1897.] Fossils and Fossilization. 287 
and with the entrance of the silica between the cleavage plains 
of the same, that doubtless we have here one and the same 
process of alteration. Therein, however, there is a difference 
between the displacement of the carbonate of lime in the mol- 
luscan shell and that in the cale-spar, in so far as the latter is 
frequently hollow and the former not; in the shell the space 
of the removed carbonate of lime is entirely, in the calc-spar 
only partially filled. We might ascribe this difference to the 
well-known inclination of silicic acid to unite itself with organic 
substances, if the amount of organic matter were not so small. 
The true explanation of this difference must await further in- 
vestigations” (Chemischen and Physikalischen Geologie). 
This latter contrast in the silicification of fossils and the 
pseudomorphism of calc-spar seems to arise principally from 
contrasted quantitative conditions. The solid replacement of 
the carbonate of lime in fossils by silica is connected with the 
former’s slow solution, as compared with that of exposed crys- 
tallized calcite, which may be rapidly invaded by carbonated 
or acid waters, whereas the imbedded fossil receives the access 
of terrestrial waters but slowly, and also retains its carbonate 
of lime somewhat intermixed with organic envelopes, the sar- 
codic filaments that penetrate the hard parts of invertebrates, 
and so surrenders it to solution less quickly, with the result of 
acquiring a dense and complete molecular replacement. In 
regard to the view of thesolution of carbonate of lime in water 
expelling its dissolved silica Bischof remarks, that in the case 
of the hollow crystals of calcite we are shown how the water 
has dissolved and removed more easily the soluble carbonate of 
lime than it has deposited the less easily soluble silica, an ex- 
pression, which, from the point of view taken here, is simply 
equivalent to saying, that the amount of silica in the waters of 
solution was insufficient to fill the space occupied by the calcite. 
Subsequent crystallization of silica within these hollows would 
simply produce drusy surfaces on a crystalline texture of inter- 
locked crystals. Fossils also undergo so-called secondary re- 
placement, when their forms become distorted and rough, and 
little circular monticules of silica are distributed over and 
through their shells and skeletons. This “ orbicular silica ” 
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