1874.1 333 [Hunt. 



pseudomorphs, or false forms, is given to mineral masses resulting 

 from very unlike processes. Of these, the first class has a very simple 

 origin. In a vein where the process of filling up by deposition is 

 occasionally varied by the opposite process of solution, the re- 

 moval of certain crystals from their matrix leaves cavities which are 

 afterwards filled, up by another species. An example of this is seen 

 in the well-known cubes of chalcedonic quartz cast in the moulds of 

 fiuorite. The silicification, or so-called petrifaction, of wood, is the 

 result of a similar process. The pores of the wood, are in the first 

 place filled with silica, while the woody fibre remains. This is sub- 

 sequently removed by decay, leaving a porous mass, the cast of the 

 original spaces in the wood ; but in many cases this, in its turn, be- 

 comes filled up, and there thus is at last produced a solid mass, in 

 which both the pores and the fibre of the wood are represented by 

 silica. 



The second class of pseudomorphs is illustrated by the alteration of 

 feldspar, and various feldspathides, which by a loss of protoxyd bases 

 with a portion of silica, and the taking up of water, are converted 

 into kaolin. The change of chalybite and pyrite into limonite, and 

 of cuprite into malachite, are examples of similar processes; all of 

 which take place under atmospheric influences. 



Analogous alterations may be produced in veins as a result of 

 changes in the composition in the circulating waters, as when crystals 

 of pyromorphite, a phosphate of lead-oxyd, are converted into 

 galena. Changes of this kind being effected from without, nuclei of 

 the unaltered species are often found in the centre of the altered 

 crystals. This process is obviously very different from replacement, 

 and is properly designated epigenesis. 



The notion of epigenic alteration has been extended to a great 

 number of cases which have nothing in common either with it or with 

 replacement, but are simply examples of the association or envelop- 

 ment of different and unlike species. This envelopment is of two 

 kinds: as an example of the first, carbonate of lime crystallizing 

 from solution in the midst of silicious sand may include so much of 

 this that the resulting crystals, though having the geometrical forms 

 of calcite, contain less carbonate of lime than sand, as is seen in the 

 so-called crystallized sandstone of Fontainebleau. The nature of the 

 process is not, however, in all cases, so obvious as this ; but it is clear, 

 from numerous examples, that small proportions of certain substances 

 may, in the act of crystallizing, give their own forms to large and pre- 



