136 Reports and Proceedings. 



to indicate this order. It was, therefore, a great pleasure during the 

 last summer to see the phenomenon so well sustained by a fresh 

 appeal to the successive rock formations on the north and south 

 flanks of Loch Broom." 



Mr. Archibald Geikie, F.E.S., communicated the second paper, 

 entitled " Notes for a comparison of the Volcanic Greology of Central 

 Scotland with that of Auvergne and the Eifel." The author began 

 by alluding to the labours of Boue, Forbes, Scrope, Daubeny, and 

 others. He then sketched the area occupied by rocks of volcanic 

 origin between the Grampians and the Silurian uplands of the 

 southern counties. The rocks which he proposed to make the sub- 

 ject of more special remark in this paper were of Carboniferous age. 

 They were capable, he said, of being broadly treated under two 

 groups : 1st, Plateaux ; and 2nd, Points of local eruption. 



1. Plateaux of Carboniferous volcanic rocks are extensively de- 

 veloped in the western part of the midland valley. They form 

 the range of the Campsie and Kilpatrick Fells, and, crossing the 

 Clj'^de into Eenfrewshire, sweep for many miles through the north 

 and north-east of Ayrshire. They occur likewise as fragments on 

 the Clyde islands, Arran, Bute, and Cumbrae. Extensive as the 

 present area of these rocks is, there can be no doubt that it once 

 covered a much greater surface, and that one great plateau of lavas 

 and tuffs stretched from the Ochil Hills to the south of Cantyre. 

 Throughout the wide district where the rocks still remain, they 

 retain a remarkable horizontality. They consist of various porphy- 

 rites, melaphyres, and tuffs arranged in beds, which are placed over 

 each other with great regularity. Hence the hillsides wear a ter- 

 raced appearance from the alternation of harder and softer beds. 

 This feature characterizes the Campsie Fells and the hills south-west- 

 ward to Ardrossan, but is most conspicuously displayed in some of 

 the valleys at the south end of Bute. One of the distinguishing 

 features of these plateaux is the comparative infrequency with which 

 any vent or true point of eruption can now be detected. Occasionally 

 such a vent is found as a boss of coarse volcanic agglomerate, or of 

 porphyrite or melaphyre ; but, as a rule, all the foci of eruption are 

 now buried under the materials which they emitted. Another 

 feature which runs through the plateaux is the apparent continuity 

 of the several beds. Viewed from a little distance, the terraces of 

 trap seem each perfectly continuous for long distances. A closer 

 examination often shows that, though the terrace may run on, the 

 rock of which it consists is formed of different sheets, 'which, though 

 lying on the same plane, have proceeded from different vents. Mr. 

 Geikie then pointed out the structure of some of the volcanic plateaux 

 of central France as illustrative of those features of the Scottish 

 plateaux to which he had referred. 



2. "While the western half of the Scottish Carboniferous area is 

 characterized by the wide extent of its volcanic plateaux, the eastern 

 half is as strikingly distinguished by the abundance of its points of 

 local eruption. Traces of these independent but closely segregated 

 vents are scattered over almost the whole extent of Fife and the 



