ME. E. WILSON ON THE DURHAM SALT-DISTRICT. 777 



of salt. This is apparently due to the insoluble earthy residue of 

 the rock-salt (which in the Durham salt-bed seems often to attain 

 rather large proportions), coupled with falls from the roof, forming in 

 time over the floor of the cavity eaten out of the salt-bed a thickish 

 earthy layer which is impervious to the solvent water. Tims, 

 after a while, the brine is found to become weaker and weaker, until 

 in time it will not pay to raise. Again, the bed of rock-salt appears 

 to be dissolved away by this process in a very unequal manner, viz. 

 much more rapidly above than below, owing to the fact that the 

 saturated brine which sinks to the lowest depths of the borehole 

 has not nearly such solvent power as the comparatively fresh water 

 which floats upon it. Hence the cavities eaten out in the rock-salt 

 at the bottom of a brine well assume the form of inverted cones, 

 of which the bottom of the well is the apex. This leads us to infer 

 that in the course of time, when the inevitable subsidences set in, 

 a number of cavities will be formed at the surface which will con- 

 form to the general contours of these subterranean cavities, and of 

 course the unequal character of such subsidences would be particu- 

 larly destructive to surface properties. It appears further that, as 

 the law now stands, owners of land adjoining these wells, unlike 

 owners of land undermined by coal-workings, have no legal claim 

 for compensation on account of the damage done to the surface, nor 

 for the loss of the mineral which has been abstracted from beneath 

 their property — a palpable injustice which it seems impossible to 

 suppose can be allowed long to continue. These special evils would 

 be removed if the salt-rock were mined and in other respects treated 

 in the same manner as coal. By that mode of working, too, a 

 much larger proportion of the bed might be extracted, as well also 

 as a good deal which extends beneath the sea ; but whether it would, 

 by any method of working, be practicable to mine the whole or even 

 the greater portion of an immense mass of rock-salt 100 feet in 

 thickness, lying at depths of from 1000 to 2000 feet, I am not able 

 to say, nor can one forecast the precise limits of the destruction 

 which might result through subsidence, were such a thing done. I 

 would conclude with the remark that, vast as are the stores repre- 

 sented by this thick and widely distributed bed of Durham salt 

 (about one hundred million tons per square mile), the supply of the 

 mineral is not absolutely unlimited, and that the interests of future 

 generations as well as those of ourselves and our own immediate 

 successors ought in a matter of this kind to receive due considera- 

 tion. 



In addition to acknowledgments already made, I am indebted 

 either for valuable information or for references to Mr. Horace B. 

 Woodward, F.G.S., Mr. Alfred Allhusen, M.I.C.E., Manager of the 

 Newcastle Chemical Works Company, to Mr. John Harrison, Secre- 

 tary of the Cleveland Salt Company, and to Mr. Rowland Gascoyne, 

 F.G.S., of Mexborough. I am also specially indebted to Mr. W. J. 

 Bird, Mining Engineer of Sunderland, for the section and loan of 

 cores of the Seaton-Carew borino'. 



