and Recent Calcareous Deposits. 53 
unlimited quantity of carbonate of lime in the surrounding 
matrix to work upon, and an unfettered access to water. 
Hence they have virtually been in the same position they 
would have occupied if simply imbedded in the chalk itself, 
where they occur in multitudes. 
In the other case the silica, carbonate of lime, and proto- 
plasm are imprisoned within a closed wall of adamant, and 
have had to come to terms as best they could without the least 
extrinsic aid or interference. 
It is moreover specially deserving of note that the occur- 
rence of s¢licified coccospheres in the closed cavities furnishes 
the strongest presumptive evidence that they and the materials 
associated with them are of extrinsic origin, and are not, as 
it were, effete or residuary products derived from the nodular 
mass itself. For, obviously, these extremely minute structures 
would have been the first to succumb to silicification by 
replacement, and consequently to absorption and obliteration 
in the substance of the flint, had those now found in the nodular 
cavities ever formed part of it. 
Another point upon which I lay great stress is the unmis- 
takable evidence, already alluded to, of the various changes 
that take place amongst the imprisoned materials, continuing 
in operation, in certain cases, to the present day. 
It is an important circumstance that, in such closed cavities 
as contain a large proportional quantity of water, the advance 
from the initiatory condition of the component materials asso- 
ciated with it ¢s at tts minimum—an apparently paradoxical 
assertion in view of what was said a few lines back, as to 
water being the determining agent in the chain of metamor- 
phism under notice. It is, however, no paradox; for what 
happens is, that the larger the relative quantity of water the 
larger is the quantity of silica maintained in solution, and the 
less tendency is there in it to supplant the coexistent car- 
bonate of lime. In this wise the carbonate of lime, obviously 
the more yielding of the two mineral elements throughout all 
these operations, is left alone for a longer period. 
It is a very remarkable fact, moreover, that the water-con- 
taining, cavities seem to be most common, and the quantity of 
water greatest, in those flints which have been obtained from 
gravel beds, and must therefore, during a very considerable 
period of their mearceration, have been subject to the mighty 
turmoil of some ancient coast-line. Unfortunately I have as 
yet neither had the time nor the appliances for analyzing this 
water. I can, however, state that it is perfectly clear and 
limpid when allowed to settle, has no perceptible saline taste, 
and evaporates with extraordinary rapidity on being laid bare 
to the atmosphere. 
