Dr. Blum on Pseudomorphous Minei'als. 79 



same by another, there are two distinct modes of formation. The 

 substitution takes place either during the removal of the original 

 mineral, or, afterwards, by infiltration 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 imbedded in clay were 

 removed by solution, and the place afterwards occupied by the 

 infiltrating anhydrite, gypsum, or polyhalite.* 



Of the former mode, are most of the pseudomorphs of quartz. 

 Implanted crystals are thus altered when there are no moulds to 

 be filled up. 



Another kind still, proceeds from the incrustation of crystals by 

 another mineral and their subsequent removal, leaving a thin 

 hollow mould with the external and internal form of the original 

 crystal. 



The various siliceous pseudomorphs are the most numerous 

 and most interesting of this class. We have already enumerated 

 them on page 68. The change is often complete through the 

 crystal, the whole being altered to quartz. In other instances 

 the interior is vacant, or consists of the original mineral still un- 

 altered. Crystals of fluor spar, consisting of quartz except a 

 small fluor nucleus, are noticed by Blum. An instance is given 

 by our author of altered calc spar in which the silica penetrated 

 between the laminae, forming a series of plates, and the remain- 

 ing lime afterwards removed left a cellular crystal with the gen- 

 eral structure of the calc spar. Their surfaces are generally 

 rough or drusy, but sometimes are smooth, with the edges and 

 angles sharp. 



An instance is stated of a crystal of heavy spar, containing a 

 concentric layer of brown iron ore about the distance of a line 

 from the exterior surface, which was altered to quartz and still 

 retains unchanged the layer of brown iron ore. The silica was 

 infiltrated through this layer to the very centre of the crystal 

 without disturbing it. 



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

 region, appear tobecastsof hopper-form crystals of salt. These hollow, pyramidal 

 crystals were imbedded 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 forms. 



