CHAP. XXIII 
THEIR ARRANGEMENT IN SCOTLAND 
365 
vent, and even where the vents were more numerous and the outpourings 
of lava and showers of ash more copious, the ejected material usually 
covered only a small area round the centres of eruption. Occasionally 
streams of basic lava and accumulations of tuff were piled up into long 
ridges. Volcanoes of this character were specially abundant in the basin 
of the birth of Forth, and more sparingly in Ayrshire and Eoxburghshire. 
They form the persistent type throughout the rest of the British Isles. 
The Buys also occupy a well-detiued stratigraphical position. They did 
not begin until some of the volcanic iilateaux had become extinct. From 
the top of the Cement-stone group up into the Carboniferous Limestone 
series, their lavas and tuffs are met with on many platforms, Init none occur 
above that series save in Ayrshire, where some of the eruptions appear to 
have been as late as about the beginning of the Coal-measures. ^ 
Arranged in tabular form the stratigraphical and geographical distribu- 
tion of the two great volcanic types of the Carboniferous system in 
Scotland will be more easily followed. I have therefore drawn up the 
accompanying scheme : — 
[Table 
