ORIGIN OF CRYSTALLINE ROCKS. 489 
molecular, or, at most, confined in certain cases to reactions be- 
tween the mingled elements of the sediments, with the elimination 
of water and carbonic acid. It is proposed to consider briefly, 
these two opposite theories, which seek to explain the origin of 
the rocks in question respectively by pseudomorphic changes in 
preéxisting crystalline rocks, and by the crystallization of aqueous 
sediments, for the most part chemically formed precipitates. 
Mineral pseudomorphism, that is to say, the assumption by one 
mineral substance of the crystalline form of another, may arise in 
several ways. First of these is the filling up of a mould left by 
the solution or decomposition of an imbedded crystal, a process 
which sometimes takes place in minera? veins, where the processes 
of solution and deposition can be freely carried on. Allied to 
this, is the mineralization of organic remains, where carbonate 
of lime or silica, for example, fills the pores of wood. When sub- 
sequent decay removes the woody tissue, the vacant spaces may, 
in their turn, be filled by the same or another species. * In the 
second place, we may consider pseudomorphs from alteration, 
which are the result of a gradual change in the composition of a 
mineral species. This process is exemplified in the conversion of 
feldspar into kaolin by the loss of its alkali and a portion of sil- 
ica, and the fixation .of water, or in the change of chalybite into 
limonite by the loss of carbonic acid and the absorption of water 
and oxygen. 
The doctrine of pseudomorphism by alteration as taught by Gus- 
taf Rose, Haidinger, Blum, Volger, Rammelsberg, Dana, Bischof, 
and many others, leads them, however, to admit still greater and 
more remarkable changes than these, and to maintain the possi- 
bility of converting almost any silicate into any other. Thus, by 
referring to the pages of Bischof’s Lehrbuch der Geognosie, it will 
be found that serpentine is said to exist as a pseudomorph after au- 
gite, prre TEM divine; chondrodite, garnet, mica, and probably 
also a and even orthoclase. Serpentine rock or oph- 
iolite is prieten to have resulted, in different cases, from the al- 
teration of hornblende-rock, diorite, granulite and even granite. 
Not only silicates of protoxyds and aluminous silicates are con- 
ceived to be capable of this transformation, but probably also 
quartz itself; at least, Blum asserts that meerschaum, a closely re- 
ey iia eg aie 
* Hunt on the Silicification of Fossils, Canadian Naturalist, new series, I, 46. 
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