Ixvili PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 
been removed and replaced by various mineral substances forms a 
subject of much interest, but upon which it would be out of place 
here to enter. It will be sufficient to observe, that the matter of a 
shell or of any other organic remain being removed, and the particles 
of matter surrounding the organic remain having been sufficiently 
consolidated to preserve its form or cast, the hollows are in the con- 
dition of any other cavity, whether produced by dislocations, cracks, 
or in any other way, so that the solutions of any substances entering 
them, it depends upon a variety of conditions what those substances 
may happen to be, or how far any of them may be retained to fill 
up the cavity. The siliceous fossil shells, so to speak, of the green- 
sand of the Blackdown Hills, are familiar examples of the substi- 
tution of silica for the carbonate of lime of the original remains, and 
similar examples are common elsewhere. Sulphuret of copper was 
found to have replaced some shells adjoming a copper lode near Ne- 
ther Stowey, Somersetshire, and sulphuret of lead may be seen oeca- 
sionally filling up the cavities of shells m the lias of Glamorganshire. 
In a trial for coal near Cairo, bitumen was found fillmg up the cavi- 
ties left by shells in the nummulitic limestone of that vicinity. All 
such changes are of an order similar to that of that section of pseudo- 
morphous crystals, where one mineral disappearing from a cavity, its 
form is given to another mineral of a different kind of crystallization, 
when the particles of the latter could freely adjust themselves. The 
pseudomorphous crystals of tin from St. Agnes, Cornwall, is a well- 
known example of this change in the body of a rock. The original 
crystals are those of felspar, in a granitic substance; these disap- 
pearing, peroxide of tin has percolated in solution into the cavity, 
and occupied it. Mineral veins, and indeed the contents of common 
faults, often show us one mineral cast, as it were, in a mould left by 
another, the mould being apparent. While thus adverting to pseudo- 
morphous crystals, it is necessary to observe that there is another 
section of them, where moulds are not apparent, and where, particle 
by particle, one mineral seems to have been replaced by another, so 
that the form of the first is preserved. 
In his memoir on the agate quarries of Oberstein, our Secretary, 
Mr. Hamilton, shows that while agates are discovered in the cells or 
vesicular cavities of igneous rocks, chalcedonic deposits are also found 
in veins traversing a red conglomerate ; whether referable to the base 
of the palzeozoic carboniferous, or to the new red sandstone series, 
his researches did not enable him to determine. After pointing out 
that the agates taken from the cells or vesicles of the igneous rocks 
could not, from the nature of their formation, contain organic re- 
mains, as it has been supposed the Oberstein agates did, Mr. Hamilton 
describes the chalcedonic veins, which evidently filled cracks formed 
in the conglomerate since its consolidation, not only the red cementing 
matter, but occasionally the pebbles themselves being traversed by 
the chalcedonic veins. The successive siliceous coatings of the fis- 
sures are sometimes clearly shown, with even a space left in the 
centre, where the mammillated surface of the chalcedony is well-ex- 
posed. 
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