124 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



material with the washed-down tuff of the cones, and probably 

 with volcanic dust and lapilli thrown out by the latest eruptions. 

 Thus along the flank of the hills from Barrhead to Strathavon 

 yellow and green ashy sandstones, grits, and conglomerates are 

 succeeded by ordinary sandstones, black shales, and ironstones, 

 while here and there true volcanic tuif and conglomerate make 

 their appearance.^ Farther west, in the Kilbirnie district, the 

 limestone lies directly on the tuffs that rest upon the porphyrites. 



But perhaps the most striking contrast between adjacent localities 

 in regard to the distance between the limestone and the top of the 

 volcanic series is to be observed along the southern front of the 

 Campsie Pells. In spite of the abundant faults which have there 

 so broken up the regular sequence of the rocks, we can see that at 

 Banton the limestone lies almost immediately on the tuffs. But a 

 little to the westward, sandstones, conglomerates, shales, and thin 

 limestones begin to intervene between them and swell out so 

 rapidly that on Craigmaddie Muir and South Hill of Campsie, only 

 some five miles off, they must form a total thickness of not less 

 than from 600 to 800 feet of ordinary non-volcanic deposits, chiefly 

 thick pebbly sandstones. Such local variations not improbably 

 serve to indicate hollows on the flanks of the plateaux that were 

 filled up with detritus before the depression and clearing of the 

 water that led to the deposition of the Main Limestone. 



I have already remarked that the eruptions of the plateau period 

 lasted longer in the western than in the eastern parts of the region. 

 In the Gaiiton district, where the peculiar viscous trachytic lavas 

 probably gave rise to a more uneven surface or more prominent cones 

 than was usual among the porphyrite plateaux, the eruptions ceased 

 some time before the deposition of the Main Limestone. As the 

 area sank, the successive zones of the Calciferous Sandstones crept 

 over the flanks of the trachytes until at last they had completely 

 buried these before the limestone spread over the area. In the 

 Berwickshire and Solway districts this extinction of the plfiteau- 

 vents appears to have taken place at a still earlier part of the 

 Carboniferous period, for there the porphyrites, while they rest on 

 the Upper Old Bed Sandstone, are covered with at least the higher 

 group of the Calciferous Sandstone. The equivalent of the Main 

 Limestone of Central Scotland must lie many hundred feet above them. 



The submergence of the plateaux, and their entombment under 

 the thick Carboniferous Limestone series, did not mark the close of 

 ^ Explanation of Sheet 22, Geol. Surv. Scotland, p. 12. 



