17 
If the Arragonite had been covered by a thin film of quartz, it would thereby 
have been decomposed, and the deposition of the calcite upon the film we should 
easily understand, but not a trace of a siliceous film can be detected. 
Nicol, in his mineralogy, states that Arragonite falls to pieces at a low tempera- 
ture, but admitting that a low temperature had occurred should we not expect to 
find the fragments of the Arragonite left in the Pseudomorph. The only feasible 
idea that presents itself to my mind is, that after considerable decomposition — 
which left the surfaces of the Arragonite very uneven and jagged — a deposition of, 
it may be, the same Carbonate of Lime took place, covering the salient points of 
the original crystal (not entirely investing it) with crystals of Calcite. 
After the Carbonate of Lime in solution had thus been deposited, the liquid 
became acidified and decomposition was renewed, no doubt the newly formed 
crystals of the Pseudomorph were now affected, but not sufficiently so as to dis- 
solve them entirely. As soon as the solvent became saturated, a second deposition 
took place ; this time, it may be, to largely increase the number of the crystals and 
to connect them together, still however, leaving many interstices such as we see in 
the Pseudomorphs on the table. A repetition of these actions would in time 
entirely decompose the Arragonite and form the Pseudomorph. 
But whether this is nature's modus operandi I wish to know, and I sincerely 
hope that the members of the chemical section here present will be able to give us, 
if not a full explanation — at all events some suggestion as to the probable manner 
in which this highly interesting and somewhat rare pseudomorph was formed. It 
is a remarkable fact that no work on mineralogy with which I am acquainted 
explains the pseudomorph. Brooke and Miller, in their last edition of " Phillips' 
Mineralogy " (which probably is the best work of the kind published in the English 
language) simply mention it, but offer no explanation or remark as to its probable 
formation. 
Mr. Jordan concluded his paper by indulging in a few aesthetieal consider- 
ations suggested by the analogy between the mutations which occurred in 
the mineral kingdom with those that occurred in the vegetable world ; 
which all tended to show the harmony of Nature and the ineffable wisdom 
and skill of Him " who made all things, and by whom all things consist." 
The President remarked that the subject of Pseudomorphs was very 
difficult, and required much thought and study. Mr. Jordan had well 
described the six systems of crystallisation, and it was a great misfortune 
that there were several names in use for the same system, by different 
writers. So far as he could see, he considered that the formation of pseu- 
domorphs was due to a gradual replacement of one set of atoms, singly 
and separately, by another. In the case of the stone-lilies, or Encrinites, so 
abundant in the limestone quarries of the neighbourhood, the stems of the 
living animal were formed of detached plates, but they had become truly 
petrified, and calcareous spar had taken the place of the whole substance 
of them so completely, that planes of cleavage in the rock would frequently 
go through several of these individual plates without detaching them from 
one another. He apprehended that the formation of pseudomorphs might 
go on in somewhat the same manner. 
