the Cheshire Reck- Salt District. 341 



spect to the rate and progression of this decrease, the ge- 

 neral fact seems to be sufficiently confirmed by observations 

 taken from different mines. In those wh ch have been 

 sunk near to the western or north-western side ot 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 ge- 

 neral direction Jrom 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 con- 

 siderable irregularity 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 appearances 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 fri)m 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 estimate of level from this mine shows that the 

 upper surface of the salt is at least twelve or thirteen yards 

 below the low-water mark of the sea at Liverpool ; a tact 

 perhaps not wholly unimportant as regards our ideas of the 

 formation of this mineral. 



The thickness of the upper bed of salt at Northwich has 

 been already stated to vary from twenty to thirty yards : 

 that of the 'lower bed has never yet been ascertained in any 

 one of the mines in this district. The workings in this 

 lower stratum are usually begun at the deptb of from 

 twenty to twenty-five yards, and are carried down for five 

 or six yards, through what forms, as will afterwards be 

 mentioned, the purest portion of the bed. In one of the 

 mines a shaft has been sunk to a level of fourteen yards 



still 



