and Recent Calcareous Deposits. 5a 
obtained in the first instance from the Cretaceous ocean itself, 
had originally formed part of the ‘combined water”’* of the 
nodule when in its still colloidal nascent condition. On the 
other hand, itis by no means improbable that, in every case, a 
limited portion of the contents of the nodular cavities has, during 
the yet unconsolidated state of the nodules, been expended in 
thickening, from within, the cherty lining of the interior of 
these cavities. But it is manifest that, although this would to 
the same extent diminish the quantity of included material, it 
could not exercise any material effect on the relative percentages 
silica and carbonate of lime left within the cavity. 
Lastly, where any channel of communication existed be- 
tween the interior of the consolidated nodular cavities and the 
outer world, through which water from without could have 
penetrated, it 1s evident that in the case of flint nodules ob- 
tained from gravel beds, or where a stratified layer of flints 
had been left exposed to atmospheric influences, such water 
must have carried along with it foreign substances in a minute 
state of division or in solution, and that these must have 
produced a distinctly observable effect upon the imprisoned 
masses. Under these circumstances, moreover, a means of 
egress as well as ingress would have been established, so as 
to permit of the escape of portions of the material from the 
interior of the nodular cavities, as well as the entrance of 
some of the materials from without—a state of things which 
could not for a moment deceive any experienced observer. 
It will be seen that in my analysis I do not pretend to have 
arrived at strict accuracy, but have rested satisfied with sup- 
plying an approximate index to the relative proportions of 
soluble and insoluble materials only. It must be borne in 
mind moreover that, on the one hand, there is no question just 
now pending as to the quantity of residuary substances known 
to exist only in extremely minute proportions in the materials 
under examination, and, on the other hand, that silica and 
carbonate of lime constitute, as a matter of fact, at least from 
90 to 95 per cent. of the mass. ‘Ihe point to be determined, 
therefore, is not whether more or less than from 4 to 3 or, at 
the utmost, 4 per cent. of the silica present in the chalk is 
doubled, trebled, or even quadrupled in the material from the 
flint-cavities, but whether the silica present in the latter cor- 
responds approximately with that met with in the calcareous 
mud of the existing sea-bed T. 
* See a paper “On the Properties of Silicic Acid,’ by the late Mr. 
Graham (Proc. Roy. Soc. for June 1864). 
T The analysis given below was made at my request, in February 1880, 
by Mr. W. F. Ward, of the Royal School of Mines. It is intended to 
