﻿160 



THE ISLAY A^TICLIJTE. 



[vol. Ixxii, 



rocks are shown to be divisible into zones of increasing and de- 

 creasing thermal alteration, the speaker pointed out that the thrust 

 drives the outer zone, in which there is no crystallization, on to a 



■zone of high alteration ; the absence of crystallization in a con- 

 siderable part of the Highland rocks of Islay is thus no proof at all 

 of newer age — it is natural to the thermal zone, to which they 

 belong. The limestone is so little altered, that its original oolitic 

 structure is locally preserved despite its great age. 



. . He agreed with the Author that the rocks in Western Islay 

 were of Lewisian age ; but, in addition, they were part of the 

 Highland rocks* or the series of sediments of "which the Moine 

 Gneisses formed a part, and had the more common strike of the 

 Highland folding, north-east and south-west. 



The speaker complained .of -the introduction by the Author of 

 new names for rocks which had long-established names. Thus, in 

 the south-east of the island the - Port Ellen Phyllite ' has long 

 been known to.be the 'Ardrishaig Phyllite ' or ' Canlochan Schist,' 

 This band of shale is known to continue across Scotland, and 

 emerges on the, coast to the south-east of the Portsoy Quartzite. 



.It is more of a true shale- in the latter area, and contains thinner 



- if not fewer sandstone-bands than in Islay. Similarly, the quartzite 

 is the well-known Highland Quartzite, which again crosses the 

 country to Portsoy ; it is also finer and thinner at Portsoy, 

 thicker and coarser in Islay. 



Again, the limestone is the Blair Athol Limestone, which also 

 occurs on both coasts ; the course of it and the Quartzite are fairly 

 well shown on the most recent edition pf Sir Archibald Geikie's 

 10-mile to the inch geological map of Scotland. The limestone in 

 Islay also is affected by changing conditions ;' while the associated 

 dark schist is, for the greater part, more sandy and lighter-coloured 

 than in areas to the north-east. 



0 With regard to the stratigraphy of the area, it is known that the 

 side of the Quartzite on which the limestone occurs is the opposite 

 to that on which the Ardrishaig Phyllite (Port Ellen) occurs. 

 This was proved many years ago, although the evidence has never 

 been published. Inspection of the map already mentioned will 

 show that the Blair Athol Limestone crops out, more or less con- 

 tinuously, for many miles on the north-west side of the first outcrop 

 of the Quartzite. On the south-east side of it another persistent 

 outcrop occurs, of , the limestone known as the Loch Tay ' or 

 Pitlochry Limestone. For many years these were thoiight to be the 

 same limestone ; but, after a short period of mapping, the speaker 

 found that this could not be the case, ' and, after a considerable 

 'argument,' Sir Archibald, Geikie was satisfied that the new view 

 was correct, and that they were stratigraphically on opposite sides 

 of the Quartzite. There was no question of their not being different 

 limestones — that was settled ; it then became incredible that there 

 could be these two limestones continuing for miles, each confined 

 to one side only of the Quartzite, without their being strati- 

 graphically on opposite, sides of that rock. These results were 

 embodied in Sir Archibald's Presidential Address to the Geological 



