il6 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



ordinary sediment of the sea-floor, spread out over a considerable 

 area, for they extend from Dunbar to beyond North Berwick — 

 a distance of some twelve miles. They now form the picturesque 

 cliffs at these two towns, where the composition and arrangement 

 of the volcanic tuffs are so admirably exhibited. Next in order 

 came the outflow of basic lavas (oli vine-basalts with probably picrites) 

 and porphyrites (andesites), which form a thin but continuous sheet 

 all over the area of the plateau. Lastly rose the successive flows 

 of trachyte, less regularly bedded than the more basic rocks, and 

 apt to assume lumpy irregular forms on the surface. During the 

 period of the emission of these rocks there seem to have been occa- 

 sional outflows of andesite. Three of the chief vents are recog- 

 nizable in the picturesque trachytic cones or domes of Traprain Law, 

 North Berwick Law, and the Bass Rock. The latest eruptions had 

 ceased and the cones and lava-sheets had probably been buried 

 under sediment before the commencement of the deposition of the 

 thick Main (or Hurlet) Limestone of the Carboniferous Limestone 

 series which lies immediately to the west of the plateau.^ 



Although the tuffs form on the whole a comparatively unimpor- 

 tant part of the constituents of the Plateaux, they attain in a few 

 localities an exceptionally great development, and even where they 

 occur only as thin partings between the successive lava-flows they 

 are always interesting memorials of the volcanic activity of a 

 district. In many portions of the plateaux the lowest members of 

 the volcanic series are tuffs and agglomerates, showing that the erup- 

 tions generally began with the discharge of fragmentary materials. 

 Thus the great lava-escarpment of the Kilpatrick Hills rests on a con- 

 tinuous band of tuff which is thickest towards the west, near the group 

 of vents above Dumbarton, while it thins away eastward and dis- 

 appears in Strathblane. At the west end of the Campsie Pells the 

 porphyrites form the base of the volcanic series. But perhaps the 

 most remarkable group of basal tuffs is that which underlies the 

 lavas of the Garlton plateau to which reference has just been made. 



Extensive accumulations of tuff form in one or two localities a 

 large proportion of the thickness of the whole volcanic series of a 

 plateau. Thus in the north-eastern part of Ayrshire, between 

 Eaglesham and the valley of the Irvine, the lavas die out for a space 



1 This important stratum, which can be recognized all over the lowlands of 

 Central Scotland, serves as a good base for the Scottish Carboniferous Limestone 

 series. In the plateau-districts it lies immediately, or nearly so, above the 

 volcanic rocks. 



