part 2] THE SOUTH-WEST HIGHLANDS OE SCOTLAND. 101 



Original order of deposition. — Three reasons can be 

 adduced for regarding the column on pp. 98-99 as arranged in 

 descending order of age, with Lenj Grits at the top and Maol-an- 

 Fhithich Quartzite at the bottom : — 



(1) Beannan Dubh, a little hill halfway along the eastern coast of Islay, 

 is composed of an outlier of typical Portaskaig Conglomerate lying flatly 

 upon an extensive outcrop of Islay Limestone. Sandy cream-coloured dolo- 

 mites are conspicuous as intercalations in the conglomeratic series, and appear 

 to have suffered ' contemporaneous erosion,' yielding fragments to the overlying 

 interstratifications of conglomerate. One of these dolomite -bands, whiter 

 than usual, can be traced for a couple of hundred yards. It has a regular 

 base resting upon shale, and a highly irregular top with cavities choked by 

 downward extensions from an overlying stratum of well-bedded gritty dolomite. 

 Along the jagged junction there is often a foot or two of coarse breccia 

 consisting of angular blocks of white dolomite enclosed in a brown gritty 

 matrix (1917, p. 143). 



There seems no room for doubt, when one is faced with this exposure, that 

 the Portaskaig Conglomerate is here ' right way up,' and therefore of later 

 date than the subjacent Islay Limestone, 



(2) A very suggestive sequence, from grey slate, to black slate, to con- 

 glomerate — charged with black slate-fragments, to quartzite, is found at 

 the southern extremity of Islay. I have described and figured the section 

 (1917, p. 154), and need only repeat that this particular conglomerate appears 

 to be younger than the associated black slate ; and that such an inference, 

 combined with my reading of the local stratigraphy, leads to the further 

 conclusion that the Port-Ellen Phyllites (and therefore also the Easdale Slates) 

 are younger than the Islay Quartzite. 



(3) Clough's evidence for repeated south-eastward movement in Cowal has 

 already been discussed (p. 89). It seems highly probable that his Carrick- 

 Castle Fold (pp. 86 & 103) developed during some phase of these south-eastward 

 directed disturbances. Accordingly, since the fold closes south-eastwards, it 

 is natural to regard it as a recumbent anticline,^ and to expect it to contain a 

 core of relatively old rocks — in other words, to regard the Loch-Tay Limestone 

 and its associates as older than the Ben-Ledi Grits and Aberfoyle Slates. It 

 is only fair, however, to state that this apparently justifiable inference 

 escaped Clough's notice. He did, indeed, refuse to follow the common custom 

 of assuming that the reverse succession had been established ; but his position 

 was entirely negative — in fact, he said that the schists of Cowal had afforded 

 him no clue whereby to determine their age-relations (1897, p. 86). 



Passing reference has just been made to a prevalent opinion that 

 the order of deposition is the exact reverse of that which is given 

 above. I believe I am justified in saying that the sole foundation 

 for this view is a widespread structural superposition of the Loch- 

 Tay Limestone, Pitlochry Schists, and Green Beds upon the Ben- 

 Ledi Grits and Schists in a tract of country reaching from Kin tyre, 

 north-eastwards past Loch Tay. When Sir Archibald Geikie, in 

 his Presidential Address to the Geological Society, spoke of the 

 Loch-Tay region (1891, Proc. p. 73), he said : 



' It is difficult to resist the belief, though it would be premature to conclude, 

 that this obvious and persistent order of succession really marks the original 

 order of deposition.' 



^ The Carrick-Castle Fold closes downwards in its type-exposure, and 

 Clough calls it a ' syncline.' The apparent contradiction is one of terms, not 

 of ideas. 



