SKETCH OF THE GEOLOGICAL HISTORY OF THE BRISTOL DISTRICT. 13 
of layers of deposit, sometimes insufficient in amount to keep pace 
with the subsidence, sometimes in excess of it and causing shallow- 
ing of the water. Although it is true that in the Llandovery period 
there was a volcanic episode, and that there is evidence of upheaval 
and erosion during the latter part of the Silurian and beginning of 
the Old Red Sandstone periods, while further the Carboniferous 
dolomites and volcanic rocks point to the Bristol distiict being 
affected by the widespread Mid-Avonian disturbances, yet, on the 
whole, throughout this enormous lapse of time, during which strata 
to the thickness of probably 12,000 feet were accumulated, the 
Fig. a — periods OF EROSION AND DEPOSITION IN THE 
BRISTOL DISTRICT. 
Post-tertiary\ marine deposits f Second 
and Tertiary/ unrepresented \ period of erosion. 
Cretaceous. 
Jurassic- 
Upper Trias 
mn 
■ 
Second 
period of deposition. 
Lower Trias f 
and Permian j 
unrepresented {j,^^.^UoLrosion 
Carboniferous 
Old Red 
Sandstone' 
Silurian 
First period 
of deposition. 
Base not seen. 
