﻿part 2] 



THE IS LAY ANTICLINE. 



163 



on its west side, of the characteristic band of congloinera e conti. ining 

 granitoid pebbles near the top of that zone. The fault separt t 

 .a' Chotain quartzite from the dolomitic beds on the west does i 

 reasoning. If there is an upward succession from the doloniiti 

 the main quartzite from this supposed inlier, it can only be found < 

 margin, where the dolomitic group with a south-eastward dip is ex} 

 beach followed by the quartzites of Rudh a' Mhail. But here thi 

 beds and the quartzites are traversed by shear-lines, showing thatt 

 relation has been disturbed by movement. 



' The evidence at the western margin of the dolomitic group of a. > 

 passage into the quartzites of Rudh a' Bholsa on the western limb of 

 pound arch is also unsatisfactory — because near the margin of the qua 

 the section is interrupted by a fault forming a wide shatter-belt. East oi <r- 

 fault, pebbly quartzites occur for a few yards in contact with the do] 

 group ; but, in my opinion, these pebbly quartzites may represent the boulder- 

 bed near the top of the lower quartzite. 



1 As regards the Author's statement that the Islay Anticline is app: 

 from the dips hi his main quartzite-belts, too much reliance ought not t< 

 placed on this line of evidence in so highly folded a region. In his own map 

 and sections he is obliged to invoke inversions of the strata north of Loci 

 Skerrols, where his so-called 'main (upper) quartzites ' dip south-eastwards ae 

 if passing, in normal sequence, below the dolomitic group ! 



' 2. According to the Author's hypothesis, Ins main quartzite is overlain by 

 the Jura black slates, and these in turn by the Scarba Conglomerate Group in 

 Islay, Jura, Scarba, and Lunga. If there is a succession upwards from the 

 main quartzite in Islay and Jura, it does not extend into Scarba. For, in 

 Scarba and Lunga, the Scarba Conglomerate contains large masses of the black 

 slates, limestones, and pebbly grits lying to the east of it. If the black slates 

 of Scarba and Lunga belong to the Luing and Easdale black-slate group, 

 there can be no upward succession from the Scarba quartzite into the Easdale 

 black slates. 



'3. The Author states that black slates do not occur in the island of Shuna, 

 between the Craignish phyllites and the Shuna limestone. Black slates 

 occupying this position were officially mapped by him at the southern end of 

 the island, though now repudiated by him. In my opinion, black slates 

 occupying this position occur not only at the southern end of the island, but 

 in the middle and at the northern end, where they were formerly wrought. 



' 4. In the quartzite-belt north of Loch Skerrols, and in the belt between 

 Port nan Grallan and the Sound of Islay, the lower and upper (or main) 

 quartzites are represented in his map and sections as coming together, without 

 the intercalation of the dolomitic group. No exjDlanation of this feature is 

 given by the Author. 



' 5. As regards the Maol an Fhithich Quartzite, which is supposed by the 

 Author to underlie the Mull of Oa Phyllites, I think that his conclusion is 

 erroneous : for the junction, where exposed at the northern end, is certainly 

 a line of faidt. In my opinion, it is merely a fatdted portion of the same 

 quartzite as that which overlies the Loch Skerrols Thrust, north of Bridgend. 



' 6. The Author refers to the Bowmore Sandstones south-east of Loch in 

 Daal as a fine-grained series, but in that very area there are marked pebbly 

 bands to which reference is made in the Islay Memoir (p. 27). 



' 7. The occurrence of Bowmore grits and sandstones west of Loch in Daal 

 has been questioned by the Author. These grits are to be found on the shore 

 near G-ortan schoolhouse, 1 mile north, of Bruichladdich, where they show 

 the same stage of deformation as the other rocks of the Rhinns, facts which 

 have an important bearing on the position of his supposed Loch Gruinait 

 Fault. 



' 8. Regarding the Author's suggestion that the Loch Skerrols Thrust may 

 represent the Moine Thrust of the North- West Highlands, I wish to call 

 attention to the fact that the rocks above and below this line of disruption in 

 Islay are in the same low grade of metamorphism — a feature quite unknown 

 in any region where the Moine Thrust has been mapped. 



