10 SKETCH OP THE GEOLOGICAL HISTORY OF THE BRISTOL DISTRICT. 
The Gleistopora Zone^ so called after a small compound coral, 
corresponds to the Lower Limestone Shales of previous writers. The 
lower beds consist of shales with thin limestones well exposed in the 
Avon section, and forming passage beds into the Old Red Sandstone. 
They contain bands with larnellibranchs and ostracods, and are succeeded 
by a massive red limestone, known as the “Bryozoa Bed.” In this 
the organic remains have been converted into oxide of iron, and though 
crinoidal ossicles greatly preponderate, polyzoa are also fairly numerous. 
The occurrence of bands of limestone in which the fossils are similarly 
converted into oxide of iron, at about this horizon, throughout the dis- 
trict, from Tytherington on the north to Burrington Combe in the 
Mendips on the south, seems to indicate the prevalence of some general 
conditions in the shallow sea — conditions which are not easy of 
explanation. Just above the “ Bryozoa Bed ” in the Avon section is 
the well-known “ Palate Bed,” from which great numbers of teeth and 
coprolites of elasmobranch fish have been obtained. The “Palate Bed” 
has also been detected in the Sod bury cutting. Succeeding the “ Palate 
Bed ” are other shales and thin-bedded limestones forming the upper 
beds of the Cleistopora Zone, The succeeding Zaphrentis Zone consists 
of masvsive limestone characterised by the great abundance of Spirifers 
and, especially in the upper part, of Zaphrentid corals. It is well ex- 
posed in the Black Rock quarry and the two northern quarries on the 
left bank of the Avon, and also at Wickwar, Clevedon, Woodspring 
(near Weston-super-Mare), and elsewhere. The upper part of the 
Zaphrentis Zone contains the well-known “Fish Beds” from which the 
vast numbers of teeth of elasmobranch fish to be met with in most col- 
lections have been obtained. 
Following the Zaphrentis Zone is the Syringothyris Zone, which in 
the Avon section shows a strong lithological contrast to the beds both 
above and below, being relatively unfossiliferous, and consisting of a 
mass of very pure white oolitic limestone — the “Gully or Caninia 
Oolite ” — both overlain and underlain by beds consisting largely of 
dolomite. These beds are clearly of relatively shallow-water origin, and 
point to some physical disturbance which caused a check to the regular 
deposition of deep-water limestones. In this connection it is interest- 
ing to note that there is further evidence of disturbed conditions 
afforded by the occurrence of a volcanic episode ^ in the Syringothyris 
Beds of Weston-super-Mare. The rocks which are exposed at Spring 
Cove include a mass of basaltic lava about 40 feet thick, while the over- 
lying limestone contains disseminated ashy particles up to a height of 
at least 8 feet from the base. The basalt is associated with a peculiar 
mass of agglorneratic material which Prof. W. S. Boulton, who has paid 
much attention to the section, ^ regards as a fluxion tuff. This deposit is 
further noteworthy for containing numerous masses of limestone which 
1 See Sir A Geikie and A. Strahan ‘ Summary of Progress of Gpol. 
Survey of the United Kingdom for 1898’, p. 104 (1899), C. Lloyd Morgan and! 
S. H. Reynolds ‘ Q. Journ. Geol. Soc.’, LX. (1904), p. 137, see also ‘ Proc. 
Bristol Nat. Soc.’, New Series, X., pt. hi (1904 issued for 1903), p. 188. 
‘ Q. Journ. Geol. Soc.’, LX. (1904), p. 158. 
