SKETCH OF THE GEOLOGICAL HISTORY OP THE BRISTOL DISTRICT. 7 
of pyroxene andesite, which has long been quarried at Moon’s Hill, and 
was till lately regarded as an intrusive mass. Interbedded in this lava 
is a well marked band of tuff, and the base further rests on a varied 
series of tuffs of which a thickness of nearly 100 feet is seen in the 
Sunnyhill quarry at Stoke Lane. Certain bands in the tuJff at Sunny- 
hill, and at other points further to the east, have yielded a considerable 
series of fossils of Llandovery type. 
A further series of fossils of the same general character occurs in 
bands of sandy shale which were exposed in laying a line of rails from 
the Downhead quarry, and which are apparently the youngest Silurian 
rocks occurring in the district. It appears, therefore, that the eastern 
Mendip Silurians do not include any representatives of the Wenlock or 
Ludlow strata. An explanation of this is suggested later on in the 
section dealing with the Old Red Sandstone. 
One other Silurian stratum occurring in the eastern Mendips requires 
mention, namely, a remarkable mass of coarse ashy conglomerate, con- 
sisting in the main of blocks of andesite embedded in a matrix of tuff. 
This deposit, which shows little or no sign of stratification, forms two 
masses, the relation of which to the other rocks of the district is 
unfortunately nowhere visible. Although the generally well-rounded 
character of the blocks tells against this view, it is possible that the 
masses of ashy conglomerate mark the vents or necks from which the 
lava and tuff were ejected. 
THE OLD RED SANDSTONE. 
The succeeding beds of Old Red Sandstone indicate a change of 
physical conditions. For the remains of marine organisms which 
characterize the Silurian strata do not occur in the Old Red Sandstone. 
A few fish scales alone break the palaeontological barrenness of these 
deposits, which would seem to have been laid down in a land-locked 
area, cut off from the Devonian sea by some physical barrier. The 
nature of this barrier we cannot determine. Barren conglomerates, 
sandstones, and red shales occur throughout the district as far south as 
the Mendips, where they form the arched summits of the hills. But in 
the not distant Quantocks somewhat further south there are found 
marine beds of the normal Devonian type. Perhaps shoals and sand- 
banks stretched between the present sites of these two ranges of hills, 
and separated the southern sea from the fresher waters of the Old Red 
Sandstone area, shifting at times, no doubt, further south, but never, 
so far as we know, allowing the marine deposits to advance further 
north. 
The best places for examining the Old Red Sandstone are on the 
shore to the south of Woodhill Bay, Portishead, and in the railway cut- 
tings on the Portishead and Avonmouth lines. In each of these locali- 
ties the rocks have yielded fish scales, and in each they contain, in 
addition to the usual beds of sandstone, shale, and conglomerate, certain 
calcareous deposits, sometimes forming large irregular masses of lime- 
stone, sometimes acting as a calcareous cement for the conglomerate. 
No trace of fossils has been found in these limestones, and their origin 
