48 Dr. Wallich on the Identity of the Chalk 
contractile, but as yet unconsolidated, nascent nodules had 
closed in so as to completely imprison them, I argued as 
follows :—Inasmuch as the stratified flints were formed at the 
immediate surface of the ancient sea-bed, from an already highly 
saturated combination of sponge-silica and sponge-proto- 
plasm *, the portion of organic débris entrapped within the as 
yet unconsolidated masses of nascent flint must, in like manner, 
have been derived from the immediate surface-layer ; and 
consequently, since the nascent nodules were, under the con- 
ditions specified, incapable of appropriating more silica from 
any source, or of transferring any of their own silica beyond 
their own boundary walls, whatever quantities of silica and 
carbonate of lime were originally present in the imprisoned 
masses, must have remained locked up, from the Cretaceous 
period down to our own time, in the cavities in which we now 
find them. 
It will be seen hereafter that the examination made by 
me, in pursuance of the plan thus sketched out, of a very 
extensive series of carefully selected nodular flints, fully sub- 
stantiates the main conclusions at which I had previously 
arrived on the basis of collateral evidence only. 
As regards the abstract possibility of the walls of the 
nodular cavities (reduced as they occasionally are to a thick- 
ness not exceeding a third or even quarter of an inch {) being 
sufficient to prevent the dialytic translation of any of the 
mineral substances held in solution within them, it is, I pre- 
sume, almost unnecessary for me to offer any further proof 
than the fact, well known to every mineralogist and chemist, 
that minute quantities of fluid have continued pent up in 
quartz, for periods, and under pressures, far exceeding those 
we have now to deal with. But, even apart from this fact, 
the condition in which we find the contents themselves, and 
the appearance presented by the perfectly unbroken walls of 
flint, tend to prove that no such dialytic translation of mate- 
rial has, or indeed could have, taken place. On this point, 
however, I shall have something more to say further on. 
It has already been stated that my observations serve to 
establish the lithological identity of the ancient and recent 
calcareous formations. But they do more than this; they 
lend the strongest support to the view, if they do not abso- 
lutely prove, that the final stage arrived at, in the consolidation 
of the contents of the nodular cavities, is wholly dependent 
* Quart. Journ. Geol. Soc., Feb. 1880, pp: 68-91. 
+ I mention these, not as indicating thicknesses present in the 
nodules the contents of which I have selected for analysis, but merely as 
the minimum compatible with safety. 
