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totally do they differ, except, perhaps, in one or two instances, from 
any other known substance, that although it is sufficiently evident 
that they are organized masses, which have derived their forms from 
the energies of animal life ; yet, in most of the substances which we 
shall examine, we shall find ourselves very much at a loss respecting 
their original nature and the mode of their formation. 
The matrix in which these curious substances are found, is a fine 
tophaceous and pure calcareous substance, deriving a yellow tinge 
from its impregnation with iron. It breaks, of course, into irregu- 
lar fragments, an examination of which often shews the traces of va- 
rious coralline bodies, as well as very small pieces, and sometimes 
the casts of various shells. 
Mr. Walch, who appears to have derived some aid from the inqui- 
ries of Mr. Hoffman, Surgeon Major at Maestricht, and who has 
evidently paid considerable attention himself to these fossils, is of 
opinion, that they may be divided into three classes. In the first of 
these classes he places those which are really petrifactions, in which 
the actual substance of the marine body has acquired a stony na- 
ture. In the second class he places those which are merely casts ; the 
lapideous matter, which had filled the several cavities of the coral- 
line bodies, having remained, after the removal of the original sub- 
stance, by the action of some chemical agent. And, in the third 
place, he disposes of those, which he considers as merely the impres- 
sions of substances which have existed on the surface, or in the sub- 
stance of these masses, and which have been entirely removed by the 
action of some menstruum. 
Perplexing as we shall hereafter see Mr. Walch found the investi- 
gation of these substances, he was yet of opinion, that although 
there exist among them so many new species of animals, yet he does 
not see any reason to suppose, that for the arrangement of any of 
them, the formation of a new genus is necessary. He, however, ac- 
knowledges, that he found many bodies which, although he consi- 
