362 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
and there common enongh. The sediments of these early Carboniferous 
waters are met with all over the southern half of Scotland, but in very un- 
equal development, and constitute what is Imown as the “Cement-stone 
Group.” 
It was while these strata were in course of deposition that the earliest 
Carboniferous volcanoes broke into eruption. In some localities a thickness 
of several hundred feet of the Cement-stone group underlies the lowest lavas. 
In other places the lavas occur in and rest on the Upper Old Bed Sandstone 
and have the Cement-stone group wholly above them ; while in j-et other 
districts the volcanic rocks seem entirely to take tlie place of that group. 
So vigorous was the earliest display of ' volcanic action in Carboniferous 
times that from the borders of Northumberland to the uplands of Galloway, 
and from the slopes of the Lammermiiirs to Stirlingshire and thence across 
the estuary of the Clyde to Cantyre, innumerable vents were opened and 
large bodies of lava and ashes were ejected. 
The Cement-stone group, save where succeeded by volcanic intercala- 
tions, passes up conformably into the lowest crinoidal limestones of the 
Carboniferous Limestone series. In the basin of the Firth of Forth, however, 
the ceiiient-stones, feebly represented there, are overlain by a remarkable 
asseinblage of white sandstones, black carbonaceous shales, or “ oil-shales,” 
cyprid limestones, occasional marine limestones and thin seams of coal, the 
whole having a thickness of more than 3000 feet. These strata, unlike the 
tpical Cement-stone group, abound in fossils both vegetable and animal. 
They prove that, over the area of the Forth, the insalubrious basins wherein 
the red and green sediments of the Cement-stone group were laid down, 
gave place to opener and clearer water with occasional access of the sea. 
The peculiar lagoon-conditions which favoured the formation of coal were 
thus devebped in Central Scotland earlier than elsewhere in Britain. We 
shall see in later pages that these conditions were accompanied by a fresh 
outbieak ol volcanic activity, in a phase less vigorous but more endurino- 
and extensiA^e than that of the first Carboniferous eruptions. 
The Carboniferous Limestone sea over the site of the southern half of 
Scotland appears neA^er to have reached the depth which it attained in 
England and Ireland. To the north of it lay the land from which large 
quantities of sand and mud Avere carried into it, as shoAvn by the deep 
accumulations of sandstone and shale, which far surpass in thickness the 
few comparatively thin marine limestones intercalated in them. There is 
thus a strikiiig contrast between the thick masses of limestone in central 
and south-Avestern England and their dwindled representatives in the north. 
Another marked difference between the Scottish and English developments 
of this formation is to be noticed in the abundant proof that the compara- 
tively shallow Avaters of the northern basin were plentifully dotted over Avith 
active volcanoes. The eruptions were especially vigorous and prolonged in 
the basin ol the lirth of Forth. They continued at intervals, even after the 
peculiar geograibical conditions of the Carboniferous Limestone had ceased. 
But they had died out by the time of the beginning of the Coal-measures. 
