56 REPORT — 1875. 



dated with tlie coral masses are other organisms which lived on the reefs or in 

 shallow lagoons. The coral beds are corered by strata formed of Oolitic limestone 

 and other detrital materials derived from the debris of wasted reefs, and formed 

 along the shores of the ancient coral strand ; sections of these oolitic Ijeds prepared 

 as slides for the microscope disclose the fact that the nucleus of the oolitic granules 

 is often the shells of Foramiaifera. 



Millstone Grit is well seen at Brandon Hill ; it rests upon the Limestone, and 

 attains a thickness of 1000 feet. On this repose the Coal Measures of the Bristol 

 Coal-field, which I have already described in connexion with the Mendip and Bad- 

 stock districts. 



Dolomitic Camihmerate. — The Palfeozoic rocks of the Bristol Coal-field are here 

 and there covered over by patches of Dolomitic Conglomerate lying unconformably 

 on their uptm-ned edges, at heights varying from 20 to 300 feet above the Avon. 

 This remarkable formation is very well seen in the new road leading fi-om the Hot- 

 wells to Clifton and Durdham Down. It has been long well known to geologists, 

 and was in former days described by Bright, Gilby, Buckland, and others. 



Rh<etic. — Between the uppermost beds of the grey marls of the Keuper and the 

 lowest beds of the Lias there lies a remarkable assemblage of strata, which I for- 

 merly described * as the " Avicula-contoi-taheds," from that shell forming the leading 

 fossil therein. The name Rhastic has since been given to the series, from a supposi- 

 tion that the beds are identical with some that occur in the Rhtetian Alps, which is, 

 however, more than doubtful. Typical sections of the Avicidn-contorta series are ex- 

 posed at Garden Clift", Aust Cliff, Penarth, and Watchet on the Severn, and at Wes- 

 ton, Keynsham, Willsbridge, and Saltford near Bath, and Puriton, Uphill, and "Wells 

 in Somersetshire, as well as at many other localities. Two of the most classical of 

 the series are Garden Clitt'and Aust Cliff"; the latter has been long known to conti- 

 nental geologists as the Bristol Bone-bed. In the upper part of the section are dark 

 grey shales, intersected by bands of limestone ; Avicula contorta, Cardium Rhceticum, 

 Pecten Valoniensis, Axinus, &c. are found in these. The Bone-bed consists of a 

 hard dark grey siliceous grit full of the bones, spines, scales, and teeth of fishes 

 belonging to the genera Nemacanthus, Acrodus, Sargodon, Hybodtis, Ceratodus, &c. 

 Beneath this thin Bone-bed with its ichthyic debris is a bed of shale which rests upon 

 the gi'ey marls of the Keupei". A similar succession of strata is repeated in most of 

 the other typical sections. I have named especially those at Garden Oliftj Penarth, 

 Uphill, and Watchet. 



Aust has been long famous for its Cemtodtts-teeth, and is, I believe, the only 

 locality where they are collected. You will find a tine series of them in the Bristol 

 Museum. This wonderful collection is quite unique and will well repay an atten- 

 tive examination. 



The only living representative of the genus Ceratodus now lives in the rivers in 

 Queensland ; and a fine specimen was lately piuchased for and presented to the 

 Museum by W. W. Stoddart, Esq., F.G.S., for the purpose of showing the com- 

 parative size of the recent and fossil teeth. 



5. Dundhy District. 



The Oolitic Formations. — The Oolitic formations will long remain classical ground 

 of English geologists, as it wa,swhil8t studying these rocks in Wilts and Somerset 

 that Dr. William Smith first acquired that knowledge which enabled him to " iden- 

 tity strata by organic remains, " and establish a true natural system of stratigraphical 

 geology. 



The Oolitic period admits of a subdivision into three groups — the Lower, Middle, 

 and Upper ; each group is based on a great argillaceous formation, on which rest 

 minor beds of sands and cream-coloured Oolitic and Pisolitic limestones. The 

 Argillaceous formations form broad valleys, extending diagonally across England in 

 a direction north-east by south-west. The limestones constitute low ranges of hills, 

 with escai-pments facing the south-west and overlooking the valleys. The Lower 

 Oolites rest on the Lias, the Middle Oolites on the Oxford Clay, and the Portland 

 and Upper Oolites on the Kimmeridge Clays. 



* Quart. Journ. Geol, See. vol. xvi. p. 374. 



