and Recent Calcareous Deposits. ‘aff 
n the cavities is not soluble even in boiling nitrohydrochloric 
acid. ‘Hence it appears to be not a mere mechanical combi- 
nation of the lime and silica which produces the extreme 
hardness, whiteness, and homogeneousness of the porcelain- 
like form to which I have already called attention, but a truly 
chemical union between the two substances. 
Under these circumstances I leave the analysis to speak 
for itself. At the same time I annex to it a purely arbi- 
trary and provisional computation of the probable average 
amount of carbonate of lime that had entered into combination 
with the silica to form the large ¢nsoluble residue. One third 
may be too high or too low an estimate; but I am inclined to 
think it will eventually prove not very far wide of the mark. 
On these grounds I submit that I have made good the 
statement with which I opened this paper—namely, that the 
ancient and recent Cretaceous formations are lithologically 
identical. 
It will be noticed that I have deemed it better, for the pre- 
sent at least, to adhere to the term “ s¢lictfication” than to 
have continually to repeat the expressions conversion into chert, 
or flinty chert, or cherty flint, as the case may be—since 
one and all of these would, for the reasons already assigned, 
be too indefinite to serve any useful purpose. Our scientific 
vocabulary is already extensive enough to discountenance the 
addition to it of such a word as chertification. At the same 
time, I confess I do not quite see my way to avoiding it. 
In selecting specimens of materials for analysis I have used 
every available precaution to ensure their having been obtained 
from really hermetically closed nodular cavities, and being 
in the condition best fitted to furnish trustworthy results. 
The total number of specimens analyzed by me is ten. Of 
these, six were extracted from the cavities of nodules obtained 
from gravel beds and four from the cavities of nodules obtained 
from the Upper White Chalk. Two of those from the gravel 
beds, however, were analyzed, not for the purpose of com- 
parison, but with a view to determine the source of their ex- 
ceptionally rich sulphur tint. The number represented in the 
analysis is therefore reduced to eight. 
Taking into account 3 per cent. 
as the approximate quantity 
of carbonate of lime locked - 
up in the znsoluble residues, 
and adding it to the soluble, 
we have :— 
per cent. per cent. 
Soluble in acid .. 5237 | 6834 carbonate of lime. 
Insoluble in acid . 47,4, | 3138 silica. 
