278 Contemporaneous Deposition of Salt and Gypsum. 



water, continuing to be thrown up, runs through the bed of 

 gypsum, and carries with it a minute portion of the earthy 

 substance, which crystallises as it trickles from the roof, and 

 becomes appended thereto, forming a stalactite of gypsum, 

 Upon the floor of the cave, from the drippings before men- 

 tioned, is produced a stalagmite of the same formation. Thus, 

 in the lapse of years, are formed gypseous caves, and beautiful 

 stalactites of alabaster, from the evaporation of sea-water. 



The rock on which it is formed is augitic basalt; the con- 

 stant south-east trade wind keeps up a heavy surf on this side 

 of the island, and replenishes the pool or basin in the rock 

 before it is emptied. On the surface of the rock I found a 

 bed of earthy gypsum 2 ft. or more in thickness, with some 

 small portion of salt. On going down to the caves, which is 

 occasionally impractibable from the great swell, I saw the 

 water oozing from every pore of the rock : it was extremely 

 salt, and the cavern abounded widi the most elegant translu- 

 cent stalactites and coarser stalagmites. Some were of great 

 size, 4 ft. or 5 ft. in length, and 40 lb. or 50 lb. in weight ; 

 but others were of the most delicate proportions. The larger 

 ones were tinged within of a bright green, probably from a 

 sulphate or muriate of iron derived from the action of the 

 water on the basaltic rock. On the basement floor of the 

 cave were some little pools of very strong bitter salt water, 

 and around the edges of the rock crystallised salt : the water 

 contained sulphate of magnesia, or Epsom salts. 



Gypsum occurs in volcanic islands : we found it at South 

 Shetland. It is abundant at St. Helena, in veins among the 

 lofty ridges of basaltic rock. But it may, in these cases, 

 have a very different origin, and be a true igneous formation, 

 from the evolution of sulphureous acid coming in contact with 

 the earthy base, and thus form a hydrous gypsum : for we 

 had sulphur and alum, at South Shetland, with the gypsum. 

 Here we have demonstration of its marine origin, forming 

 extensive beds, and crystallising in a variety of ways. There 

 is, also, evidence of the contemporaneous deposition of com- 

 mon salt, and the production of sulphate of magnesia. At 

 St. Helena, they have aperient springs issuing from the rocks 

 near the gypsum veins ; but, nevertheless, I do not think it 

 an analogous formation to this now described. It is right to 

 state that, in some of the low parts of the island, near the 

 shore, casual crosier-shaped crystals of acicular gypsum are 

 picked up. 



I have been particular in stating these circumstances, and 

 in connecting them with others, because facts are valuable 

 only in reference to some general law ; and what is obscure in 



