336 Rev, Mr. Conybeare on Siliceous Petrifactions. [Nov. 
entirely siliceous, and in parts studded with minute crystalliza- 
tions of quartz. On exposing the mass to the action of diluted 
muriatic acid, I found that the siliceous matter of the coral 
penetrated but a very little way into the limestone; even at the 
point of entrance the branch became smaller, and its organic 
character much obliterated. 
2. Siliceous Fetrifactions of the Stems and Plates of the 
Encrinite beautifully preserved in Relief on the (weathered ?) 
Surface of Limestone Blocks. (Barrington Cleve, and Men- 
dips.)—On exposure to acid exhibited the same phenomena 
with the preceding. In both cases even the smallest no- 
dules or plates of limestone exhibit in their interior, organic 
portions, of the same species with those silicified on their sur- 
faces, but preserved as usual in calcareous matter. A lucky 
fracture, even while 1 am employed in transcribing my notes, 
has denuded for me a branch of coral, which is converted on the 
surface into silex, but almost from the point of entrance is con- 
tinued in calcareous spar. The limestone in which these fossils 
are imbedded gave, upon solution in a dilute acid, a residuum of 
from six to seven per cent. consisting chiefly of silex in a state 
of minute division. It struck me forcibly that these and _all 
specimens of the same nature, which I recollect to have seen 
from the mountain limestone, appear either to have been detached 
from the exterior of the strata, or to have occurred in small 
insulated masses. This, joined to the circumstance already 
noticed of the change which takes place in the substance of the 
fossil after its entrance into the limestone, induces a conjecture 
that the source of the silex is to be sought for in the iron shot 
marle which occasionally fills up the interstices between the cal- 
careous strata. On the causes which may have operated to 
produce, either in this or any other formation of limestone, the 
alternation of marly or slaty beds, I do not venture to speculate. 
3. Gryphites preserved in coarse Chalcedony, from Dunraven. 
—These are found both in the solid lias and in the marle which 
alternates with it. A portion of the former, in which a chalce- 
donic shell was totally enveloped, afforded with acid a residuum 
of silica mixed with a small proportion of alumina, iron, and 
bituminous matter. The whole residuum amounted to nearly 
15 per cent. of which two-thirds at least must have been silica. 
In the lias which furnished these specimens, I observed large 
cornua ammonis having their interior partly studded with crys- 
tallized quartz. The composition of the lias beds, especially 
towards their exterior, is so variable that other specimens might 
possibly furnish a yet greater proportion of silex. 
4, A Specimen of Chalcedonic Shell imbedded in a very Chalky 
Form of Green Sand ( from near Stourhead, Wilts)—From the 
nature of the matrix, the action of diluted acid very readily 
extricated the shell (a portion ofa large pecten.) The acid then 
attacked the shell itself, and before its action ceased, had dis- 
