Dr J. Reinhard Blum on Pseudomorphous Minerals. 249 



posed to take place generally by the formation, first, of a 

 hydrated sulphate of iron, the sulphur and iron each uniting 

 with oxygen, and then, following this, a removal of the sul- 

 phuric acid by some alkali or earth in the vicinity. Instances 

 are given of gypsum, thus produced by this acid, combining 

 with the lime of the gangue. A more direct mode, without 

 the change to a sulphate, is as follows : — The water present 

 gives hydrogen to the sulphur, to form sulphuretted hydrogen, 

 and oxygen to oxidate the iron ; and with the oxide thus 

 formed, other portions of water combine and produce the 

 hydrate of iron in brown iron ore. In this process some 

 sulphur is set free, and thus Dr Blum accounts for the in- 

 crustation of sulphur often found with decomposing pyrites, 

 or in cavities originally occupied by this mineral. 



The change producing those pseudomorphs, commences 

 with the exterior ; and it is quite common to find crystals 

 with a thin incrustation of brown iron ore. Cubes, penta- 

 gonal dodecahedrons, and compound crystals, thus altered, 

 are of frequent occurrence. 



Among pseudomorphs, in which there is a total change in 

 the constitution of the mineral, or a complete replacement 

 of the same by another, there are two distinct modes of for- 

 mation. The substitution takes place either during the re- 

 moval of the original mineral, or, afterwai'ds, by infiltx'ation 

 into its mould. 



Of the latter mode, are, probably, as our author suggests, 

 the pseudomorphs of anhydrite, gypsum, and polyhalite, with 

 the form of common salt. The cubes of salt embedded in 

 clay were removed by solution, and the place afterwards 

 occupied by the infiltrating anhydrite, gypsum, or poly- 

 halite.* 



Of the former mode, are most of the pseudomoi'phs of 



* The hopper-form clay-crystals found in the red marl of the Onon- 

 daga salt region, appear to be casts of hopper-form crj'stals of salt. 

 These hollow pyramidal crystals were embedded in the clay (after 

 being produced by the evaporation of salt water over the clay), and 

 subsequently being dissolved out, left clay casts of their singular form?. 



