776 ME. E. WILSON ON THE DTJEHAM SALT-BISTEICT. 



surface, of which fact I am by no means satisfied, the efi'ects of 

 surface-infiltration would, I believe, be limited to a small lateral 

 extent ; because on the removal of the salt the impervious roof 

 would subside on to the impervious floor and the surface-action 

 would be brought to a standstill. It has been suggested that certain 

 cavities and swallow-holes met with along the boundary of the 

 Magnesian Limestone between Hartlepool and Darlington, and also 

 near Eipon, may be due to the dissolution of saliferous beds ; but 

 it seems to me more probable that the true explanation of these 

 hollows is the same as that for similar phenomena along other lime- 

 stone boundaries, and that the peculiar forms of the cavities may 

 be due to the rapidly varying character and solubility of beds of the 

 ]y!agnesian Limestone. I therefore conclude that the present exten- 

 sion of the rock-salt in South Durham is defined by the limits of 

 its original area of deposition and not by subsequent dissolution by 

 outcrop or other infiltration. 



Method of iv inning the Salt, Waste in worTcing Sfc, 



It would be beyond my powers and outside the scope of this paper 

 to consider the chemical and mechanical details of the mining and 

 manufacture of salt in the Durham district. Por full information 

 on these matters I must refer those who are interested in the subject 

 to Sir Lowthian Bell's admirable essay " On the Manufacture of Salt 

 near Middlesborough " *. There are, however, certain consequences 

 of the method of working the salt-bed there described which cannot 

 be considered as altogether satisfactory, and to which I should like 

 to call attention. The salt is extracted from its bed by solution, by 

 a method which has for some time past been in operation at jS'ancy, 

 in France, but was introduced for the first time into England by 

 Messrs. Bell Bros, about twelve years ago at their Saltholme works. 

 The process is as follows : — A hole from 6 to 12 inches or so in 

 diameter is bored down to and through the Eock-salt, and is lined 

 with an iron retaining tube ; within this an inner tube of 2 or 4 inches 

 less diameter is let down and secured below ; both tubes are perfor- 

 ated with holes where they pass through the rock-salt ; fresh water 

 is let down the space between the two tubes, and this passing 

 through the outer holes gradually dissolves the salt ; the brine thus 

 formed enters the inner tube and rises in it as high as a column of 

 fresh water will support a liquid having a sp. gr. of 1-204 or there- 

 abouts, and is drawn up the remaining height by pumping. Now 

 it appears that this system of working the salt, although far more 

 economical for raising this mineral from great depths, both as regards 

 the capital and the labour employed, than by sinking a shaft and 

 regularly mining as in the case of coal, is extremely wasteful, having 

 regard to the proportion of the salt which is extracted from its bed. 

 It is found that a single borehole will only extract a limited amount 



* Loc. cit. See also paper on " The South Durham Salt-bed and Associated 

 Strata," read by Mr. W. J. Bird to the Manchester Geological Society, June 5, 

 1888. 



