Il8 PHOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



be at least 1500 feet thick, from where it rests upon the Lower Calci- 

 ferous Sandstones on the west to where it passes under the Main 

 Limestone on the east. This thick mass of lavas and tuffs thins 

 away southwards and probably disappears a short distance south 

 from Ardrossan in a space of about ten miles. The original south- 

 ward slope of the plateau would thus appear to have been about 1 in 

 35. Again, the northward slope of the same plateau may be estimated 

 from observations in the Campsie Pells. Above Kilsyth the total 

 depth of the volcanic sheets is about 1000 feet, while to the west- 

 ward it is still thicker. From the top of the Meikle Bin (1870 feet) 

 above Kilsyth north-eastwards to Causewayhead, where the whole 

 volcanic series has died out, is a distance of twelve miles, so that 

 the slope of the surface of erupted materials on this side was about 

 1 in 63. 



Judging from the sections exposed along the faces of the escarp- 

 ments, we may infer that the volcanic sheets had a tolerably uniform 

 surface which sloped gently away from the chief vents, but with 

 local inequalities according to the irregularities of the lava- streams 

 that were heaped up round the vents and flowed outward in different 

 directions and to various distances from them. At the beginning, 

 these flat volcanic domes were certainly submarine. While they were 

 being formed continuous subsidence appears to have been in pro- 

 gress. But the great thickness of the volcanic accumulations and 

 the rarity of any ordinary sedimentary strata among them make it 

 not improbable that at least their higher parts rose above the 

 water. 



2. Vetits. — We have now to consider the character and distri- 

 bution of the vents from which this large amount of volcanic mate- 

 rial was discharged. The position of these vents is recognized in 

 the necks so abundantly scattered over the difi*erent districts. The 

 great majority of the necks consist of coarse agglomerate. Some are 

 formed of a massive rock, generally acid, but sometimes more basic 

 in composition, while others include both agglomerates and lava-form 

 material in different proportions. 



The distribution of the necks can best be understood from the 

 maps of the Geological Survey, where they have been carefullj- 

 indicated. As might have been expected, they are not found out- 

 side the original limits within which it can be shown that the lavas 

 and tuffs were erupted. They occur most abundantly and attain 

 their largest size in and around the districts where the Plateaux 

 are most extensively developed. No doubt a large number of them 



