* 
352 DYNAMICAL GEOLOGY. — [Boox III. 
chemical changes, of a kind like those in ordinary weathering, take 
place in them, and at some depth may be intensified by internal 
terrestrial heat. This subterranean alteration of rocks may con- 
sist in the mere addition of substances introduced in chemical 
solution ; or in the simple solution and removal of some one or 
more constituents ; or in a complex process of removal and replace- 
ment wherein the original substance of a rock is molecule by 
molecule removed, while new ingredients are simultaneously or 
afterwards substituted. In tracing these alterations of rocks the 
study of pseudomorphs becomes important, for we thereby learn 
what was the original composition of the mineral or rock. The mere 
existence of a pseudomorph points to the removal and substitution 
of mineral matter by permeating water." 
The extent to which such mineral replacement has been carried 
among rocks of the most varied structure and composition is 
probably best shown by the abundant petrified organic forms in 
formations of all geological ages. The minutest structures of plants 
and animals have been, particle by particle, removed and replaced 
by mineral matter introduced in solution, and this so imperceptibly 
and yet thoroughly, that even minutice of organization, requiring a 
high power of the microscope for their investigation, have been 
preserved without distortion or dis- 
arrangement. rom this perfect con- 
dition of preservation gradations may 
be traced until the organic structure 
is gradually lost amid the crystalline 
or amorphous infiltrated substance 
(Fig. 99). The most important petri- 
fying media in nature are calcium 
carbonate, silica, and disulphide of 
iron (mareasite more usually than 
pyrite) (see Book V.). 
Another proof of the alteration 
which superficial rocks have suffered 
Fic. 99.—FossiL wooD FROM TUFF, from permeating wane me supplied by 
BURNTISLAND, SHOWING PARTS PER- the abundance of velns of calcite and 
BRE, PRIA SEP REEL quartz by which they are traversed, 
porn or Cnrerazuizamiox OF these minerals having been. intro- 
duced in solution and often from the 
decomposition of the enclosing rock. As Bischof pointed out, a drop 
of acid seldom fails to give effervescence on pieces of crystalline 
rock which have been taken even at some little depth from the 
surface, thus indicating the decomposition and deposit caused b 
permeating water. As already stated, one of the most remarkable 

1 It is not needful to take account here of such exceptional cases as the artificial 
conversion of aragonite into calcite by exposure to a high temperature. In such para- 
morphs the change is a molecular or crystallir » rather than a chemical ono, though how 
it takes place is still unknown. 
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