292 ROCK SALT. 



to have compressed each other into polyedral figures. 

 That these are formed of concentric coats, is proved 

 hy their sections ; which display analogous lines of 

 colour, arising from the varying purity of the mineral 

 in the different layers, disposed in the same mariner 

 as in the columnar sandstone formerly described. 



The position of rock salt in the general, series, has 

 not, in every instance, been ascertained. Yet, in most 

 parts of the world, it has either been found decidedly 

 associated with the red marl, as already indicated, or 

 appearing to have been once united to this widely 

 extended deposit. In Cheshire, the evidence is un- 

 questionable, as it is in many other parts of Europe ; 

 and as, in Asia and Africa, it occurs with red sand, 

 and is often accompanied by masses of gypsum, it 

 probably belongs to the same series, now decomposed, 

 and often dispersed or obscured. It must, however, 

 be added, that, in some parts of Europe and America, 

 it passes the bounds of the red marl on both sides ; 

 so as sometimes to occur in the limestone beneath it, 

 and at others in that above. In some such cases, 

 where the limestones alone have been visible, it has 

 been described as belonging to them. It may, indeed, 

 exist in these without occurring also in the sandstone, 

 or be found where that is not present : but this will 

 not materially affect its geological history, when we 

 consider that it was the produce of partial actions, 

 during the general period of a deposit which must, in 

 the upper and lower parts, have been of nearly the 

 same dates as the calcareous rocks in contact with 

 these. 



The frequent occurrence of gypsum with salt, is thus 

 explained; as this mineral is equally associated with 

 the red marl, and as the beds of clay with which it is 

 intermixed are those which, in many places, form so 



