and Recent Calcareous Deposits. 47 
to obtain for analysis a specimen of the consolidated stratum 
resting at some depth below the surface, inasmuch as the 
excess present in the immediate surface-layer (from which 
alone all our information has heretofore been derived) repre- 
sents, in reality, the aggregate contributions extracted from 
each freshly-deposited layer as it became gradually submerged 
beneath the layer that succeeded it. And, lastly, stating the 
case in another form, I concluded that, were it possible 
under existing conditions to compare with a portion of chalk 
a portion of recent calcareous mud obtained from a consider- 
able depth * below the surface of the sea-bed, there would in 
all probability be no material difference in the relative per- 
centages of silica derived from the two sources. 
It subsequently occurred to me, however, that if my hypo- 
thesis concerning the causes which lead to the stratification 
of the flints is correct, the fossilized organic contents, so 
frequently met with in certain hollow nodules, would at once 
supply the requisite data for determining the relative quan- 
tities of silica in the two formations, especially as the alleged 
great excess of silica in one of these is so pronounced as to 
render an approximately accurate quantitative analysis—such 
as I might myself carry out—sufficient for the purpose SO 
templated. 
The line of inquiry I marked out in order to put my 
hypothesis to the test of actual experiment was, to ascertain, 
if possible, what constant distinctions are traceable between 
the contents of those nodular cavities that have never, since 
their consolidation, been so perfectly closed as to cut off all 
communication whatever between their contents and the 
medium surrounding the nodules, and the contents of those 
cavities that have remained hermetically closed since that 
critical stage in their history when they became sufficiently 
consolidated to put every subsequent change in the lithological 
combinations formed amongst the enclosed materials beyond 
the pale of any supplementary accessions of materials from 
without, or any transfer of the originally included materials 
from within. a 
Then, taking as my point of departure the fact alluded toin 
my last paper on the flint question J, namely that the contents 
of hermetically-closed nodular cavities consist of absolutely 
intact portions of the organic material, and even of the water, 
of the ancient Cretaceous ocean, around which the highly 
* By immediate surface-layer I mean from 6 inches to probably a foot 
and a half, beyond which there is no sufficient proof that any dredge, 
sounding, or probing-machine has ever yet penetrated. ic 
+ Ann. & Mag. Nat. Hist., Feb. 1881, pp. 178 and 201. 
