402 THE GEOLOGY OF 



deposition. In the Loch Lomond district and in the western part of Perthshire 

 the groups are met with in apparent descending order as given below : 



10. Blair Atholl limestone and black schists. 

 9. Quartzite. 

 8. Graphitic schist with thin lenticular bands of limestone, clay-slate, and 



calcareous sericite-schist. 

 7. Garnetiferous mica-schist. 

 6. Loch Tay limestone. 

 5. Mica-schists with sills of epidiorite. 

 4. Schistose epidotic grits (green beds). 

 3. Ben Ledi grits. 



2. Aberfoyle slates, with subordinate bands of grit. 

 1. Leny and Aberfoyle grits. 



In Cowal and the district round the head of Loch Fyne, further subdivisions 

 of the crystalline schists have been introduced by the Geological Survey 

 which need not here be given. It is sufficient to state that the groups met 

 with in Perthshire have been identified and traced across the county of 

 Argyll. For example, the Dunoon grits and phyllites are evidently the pro- 

 longations of the grits and slates at Aberfoyle ; the Ben Ledi grit group is 

 largely represented in Cowal and in Kintyre : the green beds (No. 4), mica- 

 schists (No. 5), and the Loch Tay limestone (No. 6) have been traced westwards 

 from the head of Glen Fyne by Glendaruel to Loch Tarbert, and they reappear 

 to the north of Campbeltown. The higher groups (Nos. 8, 9, 10) are evidently 

 represented by the Ardrishaig phyllites and Loch Awe series, the latter com- 

 prising limestones, black slates, grits, and quartzites. 



There can be little doubt that most of these rocks were originally sedi- 

 mentary deposits, for in many instances the original clastic grains may still 

 be recognised. Annelid tubes have been detected in the quartzites of Jura 

 and Islay (No. 9) ; similar structures were detected by the late Duke of 

 Argyll in quartzose-schists near Inveraray, and Mr. Macnair, of the Glasgow 

 Geological Society, has detected what he believes to be worm-casts in certain 

 schists apparently above the Loch Tay limestone. The geological age of these 

 eastern Highland schists (the Dalradian series of Sir A. Geikie) still remains 

 to be determined. These sedimentary rocks have been subjected to a regional 

 raetamorphism which cannot be attributed to any intrusion of igneous rocks 

 now visible at the surface. Indeed Mr. Clough and Mr. Hill, of the Geo- 

 logical Survey, have been led to believe that in the Cowal area the 

 metamorphism increases in intensity from the southeast and from the 

 northwest towards a central anticline of foliation trending in a northeast 

 and southwest direction. 



These crystalline schists are pierced by igneous rocks belonging to different 

 geological periods. One group, which is older than the regional metamorphism 

 of the sedimentary rocks, comprises gabbro, epidiorite, and hornblende- 

 schists ; the other set is later than this metamorphism, and is represented by 

 various igneous masses, as, for example, the plutonic rocks of Garabal Hill 

 near the head of Loch Lomond, and the numerous sheets and dykes of acid 

 igneous materials (quartz-porphyries and porphyrites) in the neighbourhood 

 of Loch Fyne, and in other districts. 



The geological structure of the Silurian tableland of the Southern Uplands 

 is extremely complicated, due partly to the non-fossiliferous character of many 

 of the strata, partly to the inversion of the rocks over wide areas, and partly 

 to the variation in the types of sedimentation ranging from oceanic to shallow- 

 water and shore conditions. The true order of succession has been demon- 

 strated by Professor Lapworth, who, by means of the vertical distribution of 

 the graptolites, furnished a clue to unravel the stratigraphy of the whole 

 region. As a result of the detailed examination of the tableland, it may be 



