﻿164 



the islay anticline, 



[vol. lxxiL 



' Dr. Clough's view that the Lewisian gneiss and unaltered Torridonian 

 rocks of Iona are separated frOm "the highly crystalline.. Moine Schists of the, 

 Ross of Mull by the Moine Thrust is quite in accordance with the phenomena 

 connected with this line of movement in the north. 



' I also agree with 'Dr. Clough's suggestion that the position of the Great 

 Glen Fault lies between the Ross of Mull and Colonsay, on the ground that 

 the strata on the j downthrow side of ^he faidt are in a low grade of meta- 

 jnorphism, compared, with the Moine Schists of the Ross of Mull and Morvern," 

 which existed as crystalline schists before the intrusion of the igneous rocks 

 by which they are now traversed.' 



The contributions by Mr. Barrow and Dr. Peach were forwarded 

 ■|eo the Author, who sent the following reply : 



'I believe that there is good foundation for some of Mr. Barrow's views 

 regarding the equivalence of rock-groups found in Islay and Perthshire 

 respectively. In fact, a comparison was early initiated by Macculloch. On 

 the other hand, I think that Mr. Barrow employs a very dangerous method 

 when he confidently interprets the^Islay structure and succession in the light 

 of his Perthshire experience. He comes to the conclusion, for instance,, that 

 Central Islay is of synclinal structure, although, in my opinion, it Is easy to 

 demonstrate the reverse in the field. He also regards the Islay Limestone as 

 of later date than the Quartzite, although this, too, is contrary to the local 

 evidence. : 



' I do not think that the complexity of the Highland problem has been 

 sufficiently appreciated in .the past. Before a satisfactory general solution is 

 attained, I am convinced that we shall have to i accumulate careful fieldT 

 observations for several years to come. : 

 The points raised by Dr. Peach are best considered seriatim -. — 



1-. I consider the sections along the northern coast of Islay to be quite con- 

 vincing in regard to structure and succession ; and I believe that the descrip- 

 tions given in my paper will be found to be accurate. In answer to the 

 criticism that I have to invoke inversions north of Loch Skerrols, I should like 

 to point out that this is not a part of the district where one would attempt to 

 decide the anticlinal or synclinal structure of the Islay Fold as a whole. It is 

 on a flank, whereaS'the northern coast exhibits the axial region. , 



2. Dr. Peach's second criticism is based upon the claim that the source of 

 certain boulders contained. in the Scarba ^Conglomerate in Scarba and Lunga 

 can be identified on the local u evidence. As a first suggestion, I think 

 Dr. Peach's interpretation of Scarba excellent ; but, in the light of the fuller 

 evidence afforded by Jura and Islay, I am convinced that it now needs 

 modification. . , 



3. I mapped black slates as occurring in Shuna when 1 1 accompanied 

 ; Dr. Peach to that island within a few months of my joining the Geological 

 Survey. Dr. Peach at the time told me that a particular belt consisted of 

 " bleached black slate." Since then I have revisited Shuna, and compared 

 the rocks in question with such small areas of bleached black slate as are 

 sometimes found immediately adjacent to outliers of Old Red Sandstone 

 farther north, and am convinced that a mistake was made. 



1 4. The reason why a detailed explanation is. not given, is. that it is impos- 

 sible to decide between the various alternatives that: are available. 



5. The evidence upon which my conclusion is based is given in my paper, . 



: - 6. All along the Bowmore shore the sandstone is thoroughly fine-grained, as 

 stated in my paper. 



7. It is not denied in my paper that the Bowmore Sandstone may have been 

 rightly identified on the shore near Gortan scheolhouse. The discussion of 

 the evidence for the Loch Gruinart Fault explicitly includes this possibility. 



8. The absence of highly crystalline schists immediately above the Loch 

 Skerrols Thrust is emphasized in my paper, and is taken into account in con- 

 sidering the possible equivalence of the Loch Skerrols and Moine Thrusts.' 



[September 10th, 1917.] 



