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PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Dec. 19, 



of quartz is also encrusted in some places with beautiful little cry- 

 stals of carbonate of iron or sparry iron-ore. Its course is nearly 

 vertical, with a strike about N. 25° W. It had been wrought in 

 former times as a mine, and is now opened anew. Other veins of a 

 similar nature, with some ironstone, are known to occur in the lower 

 grits between Inverneil and Barmore ; and I heard that a vein of 

 lead had once been opened, at the rocky point between Loch Gilp 

 and Loch Pyne, and also near Dunartry, in the upper grits. 



§ 7. I examined the limestones that came under my notice for 

 fossils, more particularly the thick calcareous grit of Otter, which 

 seemed a likely rock for containing them ; but in none did I perceive 

 anything of the kind. In the weathered surface of some of the 

 quartz-grits in Knapdale, both above and below the slates, I noticed 

 numerous circular cavities, and also some curious elongated stripes 

 of coarser sand, which suggested the idea of Annelide-burrows ; I 

 could not, however, satisfy myself as to whether such was actually 

 their origin, and must leave it to the decision of those who know 

 more about these matters. 



The period at which these old beds of sand and mud had been 

 thrown into undulations and changed into quartz-rock, mica-schist, 

 and slate is evidently veiy remote ; for the Old lied Sandstone 

 conglomerate in the shore beside Rothesay is seen to be made up of 

 the debris of these strata ; and it is important to remark that its 

 water-rolled pebbles of siliceous grit, mica-schist, and slate have 

 quite as metamorphic an appearance as the parent rocks from 

 whence they were derived have at the present clay. Many of these 

 imbedded fragments show the same contorted, wrinkled, foliated 

 structure, and are traversed by the same ramifying veins of quartz 

 (which had solidified before the water-rolling of the fragments), and, 

 in short, are altogether identical in their mineral complexion with 

 the present features of the rocks I have been describing. Their 

 metamorphism must therefore have been completed before the Old 

 lied conglomerate began to accumulate ; and this leads to the con- 

 clusion that a great chasm intervenes between the era when these 

 old rocks were formed and that of the Old Red Sandstone, and brings 

 us to assign to them a date anterior to that of the upper Silurian 

 beds. They do not appear to resemble, in mineral features at least, 

 either the " Cambrian sandstone," or the " fundamental gneiss " of 

 Sir Roderick Murchison, described by him as occurring in the North- 

 west Highlands ; wliilc they seem to bear a striking resemblance to 

 the quartz-rocks, limestones, and mica-schists of Sutherlandshire, ifcc., 

 shown by the same geologist to be of Lower Silurian age, — the chief 

 difference being the presence of the bedded greenstone, which, how- 

 ever, is, as I have shown, a local phenomenon, being absent in Bute. 

 So far, therefore, as lithological appearances are entitled to weight, 

 there is reason to believe them to be of similar date to those rocks of 

 the North-west Highlands. However, as mere mineral features 

 alone form a very unsafe criterion in such cases, Ave must look either 

 for evidence proving the physical synchronism of these beds of 

 Argyleshire with the North-western types of Murchison, or, still 



