MR. E. WILSON ON THE DURHAM SALT-DISTRICT. 775 



to the light recent explorations in the Durham salt-field have thrown 

 on the very considerable depths to which any coal-explorations would 

 have to be carried in the above district, even supposing productive 

 Coal-measures to be there present. For, to the 3000 feet of Lias 

 and Trias, we should have to add fully 800 feet of Permian strata, 

 besides a more or less considerable capping of Lower Oolites. This 

 would mean something like 4000 feet down to the surface of the 

 Carboniferous rocks, a depth which was held by the Eoyal Coal 

 Commission of 1871 as the limit at which it would be possible to 

 mine coal. 



To return, however, to our proper subject, I would again insist 

 on the want of all certainty there is in the distribution of so fluc- 

 tuating and unreliable a mineral as rock-salt. All that we can 

 safely say is, that the thick bed of rock-salt of South Durham has 

 already been proved to extend over an area four miles by three or 

 four in extent ; that it is highly probable that beneath the greater 

 portion, if not the whole, of this area the salt-bed maintains a 

 considerable (80 to 120 feet) and pretty uniform thickness ; that it 

 is improbable that so considerable a deposit should rapidly die away 

 in every direction ; and that, as previous explorations seem to show 

 that the bed does die away in two given directions (N. and W.), 

 there are reasonable grounds for anticipating its further extension 

 in the opposite (E. and S.) directions. At the same time I do not 

 mean to affirm that the disappearance of the salt-bed at a single 

 point on the Tees is sufficient to prove that it is absent from the 

 whole of the rest of the Triassic country beyond, stretching S.W. 

 from the Tees mouth, or that its presence at three or four points on 

 the S.E. bank of this river is sufficient to prove its continuous and 

 indefinite extension in that direction. 



It is a well-known fact that rock-salt never crops out at the 

 surface, and it has been justly observed that so soluble a mineral as 

 this is could not be expected to do so, since its outcropping 

 portions would be speedily destroyed by the infiltration of surface 

 waters. I do not, however, believe, as some have supposed, that this 

 is the exy)lanation of the absence of the rock-salt on the Tees 

 opposite Middlesborough, and still less that such dissolution along 

 the outcrop has originated the channel of that river. The point 

 referred to is between four and five miles from the outcrop, and here 

 the horizon for the salt-bed lies 1000 feet beneath the surface and is 

 bounded by impervious marls. The salt-rock has also been met with 

 at other points nearer the outcrop. At Seaton Carew which is about 

 one mile and a half from the outcrop of the Magnesian Limestone, 

 the horizon for the salt-rock would lie at about 500 feet from the 

 surface. Here also the measures were, I understand, dry, and there 

 was no evidence in the shape of brine or other springs at this horizon 

 to explain its disappearance. We may broadly assert that in the 

 South-Durham salt-district the salt-rock (or the stratum occupying 

 its horizon) is always enclosed between impervious beds and 

 is free from water, having what is known in the Cheshire district 

 as " a dry rock-head." If this salt-bed ever did crop out at the 



