n8 Peacock: Naturalists at Newark. 



principal supply of rock salt and gypsum is obtained, hence 

 they are often known as the salt-bearing- rocks. They occupy 

 the upper division of the New Red Sandstones or Triassic 

 system. To a non-geologist this explains nothing. It may be 

 better, therefore, if I ask you to let your minds travel back 

 through ages and ages to a time when this England of ours 

 formed part of a large continent, in which were great rainless 

 plains and extensive salt and bitter lakes. It was in these lakes 

 that the red marls, the salt, and gypsum were deposited. We 

 have in the Dead Sea at the present time a very good example 

 of a salt lake, similar in character to those of the Triassic 

 period. Here the Jordan and other streams, bringing down 

 large quantities of dissolved matter, pour their waters into the 

 Dead Sea. Evaporation takes place from the surface so rapidly 

 that none of the water escapes in any other manner. The salts 

 accumulate until their point of saturation is reached, and are 

 then precipitated, unless they are previously thrown down by 

 chemical reaction. Of course, the least soluble salt is thrown 

 down first, and the more and more soluble as the volume of 

 water decreases. Now, taking the two principal salts found in 

 these marls, viz., gypsum, or sulphate of lime, and rock salt, it 

 is found that gypsum is precipitated from a solution much earlier 

 than rock salt; in fact, if we take sea water as a typical example 

 it is necessary to only evaporate 37 per cent, of the water before 

 gypsum is precipitated ; before common salt was precipitated it 

 would be necessary to evaporate 93 per cent, of the water. So 

 that in these salt lakes, whether ancient or modern, if ordinary 

 evaporation and concentration went on we should find first a layer 

 of gypsum, then a layer of rock salt, and this is often found to 

 be the case in the salt districts. Where we have, as in this 

 case, only gypsum, it may be due to the point of saturation of 

 rock salt never having been reached, or to the salt having sub- 

 sequently been dissolved away. In parts of the Dead Sea 

 which have at times been exposed the mud is found to be full of 

 crystals of common salt and gypsum, and we have abundant 

 proof that similar crystals formed in the mud of this old Keuper 

 Lake. Here is a small slab of the marl from a pit a few miles 

 distant, and on it you will be able to see great numbers of cubic 

 crystals of mud, which were originally crystals of Common salt, 

 but percolating water has subsequently dissolved away the salt 

 and filled up the spaces With particles of mud, hence we get 

 these pseudomorphs of rock salt. You will easily understand 

 that in such concentrated solutions as we find in these dead 



Naturalist, 



