1852.J STRICKLAND PSEUDOMORPHOUS CRYSTALS OF SALT. 7 



tides was covered by the sea, we can easily conceive that in the in- 

 terval between two spring tides, or in the still longer one between two 

 sequinoctial 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 in- 

 terval, the tide again overflowed the spot, the returning sea-water (not 

 being saturated) would dissolve these saline crystals, leaving cubical 

 cavities in the mud which contained them. The tide would bring vnth 

 it a fresh deposit of fine sand, a portion of which would pour into the 

 cavities formed by the ciystals, and the remainder wovxld form a 

 homogeneous stratum immediately above. Such seems to me the 

 probable explanation of the phsenomenon in question, as seen on the 

 under side of the slabs of sandstone. There are, however, examples 

 on one of the slabs of similar crystals on the uppei^ surface, similarly 

 connected vpith the mass of sandstone, and less easy to be explained. 

 In the former case we suppose the sand to have poured by simple 

 gravitation into the subjacent cavity. But here the sand rises above 

 its ordinary level, filling a cavity in the superincumbent stratum of 

 marl. I can only suppose that here the crystals having formed be- 

 tween the sand and the mud which covered it, and being afterwards 

 dissolved away by the returning tide, their places were filled by a 

 portion of the subjacent sand, which being in a soft state, and inter- 

 mixed with water, might without much difliculty be pressed upwards 

 through the short space, not exceeding ^^th of an inch, in which 

 these crystals project above the surface. 



That chloride of sodium was the original substance that gave form 

 to these bodies is further shown by the fact that in one example we 

 find that peculiar concave figure so often seen in crystals of common 

 salt, and which is due to the original cube floating at the surface of 

 the brine, when a succession of smaller cubes form round its margin, 

 and ultimately give it a kind of basket-shaped form, which is di- 

 stinctly seen in the specimen in question. 



These pseudomorphous crystals of salt therefore supply us vpith an 

 additional evidence of those subaerial agencies affecting littoral de- 

 posits, of which the Keuper sandstone has already furnished examples 

 in the ripplings, the rain-drops, the mud-cracks, and the reptilian 

 footprmts so often presented on its surface. 



I may add, that on showing these specimens to Prof. J. Phillips, 

 he informed me that he had seen similar examples of pseudomorphous 

 crystals in the Keuper Sandstone at Spetchley in Worcestershire. 

 There are also some specimens of the same phsenomenon in the Mu- 

 seum of the Birmingham Philosophical Institution, presented by 

 Miss Jukes, the locality of which I have not been able to ascertain. 

 And Dr. Percy has kindly communicated a specimen from Clifton 

 Grove, Nottinghamshire, which seems to prove the extension of 

 Keuper Sandstone, affected by similar conditions, over a considerable 



