IOO 
THE PERMIAN VOLCANOES 
BOOK VII 
the long volcanic history of the South-west of England, enough is known to 
indicate the general character of the phenomena. The eruptions were on 
even a feebler scale than those of the Permian period in Scotland, but they 
seem to have resembled them in their general character. Small puy-like 
vents were opened, from which dark scoriaceous lavas and showers of 
gravelly tuff and stones were discharged over the floor of the inland sea or 
lake-basin in which the red sandstones and breccias were accumulated. 
These outflows and explosions took place too, as in Scotland, towards the 
beginning of the deposition of the red strata, and entirely ceased long before 
that deposition came to an end. In each area the eruptions mark the 
close of Palaeozoic volcanic activity in Britain. The varied and recurrent 
volcanic episodes which distinguished each successive geological period from 
the Archaean onwards now definitely terminate, not to he resumed until 
after the passing of the whole of the vast cycle of Mesozoic ages. 
2. ERUPTIVE ROCKS IN THE MIDLAND COAL-FIELDS 
Between the thick and thoroughly marine development of the Carbon- 
iferous Limestone in Derbyshire and in South Wales, there lies the region, 
already referred to, wherein both the Carboniferous Limestone and Millstone 
Grit die out against what must have been a ridge of land or group of islands 
that stretched in a general east and west direction from the high grounds 
of Wales through Shropshire, Staffordshire and Leicestershire. On the 
slopes of this ridge the limestone is gradually overlapped by the Millstone 
Grit, and both are in turn overlapped by the Coal-measures, which are then 
found lying immediately on the more ancient rocks of the region — Cambrian 
or pre-Cambrian, Silurian and Old Bed Sandstone. The gradual subsidence 
that led to the deposit of several thousand feet of Carboniferous strata over 
the regions to north and south, before the beginning of the Coal-measure 
period, does not seem to have sensibly affected the persistence of this old 
terrestrial surface, which probably lay on an axis of upward movement, so 
that, amidst the surrounding depression, its position above water was on the 
whole maintained. . But there are indications that the inequality of move- 
ment in this part of the earth’s crust was of much older date than the 
Carboniferous period. The Old Red Sandstone is conformably continuous 
below the base of the Carboniferous system, and in Wales is estimated to be 
some 10,000 feet thick. No break has yet been detected in this vast 
accumulation of sedimentary material, though it is highly probable that some 
such uneonformability must exist in it as that between the Scottish Lower 
Old Red Sandstone, which passes down into the Upper Silurian shales, and 
Upper Old Red Sandstone, which graduates upward into the base of the 
Carboniferous formations. But even if such a break should be discovered, 
it will not account for the position of the Coal-measures on Cambrian or 
even perhaps older rocks. It is hardly conceivable that, had these rocks 
been covered with a full development of Old Red Sandstone, they could have 
