ROCK SALT. 



V P*111U. 



37 



t 



■ ■ tlle s 



( ' arl y40f 



m 50 to i 

 °°0 tons , 

 ro quantit 



other 



o. 



; equal to: 



river wi 



, and by 



lake (hav 



after wH( 



n the mam 



salt lab 



jfo/i spines, and a small bi-valve crustacean Estheria Vtpea 

 ?, formerly called Posidonomya minuta. The ma- — 



joritj of the crystals " are cubes, or modifications of cubes." 



" Of substances which crystallize in cubes, the only ones 



which usually occur in the Triassic formations are sulphuret 



J of iron, or iron pyrites, and chloride of sodium, or common 



N salt. It is hardly possible that sulphuret of iron can have 



(Offic supplied the mould into which the sand was afterwards 



poured, as it would require a considerable time both for the 

 [ 1> and oct formation and the removal of crystals of that mineral, 

 atcr part whereas it is evident that the crystals in question must 

 1 v and ferr have been formed, and must have been afterwards removed, 

 ncls, althou; leaving an empty cavity, in the short interval between the 

 ie manner deposition of one bed of sand, and of the one immediately 

 ck is obsct superimposed. All these conditions, however, are supplied 

 )sited in a i Q the most satisfactory manner by supposing chloride of 



Wall-case 40. 



[e of sodi 

 9-5 



1st 



crC 







iou- 



crf 



icUllc 



o 



;>» 



sea, abounding with sand- 



sodium to have been the material which formed the mould 

 for these pseudomorphous crystals. The ripple-marks — the 

 cracks formed by desiccation in the argillaceous beds, and 

 afterwards filled with sand poured in from above, and the 

 not unfrequent impressions of the feet of air-breathing 

 reptiles, all of which phenomena especially characterize the 

 Keuper sandstones of our English counties, seem to point 

 to a very shallow state of the 

 banks, and extensive salt-water marshes, often laid bare in 

 the intervals of the tides. If now we suppose that at the 

 locality in question a sandy marsh existed, which at high 

 spring tide was covered by the sea, we can easily conceive 

 that in the interval between two spring tides, or in the still 

 longer one between two equinoctial tides, the sea water, 

 ponded up in such a marsh, had time to evaporate and to 

 deposit its crystals of chloride of sodium, which being slowly 

 and tranquilly formed would assume their normal shape of 

 cubes. As the desiccation proceeded these crystals would 

 be enveloped by the fine muddy sediment which usually 

 forms the last deposit of water as it evaporates to dryness ; 



when after a given interval, the tide again overflowed the 



