8 SKETCH OF THE GEOLOGICAL HISTORY OP THE BRISTOL DISTRICT. 
was probably chemical. The pebbles of the conglomerate are of distant 
origin, and consist of quartzite, jasper, and rarely igneous rocks, such as 
quartz-felsite. 
One of the most interesting points about the Old Red Sandstone of 
the Bristol district is its remarkable variation in thickness. While in 
several places, as in the Avonmouth and Portishead Railway cuttings, 
and formerly in those of Sodbury and Tytherington, the upper beds 
passing into the Carboniferous rocks are well seen, at one locality only, 
Tite’s Point on the Severn, does there seem to be the possibility of a 
conformable passage down into the Silurian, and nowhere in the district 
is there a complete section of the Old Red Sandstone available. The 
thickness of the rocks as exposed also shows a surprising amount 
of variation. While in the cuttings on the Avonmouth and Portishead 
lines a thickness of over 1500 feet is seen, in the Tort worth district the 
thickness is reduced to some 200 to 300 feet at most. This attenuation 
of the Old Red Sandstone, and the probable absence of the greater part 
of the Ludlow series in the Tortworth district, is probably to be 
attributed to upheaval and erosion of the area in late Silurian and 
early Devonian times. The apparent absence of Wenlock and Ludlow 
strata in the eastern Mendips admits of a similar explanation. 
CARBONIFEROUS ROCKS. 
From whatever point of view they are considered, whether the 
economic, scenic, or stratigraphicai, the Carboniferous strata yield to 
no others in the district in point of importance. They may be readily 
divided into an upper series, in the main non-marine in character, the 
Coal Measures and Millstone Grit, and a lower completely marine 
assemblage of strata, the Carboniferous Limestone series. It will be 
convenient to treat these separately. 
Carboniferous Limestone Series. ^ 
The Bristol district is fortunate in possessing what are probably the 
two finest and most continuous sections of the Carboniferous Limestone 
series in the British Isles, viz., those of the Avon and of Burrington ; 
while at a number of other points, the Sodbury and Tytherington rail- 
way cuttings, the Cheddar Gorge, the Wick war quarries, the coast sec- 
tions of Clevedon and of the Weston-super-Mare district, a large part 
of the succession is finely exposed. 
The detailed palieontological work of Dr; Arthur Vaughan has led to 
the successful zoning of the Carboniferous Limestone, and his methods 
have been most ably applied by Dr. T. F. Sibly ^ in the Mendips and 
Weston-super-Mare districts. 
^ See A. Vaughan, “ Palieontological Sequence in the Carboniferou.s Lime- 
stone of the Bristol Area”, ‘ Q. Journ. Geol. Soc.’, LXL, 1905, p. 181, and “ The 
Carboniferous Limestone Series (Avonian) of the Avon Gorge ”, ‘ Proc. Bristol 
Nat. Soc.’, 4th Series, I., part ii., 1906 (issued for 1905) p. 74. 
‘ Q. Journ. Geol. Soc.’, LXT. (1905), p. 548, and LXII. (1906), p. 324 ; also 
‘ Proc. Bristol Nat. Soc.’, 4th Series, L, pt. i. (1905, issued for 1904), p. 14. 
