SALMON — ON ROCKS. 



223 



While in many minerals these isomorphs replace each other to a 

 large extent, the student must guard against supposing they can do 

 so ad injlnitum, or in all instances. The example which I have given 

 of garnet is an extreme case. In the majority of minerals iso- 

 morphism can only take place within narrow limits, and is confined 

 to certain constituents : any increased alteration carrying with it a 

 change of crystalline relations. No definite laws can be laid down, 

 each group of minerals being characterised by particular powers of 

 substitution. 



XXII. Pseudomorphism is another incident of crystallization im- 

 portant in its bearing on the study of rocks. A pseudomorphic crystal 

 is one with a form foreign to the substance which composes it. For 

 instance, we find Quartz in crystals which differ entirely from its 

 proper form, and which on examination we discover to be charac- 

 teristic of various other minerals, among them Fluor, Gypsum, Calcite^ 

 Pyrite, &c. These forms are not true crystals, and the quartz has 

 acquired them in a way entirely different from crystallization. The 

 number of pseudomorphs already known is considerable, and will 

 probably be yet increased. They seem principally, if not entirely, to 

 have been brought about by slow aqueous agency gradually removing 

 the original mineral, or some normal constituent, and substituting in 

 its place either an entirely different body, or such a constituent as to 

 alter essentially its original chemical character ; the whole operation 

 occurring so slowly as not to admit of the substituted or altered 

 substance taking its proper crystalline form, thus retaining the form 

 of a substance passed away. Petrifaction is a familiar instance of 

 pseudomorphic change ; there the original animal or vegetable sub- 

 stance is removed, and entirely replaced by a foreign mineral substance, 

 although the original form in all its details is little altered. 



Blum* classifies pseudomorphs as — 



1. Alteration-pseudomorphs ; produced by 

 a. Removal of constituents. 

 h. Addition of constituents. 

 c. Exchange of constituents. 



* J. R. Blum, — Die Pseudomorphosen des Mineralreichs, mit Nachtrag. 

 Stuttgardt, 1843-7. A very complete list of pseudomorphs, from Blum, is given 

 in Brooke and Miller's edition of " Phillips' Mineralogy." 



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