a2 Dr. Wallich on the Identity of the Chalk 
questionable proof that the quantity of silica now present im 
it is so small as hardly ever to exceed 3 per cent., the close 
approach to uniformity between the demonstrable percentage 
of silica in the surface calcareous mud of the existing sea-bed 
and the now also demonstrable percentage in the contents of 
the hermetically closed nodular cavities (which must in like 
manner have been derived from the immediate surface mud 
of the cretaceous sea-bed) warrants the inference that the 
relative percentages in the equivalent ‘ horizons” of the two 
formations are equal, and hence that this uniformity must, 
according to all reasonable probability, extend to every sub- 
jacent horizon. 
In order to substantiate this inference, however, it is indis- 
pensable that some distinct and constant difference should be 
shown to exist between the mineral constitution of the material 
contained in such flint-cavities as have from the beginning 
remained perfectly closed, and those which have never been 
completely so. I may state that within the past six months 
I have carefully examined at least two hundred specimens of 
each kind and that, so far as the experience thus acquired 
enables me to speak with confidence, I am quite unaware of 
a single example in which the original organic carbonate of 
lime of the Foraminifera and other débris obtained from the 
closed cavities, has not been more or less completely, but 
never completely, replaced by the organic silica. Isay never 
completely replaced because I have never yet seen any portion 
of the original calcareous material that yielded otherwise than 
very partially to the action of hydrochloric acid. As a rule 
the entire contents, without exception, are composed of chert 
—that is to say, of a chemical combination of carbonate of 
lime and silica in every proportion, from the state of nearly 
pure silica on the one hand, to the perfect form of chert 
already described by me at p. 49, anté, on the other; whilst 
in the contents of the only partially closed cavities, and 
notably in the deep external furrows present in some nodular 
flints, the silicification would seem rarely, if ever, to have 
proceeded to the same extent; and we find (what is never 
found in the perfectly closed cavities) calcareous organisms 
altogether unmetamorphosed. ‘This is most strikingly mani- 
fest in the case of “ coccoliths,” and, as I have recently 
discovered, of the coccospheres, every one of the latter obtained 
from the closed cavities being, without exception, silicified to 
such a degree as to remain perfect in all their details after sub- 
jection to strong hydrochloric acid. ‘The explanation of this is 
sufficiently obvious. In the one case the siliceous materials 
and included portion of colloidal protoplasm have had an 
