CXiV PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 
of river channels, and in volcanic districts lakes may readily be 
formed and subsequently drained, the barriers which caused them 
being cut through or removed. It will depend upon obvious con- 
ditions whether any permanent records of these levels at different 
times be left or preserved. Lakes and isolated so-called seas, as is 
well-known, are inferred to have occupied larger areas than at present 
from evidences of this kind. 
The beautiful manner in which silica has entered the interstices of 
vegetable matter, even showing the succulent parts of plants which 
must have been in a state of partial decay, is well known. We have 
the finest vegetable tissues most perfectly preserved by means of silica. 
It will be in the recollection of the Society that Dr. Mantell pointed 
out to us in this room what he considered the soft parts of molluscs 
also preserved in silica. Yesterday only I received a communication 
from Mr. Charlesworth, in which he informs me that im a collection 
which constitutes part of the well-known museum of Miss Benett of 
Wiltshire, he had found several examples of Trigonie with their 
branchiee well-preserved in silica. As silica may have, and has filled 
up cavities left by shells, thus giving us the most perfect representa- 
tion in silica of that which was once carbonate of lime, great care is 
of course required, so that the mere filling up of the interior of uni-- 
valve and bivalve shells, before the matter of the shells themselves 
disappeared, or even when these are still left, be not taken for the 
preserved remains of the fleshy portions of the molluscs. We should 
expect, in cases of real preservation, as in those of vegetables, that 
the original tissue would be found by slicing in the usual manner. 
Mr. Charlesworth, who also forwarded a lithographic plate to appear 
with descriptions in the next number of his Geological Journal, con- 
siders that in the specimens he notices the fleshy parts of the Tri- 
gonie are really silicified, and states that the silica has only preserved 
some of the soft parts, without filling the entire cavity of the shell, 
and so that the filaments of the branchiz have all the appearance of 
an elaborate piece of dissection. Certainly the entire cavity of the 
shell not beimg filled up is very important, and as we find that the 
tissue of succulent vegetables has been preserved im silica, it may 
be fairly asked, why may not the fleshy parts of molluscs be thus also 
preserved ? 
The slags from our furnaces are often, as is well known, found 
crystallized in drusy cavities, their component parts havmg in such 
situations been placed under the needful conditions for adjustment 
according to their affinities, and for taking the definite forms due to 
their combinations. In a Report to the British Association on Cry- 
stalline Slags by Dr. Percy (of Birmingham) and Professor Miller 
(of Cambridge), published during the past year, we have the ana- 
lyses and crystallographic and mineralogical descriptions of many 
of these bodies, some of which, in the collection of Dr. Percy, are ex- 
ceedingly interesting. Six analyses of crystallized slags from iron 
blast furnaces gave the formula Al? O%, Si0’+2(3(Ca, Mg, Mn, 
Fe)O, SiO*), which differs from that of vesuvian in containing two 
equivalents of the lime series instead of one. The crystals of these 
Bi 
