Luing, 

 and Scarba, on to Jura, subsiding gradually to lower angles, and 

 exhibiting a descending series of grits and greenstones, followed by 

 clay-slate, mica-slate, and quartz-rock, in the very same order as I 

 have described them in Knapdale ; and, although I had not time to 

 visit those islands, I have little doubt that in the quartz-rock of 

 Jura we have the western extension of those great siliceous masses 

 that form the lofty ridges of Sliabh Goil and Meal Dhu, between 

 Loch Tarbert and Loch Killisport. The clay-slate forming the 

 eastern border of Jura, and ranging north-east through Scarba, 

 Lunga, and Luing, on to Eisdale, I have as little hesitation in say- 

 ing, represents the slaty beds of Bute, which, rolling over the anti- 

 clinal fold, plunge under the upper grits of Cruach Lussa, and re- 

 appear again to the westward in the reversed curve, troughing, as it 

 were, these upper grits ; and if the limestone of Islay, Garveloch, 

 and Lismore lies at the base of the Jura quartz-rock, we have that 

 also paralleled by the calcareous masses of Otter, Barmore, and 

 West Loch Tarbert, which lie beneath the quartz-rock of these 

 localities. 



With regard to the thickness of these old rocks, taking either 

 side of the anticlinal axis, their enormous dimensions are equally 

 apparent. The mass of lower grits, judging from the Bute section, 

 must be many thousand feet thick ; and the group of thin-bedded 

 slates in that island has an apparent thickness of 2000 or 3000 

 feet ; while the upper grits, although far short of the dimensions 

 they seem to attain in Knapdale, form also a considerable body of 

 strata. 



§ 6. The greenstones, which I have so often mentioned, form a 

 most remarkable feature in the geology of Knapdale. They attain 

 their greatest development among the upper grits of the synclinal 

 trough, but are not confined to them, and alternate with the strata 



