46 STKATIGKAPHICAL GEOLOGY 



The four lower divisions represent an expansion of the Diabeg 

 Group, and it will be noticed that they include two important 

 developments of shaly deposits. From the facts recorded in the 

 Geological Survey Memoir it may be inferred that the Torridonian 

 Series originally presented a succession of overlapping beds, the 

 lower members thinning out northwards beneath the higher beds 

 as indicated in the diagram, Fig. 5. In other words we may sup- 

 pose that deposition began in the south and extended northwards 

 as land in that direction was gradually submerged. 



Small patches of Torridonian sandstone occur in the island of 

 Lewis, and show that the series had a western extension ; outliers 

 of it have also been detected beneath the Cambrian rocks of East 

 Sutherland, so that it must originally have occupied a very large 

 area, and probably extended beyond the shores of Scotland both 



Fig. .0. DIAGRAM OF THE ORIGINAL SUCCESSION OF TORRIDONIAN DEPOSITS. 



3. Aultbea Group. 2. Applecross Group. 1. Diabeg Group. 



on the east and 011 the west. It was, however, to a large extent 

 destroyed and broken up into isolated tracts during the time which 

 elapsed before the deposition of the lowest Cambrian. 



The most southerly occurrence of the Torridon Series in Scotland 

 is in the Isle of Islay, where there is an interesting and suggestive 

 collocation of Archtean rocks. 11 Hebridean and Torridonian are 

 only found in the western part of the island, while the eastern and 

 larger part is occupied by two groups, quartzites and schists, one of 

 which is a continuation of the Grampian Series. The two tracts 

 are separated by a thrust-plane which runs from Loch Gruinard on 

 the north, through Loch Skerrols and near Bridgend, to the east 

 side of Laggan Bay on the south. The Torridonian sequence 

 resembles that of Skye, having an epidotic grit and conglomerate 

 at the base, overlain by a thick group of grey slates and grits, 

 which are succeeded by the Bowmore grits, a series of red, green, 

 and grey felspathic grits or arkoses like those of the Applecross 

 Group. Owing to intense plication no estimates of thickness can 

 be made. Moreover, the whole series has been affected by dynamic 

 metamorphism and converted into slates, phyllites, and more or 

 less schistose grits. 



