12 SKETCH OF THE GEOLOGICAL HISTORY OF THE BRISTOL DISTRICT. 
the whole being known as the Millstone grit. These rocks are seen 
at the great fault below Observatory Hill, Clifton, at Brandon Hill, 
at Long Ashton, at the Wick rocks, and at various other localities in 
the Bristol district. An interesting fact which has been brought 
into prominence by Dr. Vaughan’s researches is that the grit beds 
do not everywhere come on at the same horizon. Thus in the im- 
mediate neighbourhood of Bristol the lower part of the Millstone grit 
contains a marine assemblage of fossils — Productus, LitJiost ration^ etc., 
and is clearly more closely related to the underlying limestone than 
to the overlying Coal Measures, while in the North of England the 
Millstone grit is clearly more related to the Coal Measures. Hence 
it seems desirable to use the term with a mainly lithological signifi- 
cance and not as denoting an exact horizon in the Carboniferous 
series. The Coal Measures of the Bristol district are sub-divided as 
follows : — 
Middle — Pennant series — 2,000 
The productive coal seams occur principally in the upper and lower 
beds, the middle or Pennant series consisting chiefly of hard grey 
and red sandstones, which form one of the most important building 
stones of the district. 
The Coal Measures of the Radstock district have long been famous 
for the number and excellent preservation of the plant remains, and 
for the extraordinary amount of faulting and disturbances to which 
the rocks have been subjected. Until lately no evidence was avail- 
able to show that the sea ever gained access to the swamps and 
lagoons in which the Bristol coal beds were being laid down, as it 
did in some parts of northern England ; Mr. H. Bolton has, however, 
recently described ^ an interesting marine fauna including brachiopods, 
lamellibranchs, gasteropods, cephalopods, and fish from the lowest 
Coal Measures of Ashton, proving that marine episodes occurred in 
the Bristol Coal Measures just as they did further to the North. 
A well-known point of interest in connection with the Bristol Coal 
Measures is the extent to which they are covered by overlying 
secondary strata. Hence, in mining the Coal Measures of the Ashton 
district, the Keuper is penetrated ; to reach the productive strata in the 
Radstock district, the Lias, and even the Inferior Oolite. 
With the close of the Coal Measure period the first and longest 
chapter in the geological history of the Bristol district comes to an 
end. Throughout the vast period from the commencement of Silurian 
to the end of Carboniferous times the district was one in which 
prolonged and gradual depression was accompanied by the formation 
^ ‘ Q. Joiirn. Geol. Soc.’, LXIII. (1907), p. 445. 
