MR. E. WILSON ON THE DORHAM SALT-DISTRICT. 771 



demur to the use of the term " Limestone " as applied to the whole 

 of these beds, and would designate them instead "indurated marls." 

 Although there appear to be dolomitic or calcareous, as well as dark 

 bituminous beds among them, they show no sort of resemblance to 

 any known beds of the Magnesian Limestone of Durham ; on the 

 other hand, they possess the characteristic greenish-grey colour of 

 certain Keuper Marls, as well as a very similar texture and probably 

 also mineral composition, although decidedly harder than most of 

 the rocks of that series. It is also worthy of note that they contain 

 gypsum, as well as that they overlie a thickish seam of rock-salt. 

 It was upon the supposed identification of these ' limestones ' as 

 belonging to the Magnesian Limestone that Eamsay based his 

 hypothesis of Permian salt in Durham. Whilst not prepared to 

 accept the evidence of rock-salt in the Permian formation in Eng- 

 land, I do not on abstract grounds contest the possibility of such an 

 occurrence. With the hypothesis of direct chemical precipitation 

 in inland salt lakes (or lagoons) of the dolomitic deposits of the 

 British Permians I entirely concur, and elsewhere I have advanced 

 arguments in support of this theory *. Although the idea of ' Perm- 

 ian salt ' in Britain must, I believe, be abandoned, it is worthy of 

 note that in certain of the deep borings in the Durham salt-field (see 

 Appendix, pp. 779, 781), gypsum and anhydrite are found to occur in 

 intimate association with the dolomites of the Magnesian Limestone ; 

 and in the Seaton Carew section these minerals are distributed, 

 more or less abundantly, through the greater portion of that series. 

 Surely this is a very significant fact and one that must tell strongly 

 in favour of the chemical-precipitation hypothesis. Accepting the 

 accuracy of the information as to the presence in the iSaltholme 

 section of dolomitic limestones above certain saliferous strata, it 

 would not be safe to assume, failing more decisive evidence on the 

 subject, that such beds belong to the Permian formation. Calcareous 

 beds are met with in rocks of undoubted Triassic age exposed at the 

 surface in South Durham, and dolomitic rocks are known to occur 

 to a considerable extent in the Keuper sandstones and marls of the 

 West of England and in other parts of the British area, especially 

 where these rocks approach a margin of Mountain Limestone. In 

 the same way we might naturally expect to meet with dolomitic 

 beds towards the base of the Keuper in a district where these rocks rest 

 on a margin of Magnesian Limestone. 



The view that the upper portion of the saliferous rocks of South 

 Durham belongs to the Trias and the lower to the Permian, seems 

 to me, if anything, the most improbable of all. The chances, in the 

 abstract, against two sets of beds of such an uncommon mineral as 

 rock-salt occurring at the same point, and within 200 feet of each 

 other in the same vertical section, in two distinct rock-series, are 

 assuredly very great; but the chances against such a coincidence are 

 vastly increased when we consider that there is no sort of sequence 

 between the two formations in the district in question, but that, on 



* " The Permian Formation in the North-east of England," * Midland Nat- 

 uralist,' vol. iv. pp. 202-208 (1881). 



