540 On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain. 
Bristol.—The Carboniferous Limestone in this district underlies the Bristol'Coal-: 
field, and rests conformably on the marls and sandstones of the Devonian or Old: 
Red formation. It consists of the following members :— 
Millstone Grits and Coal Measures. 
Upper Limestone Shales. 
Carboniferous Limestone. 
Lower Limestone Shales. 
Old Red Sandstone. 
The junction between the Old Red Sandstone and the lower beds of the Limestone 
Shales is an almost imperceptible one, and can only be discriminated by the gradual 
insertion of thin beds of calcareous shales and the presence of fossils of purely 
Carboniferous types. The section exposed on the banks of the Avon, near Bristol, 
exhibits the whole series as developed in Semerset and Gloucestershire, and as it 
is from the quarries near the river that rearly all the fossil fishes have been 
obtained, a short description of the Avon section may serve the purpose at present 
in view. 
The north-western extremity of the section, near Cook’s Folly, is composed of 
Devonian sandstones and marls of a red colour. Towards the upper part a quartzose 
conglomerate occurs, and about thirty feet above this is the first bed which exhibits 
a decided admixture of carbonate of lime. The base of the Lower Limestone 
shale series may be placed somewhere between the two, though it is quite 
impossible to determine its exact position. ‘The limestone shales are about 500 
feet in thickness, and are throughout more or less fossiliferous. They consist of 
shales with thin beds of limestone. About 100 feet from their base there is a thin 
breccia four or five inches in thickness; when freshly broken it has a grayish 
brown colour, which quickly changes by exposure to dark reddish brown from the 
oxidation of the iron it contains. The breccia is replete with fossil fish palates, and 
some coprolites have been found which contain the comminuted remains of the 
hard coverings of mollusca. Numerous shales and limestones with characteristic 
fossils occur between the breccia and a second stratum, which is remarkable for 
the presence of remains of Oracanthus associated with remains of Brachiopods. 
The bed is an argillaceous limestone which weathers easily, and the fossils are in 
consequence more easily and perfectly separated from the matrix. It is nearly 
200 feet above the breccia, and about the same depth below the Carboniferous 
Limestone proper. The latter attains a thickness, in the Avon section, of 2,000 
feet, and may be studied im the Black Rock and Great Quarries. ‘The change from 
the lower shales to the thick limestone is very gradual, and it is impossible to 
draw a distinct line of demarcation between the two. The limestone in the Black 
Rock Quarry is very dark in colour, due to the presence of bitumen. ‘The latter 
is occasionally so abundant as to be distinctly recognised by the smell. ‘he strata 
are very regularly and uniformly bedded, dipping at an angle between 20° and 30° to 
the S.S.E. In the Black Rock Quarry, about 100 feet above the base of the 
