OF COMMON SALT, 



153 



which they finally subside. It is precisely whilst this 

 shrinking is going on from 850 to 100 volumes that gypsum 

 is being deposited, so that when the brine reaches the centre 

 and begins to deposit salt, very little, or perhaps no gypsum 

 may be left in it. It all depends on the configuration of the 

 sea bottom. In all ordinarily shaped sea basins, therefore, 

 having shelving shores sloping to a central abyss, we will 

 have a large deposit of gypsum laid bare by the receding 

 brine, and in many cases the whole of the gypsum will be 

 thus left behind away from the salt. 



What follows is this. Common salt is deposited in a large 

 circle in the centre or deepest part of the sea bottom. This 

 may go on uninterruptedly until all the salt has been parted 

 with, in which case, we would have a large central salt 

 deposit, surrounded by a much larger circle of gypsum, 

 deposited on a slope, at a higher level. It only needs the 

 intervention of rain and flood to complete the picture ; the 

 gypsum is washed over the central salt deposit and settles 

 upon it, covering it over with a thick impermeable stratum, 

 which protects it from further injury and seals it up for 

 future use. But more likely, storms will overtake the 

 process, more than once, before its completion, and floods of 

 gypsum and mud, with mollusc shells perhaps, will contami- 

 nate the brine, and settle down upon the salt, which, in its 

 turn, will form again upon the gypserous bottom ; and so 

 alternate layers of rock salt and gypsum, or gypserous marls 

 with shell limestone, &c, will come to be formed, precisely 

 as we find them. 



After an uncertain lapse of time, this salt basin would be 

 completely filled up, by the gradual deposition of sedimentary 

 matters carried in by streams, or by the dust of aerial currents. 

 Once filled up, our rock salt basin loses its power of 

 drawing solid materials to itself ; it forms a part of the great 

 highway to the sea, and comes under the influence of 



