772 ME. E. WILSON OlS" THE DIJEHAM SALT-DISTEICT. 



the contrary, there is a decided hreak and unconformity between 

 them, indicated by the omission of the whole of the Lower Triaa 

 or Bunter Sandstone, not to mention the Middle Trias, or Muschelkalk 

 of the continent. 



I would here observe, parenthetically, that this discordance between 

 the Permian and Trias of Durham is probably in a large measure 

 due to want of conformity between the Upper and the Lower Trias, 

 coupled also perhaps with an original northerly thinning out of the 

 Bunter Sandstone. The very ample and Cheshire-like development 

 of the Keuper series in the Tees valley (1800 or 1900 feet as com- 

 pared with 600 or 800 feet in the East Midlands), taken in conjunc- 

 tion with the total absence of the Bunter Sandstone in.South Durham, 

 is certainly a very suggestive phenomenon. 



In some parts of the Midland district there are evidences of 

 rapid attenuations of the Banter Sandstone, as well as of actual 

 discordance between the Bunter and the Keuper *, and in passing 

 across Yorkshire something of the same kind evidently occurs. 



The arguments against the whole (as of any part) of the saliferous 

 rocks of South Durham being Permian are also very strong. In 

 addition to the indications of the graduation of these beds upwards 

 into undoubted " Eed Marls," and the evidence of their mineral 

 characters, which I affirm indicate that they belong to the Upper 

 Trias, we have the negative fact, that no deposits of rock-salt have 

 ever been found in any British rocks which have ever (rightly or 

 wrongly) been assigned to the Permian period. Gypsum is, indeed, 

 known to occur in certain Permian marls in this country, and, as we 

 have lately learnt, has been found associated with the Magnesian 

 Limestone of South Durham. Although beds of rock-salt occur in 

 certain continental Permians, not even a single pseudomorph of 

 common salt has ever been found in any British rock of Permian age. 

 On the other hand, rock-salt occurs in the Trias (Upper Keuper) of 

 Cheshire, Staffordshire, Shropshire, Worcestershire, &c., and in the 

 north of Ireland ; and where we do not meet with actual beds of 

 this mineral in these rocks, its former presence is verj' frequently 

 indicated by salt-pseudomorphs or by brine-springs. A very little 

 consideration wiU show that it is much more probable that beds of 

 rock-salt should occur towards the base of the upper than towards 

 the top of the lower of two discordant formations. Between the 

 Permian and Triassic epochs in Durham there was certainly an 

 interval in time unrepresented by rock-formation. Had any deposits 

 of salt been formed towards the close of the Permian epoch, and 

 thus left for long periods of time exposed near the surface, these 

 beds would almost certainly have been destroyed during this interval. 

 That the main mass of Rock-salt belongs to the overlying and not 

 to the underljing rock-series is indicated by its persistence at a weU- 

 defined horizon t in the former for a distance of at least four miles 

 (Eston to Greatham), in a direction at right angles to the average 



* "On the Unconformity of the Bunter and Keuper," Geol. Mag. 1880, 

 p. 309 ; ' Geology of England and Wales,' 2nd ed. 1887, pp. 221, 224. 

 t See Vertical Sections, facing p. 782. 



