4^ Mr. H. Holland on the Cheshire Rock-salt District. 



uncertainty exists with respect to the rate and progression of this 

 decrease, the general fact seems to be sufficiently confirmed by 

 observations taken from different mines. In those which have been 

 sunk near to the western or north-western side of the area before 

 described, the thickness of the upper bed has been very generally 

 twenty-eight, twenty-nine, or thirty yards. Proceeding towards the 

 east or south-east, we find this thickness decreasing to twenty-five 

 yards, and in the mines near the eastern boundary, the bed of rock- 

 salt comes down to twenty, eighteen, and even seventeen yards in 

 thickness. It will be observed that this thinning takes place in a 

 general direction yro;w the nearest sea coast ; the thickest part of the 

 body of rock being situated furthest down the Weaver, and just 

 above the contraction which takes place in the valley of the river 

 at Anderton. 



Besides this general variation of surface In the superior stratum 

 of rock-salt, it has been found that there is a considerable irregu- 

 larity of level on its upper surface. In one of the mines, in which 

 a tunnel was carried one hundred yards along this surface, many 

 small risings and depressions were met with ; and similar appear- 

 ances have been observed in the other mines near Northwich. 



The depth at which the upper bed of rock-salt is found, though 

 varied by several of these circumstances, depends principally, of 

 course, upon the surface of the ground above, which at Northwich, 

 from the confluence of streams there, is somewhat irregular. In the 

 greater number of the mines, it is met with at a depth varying from 

 thirty-five to forty yards. The smallest depth, at which it has been 

 found, is in a mine situated close to Witton Brook, about half a 

 mile above the entrance of this stream into the Weaver. Here it 

 appears at twenty-nine yards from the surface ; and a general esti- 

 mate of level from this mine shews that the upper surface of the 



