78 
UPPER CHALK. 
the particles of the siliceous nodules, as they were 
in the act of separation from the original compound 
mass. The distances of the siliceous strata must 
have been regulated by the intervals of precipitation 
of the matter from which they are derived : each 
new mass, as it was discharged, forming a bed of 
pulpy fluid at the bottom of the then existing 
ocean, which, being more recent than the bed pro- 
duced by the last preceding precipitate, would rest 
upon it as a foundation similar in substance to 
itself, but of which the consolidation was sufficiently 
advanced, to prevent the ingredients of the last 
deposit, from penetrating or disturbing the pro- 
ductions of that which preceded it.” * 
That the beds of chalk and flint were deposited 
periodically, cannot admit of the slightest doubt. 
Specimens are not unusual, in which angular frag- 
ments of black flint, that could not possibly have 
been originally formed in their present state, are 
imbedded in chalk. An example of this kind, in 
my possession, contains several portions of flint 
which are as sharp and translucent as if recently 
broken, and entirely destitute of the external 
opaque crust invariably seen in the perfect nodules ; 
these are imbedded in, and separated from each 
other by, tlie chalk. It is sufficiently obvious that 
the nodule from which these pieces were derived, 
must have been displaced and broken, subsequently 
to its original formation, and the fragments after- 
wards enveloped in another and more recent stratum 
of chalk. In this country the chalk very rarely 
contains traces of older deposits : the only in- 
stances of extraneous rocks that have come under 
(icological Transactions, vol. iv. p. 420. 
