456 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



in the Mendips, from several of which I have also obtained organic 

 remains. The conglomerate of Durdham Down, in which the reptile 

 Thecodontosaurus was found, was first supposed to be of Permian 

 age, but is now assigned to the Dolomitic conglomerate. It is not 

 my intention in this paper to enter into a consideration of the 

 Ehaetic fauna ; but from the fact that reptilian remains of the same 

 genera are found in a conglomerate which is lithologically identical 

 with that at Holwell, and deposited under the same circumstances, 

 I have been led to infer that the Durdham Down deposits will have 

 to be removed from the more ancient Dolomitic to the more recent 

 Rhaetic period. At Croscombe, near Shepton Mallet, some mining- 

 operations have been carried on for the discovery of manganese and 

 iron. In a gallery driven through the conglomerates at this place 

 they were found to enclose numerous masses of blue and variegated 

 marls, which I was unable to distinguish from the Upper Keuper 

 marls. One of the veins at this spot shows distinct traces of car- 

 bonate of copper. 



Conglomerates of Liassic age assist to fill many of the Carbonife- 

 rous -limestone veins previously referred to. They may be found 

 also resting on the limestone in the Frome Koad at Holwell, and 

 again in the line of the turnpike-road at Cranmore, near Shepton 

 Mallet. The most remarkable evidence, however, of the conglome- 

 ratic character of the coast-line of the Liassic sea when bounded by 

 the Carboniferous Limestone, will be shown when* I refer to beds 

 contemporaneous with the Lias of Shepton Mallet, at Sutton and 

 Southerndown in South "Wales, where there are conglomerates far 

 exceeding in thickness any with which I am acquainted, even 

 amongst the thickly massed Dolomitic deposits. The base of the 

 Inferior Oolite, again, where it is unconformable to the Carboniferous 

 Limestone is often a siliceous conglomerate ; but, in this instance 

 there is not the same difiiculty in arriving at its exact position. 



III. StEATIITED EoCKS SUBSEQUENT TO THE MeNDIP IJpHEAVAL. 



As a general rule, it may be observed that, with the New Eed Sand- 

 stone, deposits commence which have not been afi'ected by the ele- 

 vation of the Mendips, since, when these are compared with the 

 older beds in that range, or with the Coal-measures in the basin, 

 they will be found horizontal and lying against or upon the up- 

 turned edges of the latter. 



It wiU be my endeavour to show (in proof of my position that 

 the Mendips and their continuation in South Wales have to a great 

 extent jDroved a barrier to the incursion of the Secondary seas within 

 its northern borders) that, even drawing a line east and west so far 

 north as Bath, most of the beds between the Coal-measures and the 

 Oolites, where they can be examined, have been laid down either 

 under abnormal or unconformable conditions ; the latter fact will 

 fuUy account for the reasons why, in the southern portion of the 

 Somersetshire Coal-field, coal-shafts have been commenced, and coal 

 has been soon won even through the Inferior Oolite. 



I propose now to give examples of beds on certain geological 



