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PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



[Dec. 19, 



of strata which, although closely linked together, may he classed 

 into three distinct groups, namely : — 



1. A set of lower grits, of immense thickness. 



2. A great mass of thin-hedded slates. 



3. A set of upper grits, with intercalated seams of slate. 



Beds of limestone* occur here and there sparingly in all the three 

 divisions, the thickest I have met with being situated deep down in 

 the lower grits. These limestones appear to attain their greatest 

 development in the western extension of the strata, thinning out to 

 the eastward. The siliceous grits also seem to become purer and 

 more devoid of green sediment as we trace them to the west. All 

 the members of the series, namely the upper grits, slates, and lower 

 grits, have a persistent strike from S/W. to N.E. (sometimes in Bute 

 approaching to due N. and S.), following one another in conformable 

 order ; and the three groups graduate into each other where they 

 meet, in such a way as to show that they belong to one continuous 

 succession of deposits ; and not only so, but the materials of which 

 they are composed seem to have been derived from very similar 

 sources : beds of the lower grit are often undistinguishable in hand- 

 specimens, or even in mass, from those of the upper, being made up 

 chiefly of water-worn grains of quartz, many of which are of a pecu- 

 liar semitransparent bluish tint. 



I selected the region lying between Rothesay and the Sound of 

 Jura as likely to afford the best insight into the disposition of these 

 rocks, and devoted my attention to the districts of Bute, Cowal, 

 Knapdale, and the line of the Crinan Canalf . In order to render the 

 following pages more intelligible, it is necessary, before going further, 

 to mention that the outlines of the chlorite-series, as laid down in 

 the map of Macculloch, and also in the smaller one of Nicol, are 

 incorrect in some places to a considerable degree. For instance, 

 almost all South Knapdale is brought into the chlorite-series, whereas 

 the coast-section across the strike of the beds from Barmore to In- 

 verneil, being a distance of about six miles, shows no chlorite-slate, 

 but is almost all a highly siliceous grit, often forming indeed a true 

 quartz-rock. On both sides of Loch Fyne also, in the neighbourhood 

 of Otter Ferry, and apparently for some distance up and down the 



* I ought to mention that I did not myself observe any mass of limestone 

 in the middle division, or great body of slates, although it occurs closely asso- 

 ciated with slate amongst the upper grits. See, however, Macculloch, ' Western 

 Isles.' vol. ii. p. 458, where he says that the limestone " peculiarly accompanies " 

 the clay-slate. And Col. Imrie, in the Edinburgh Philosophical Transactions, 

 vol. vi., describing the section along the N. Esk in Kincardineshire, mentions 

 a bed of limestone, 6 feet thick, amongst the slate, and another, 12 feet thick, 

 bet ween two thin layers of black shale. See also an interesting short notice by 

 Prof. James D. Forbes on the geology of the parish of Fordoun, in the ' Statistical 

 Account of Kincardineshire, ' where he mentions a bed of limestone occurring 

 in the midst of the slate, and quarried at a place called Clattering-brigs. Daniel 

 Sharpe seems to have found no limestone in the slate at Brig o' Cally, Birnam, 

 Stratherne, and Aberfoyle, nor in the Loch Lomond section, these being the 

 points where he examined it : see his paper in the Quart. Journ. Geol. Soc, 

 vol. viii. p. 126. 



t A geological sketch-map of the district has been deposited by the author 

 in the Society's Library. 



