206 PROCEEDIXGS OF THE GEOLOGICAL SOCIETY. [Mar. 20, 



know where to draw the line between the Dolomitic and Lias Con- 

 glomerates. We can easily conceive that one part of a mass of 

 shingle and fragments may have been formed at the end of one 

 epoch, and another part at the commencement of another, the same 

 general physical causes for the production, accumulation, and 

 consolidation of the gravel and fragments continuing in particular 

 localities." 



The occurrence of an ore of lead in the Sutton Stone is an in- 

 teresting fact. This ore, Galena (sulphide of lead), is met with in 

 the stone, sometimes in sufficient quantity to offer inducements to 

 mining- enterprise — more especially at Llangan, where "Works have 

 been erected and a considerable amount of ore is said to have been 

 obtained. 



The ore, in some instances, is associated with fossil plants, which 

 it partly mineralizes ; but its most common mode of occurrence is 

 in strings or bunches, and sometimes disseminated through the 

 stone. Lead-ore is noted by Sir Henry de la Beche as occurring 

 in the Carboniferous Limestone of the district ; and at St. Hilary 

 the remains of old workings show that it was formerly worked to 

 some extent in the Dolomitic Conglomerate. Hence the occurrence 

 of Lead-ore in the Sutton Stone corroborates the theory of its deriva- 

 tion from the denudation of Carboniferous Limestone and Dolomitic 

 Conglomerate in which it originally existed, being carried down with 

 the calcareous mud and debris of which the Sutton beds consist, 

 and re-deposited with them in the ancient Liassic sea-bottom. 



Sir Henry De la Beche states that " at Candleston, a variety of 

 Sutton Stone, with small cavities, often the casts of shells, is very 

 plumbiferous, galena being distributed throughout it and even filling 

 up the cavities left by the disappearance of the shells"*. 



In surveying the country' between the river Ogmore and Cow- 

 bridge, the area occupied by the Lias (including the so-called Sutton 

 Stone) proved to be somewhat greater than was represented on the 

 existing geological maps, — a discrepancy easily to be accounted for 

 by the strong resemblance borne by the lower beds of that series to 

 the Carboniferous Limestone upon which they there generally repose. 

 This striking similarity had been noticed by Sir Henry De la Beche, 

 who remarks that "at Merthyr-mawr, on the west bank of the Ogmore, 

 the Lias so resembles the Carboniferous Limestone on which it rests, 

 that when the curvatures of the latter bring portions of it into a 

 nearly horizontal position, and this kind of Lias reposes upon it, it 

 is only by a very strict search for organic remains that the difference 

 can be found, especially as in some situations the Lias, from the 

 presence of abundance of Pentacrinites, has the encrinal aspect of so 

 much Carboniferous Limestone "f. 



The object of this communication not being to describe the Sutton 

 beds of South Wales in detail, but merely to define their true posi- 

 tion in the geological scale, I will merely add, in conclusion, that 

 in this district it will be found that, wherever the Lias rests upon 



'^ Memoirs of tlie Greological Survey, vol. i. p. 272. 

 t Ibid. p. 272. 



