SALMON OX nOCKS. 
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W''hile in many minerals these isomorphs replace each other to a 
large extent, the student must guard against supposing they can do 
BO ad ivjinitum, 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, 
Fyrite, &,c. These forms are not true crystals, and the quartz has 
acquired them in a way entirely different from ci*ystallization. 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 fm'm 
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. Kemoval of constituents. 
h. Addition of constituents. 
c. Exchange of constituents. 
* J. R. Bhnn, — Die Pseiidomorphosen des Mineralreichs, mit Nachtrag. 
Stuttgardt, 184.S-7. A very complete list of pseudomorphs, from Bhim, is given 
in Brooke and Miller's edition of " Phillips' Mineralogy." 
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