* 
1897.] Fossils and Fossilization. 291 
limited its process of universal secretion to creating skeletons 
and coverings of carbonate of lime, in which there has also been 
mingled in many instances phosphates and occasionally cor- 
neous layers of an indeterminate mineral and organic charac- 
ter. Most of the fossils we are required to study originated in 
calcareous bodies, and were originally deposited as such, and 
for the most part they as fossils retain their calcareous sub- 
stance to-day. A mineralogical change, however, has in many 
cases supervened, and the carbonate of lime, known as aragon- 
ite, which formed many shells when occupied by their living 
tenants, has become changed to the more stable form of the 
same salt, calcite. This change has been often hastened by 
pressure and heat, and even, perhaps, by perturbations of the. 
earth’s crust, which have reassorted the molecular units and 
brought them into the secondary state of equilibrium known 
as calcite. Sorby has shown that the calcareous portion of 
organisms is at first deposited in the form of granules of vari- 
able size. These “afterwards undergo more or less of crystal- 
line coalescence. In some cases this scarcely occurs at all; but 
in others it does to a very considerable extent during the life 
of the organism, and this produces a great difference in the 
character of the particles into which it is resolved by decay. 
The falling to powder that then takes place is the result of the 
oxidization and removal of the organic portion, and, if no 
crystalline coalescence had occurred, the shell or other body 
might be resolved into the very minute ultimate crystalline 
granules; whereas, if much coalescence had taken place, it 
would break up into much larger ones, showing in many cases 
its minute structure.” These observations were made with 
reference to the condition of the shells of Lymnea and other 
fresh water molluscs in marls, but doubtless apply to the shells 
of marine formations, and may explain the fragmentary state 
of shells in limestones, while it points to an agency in prepar- 
ing the calcareous mud in which they are embedded, though 
this latter arises more generally from partial solution of shells 
in carbonated water. A remarkable form of replacement occurs 
in caleareous fossils, as it has been shown by Zittel, Hinde and 
Sollas that the soluble silica of the siliceous skeletons of the 
