52 
Wallich_, on Siliceous Organisms, 
eartliy bodies^ and the whole diffused through water^ the 
grains of silica have_, under certain circumstances^ a tendency 
to aggregate into small nodules ; and in chalk some grains of 
quartz (fragments of siliceous spiculse^ &c.) are discover- 
able." 
This theory^ no doubt_, is perfectly correct as far as it goes ; 
and the agency described does_, in all probability, perform a 
part in the segregative disposition of the flint formation. 
But_, when it is taken into consideration that a state of unde- 
viating repose is maintained in the depths at the bed of the 
ocean, and that the calcareous particles, which go to form the 
Chalk, are deposited in a pulpy mass, presenting a homoge- 
neous surface upon which any foreign deposit would be 
equably distributed, it is clear that other agencies must also 
be at work in order to produce the results observed. Under 
such a state of repose, were the tendency of the minutely 
divided particles of silex to aggregate together the sole agency 
in the flint formation, we should find the concretions obeying 
some general law as to size, shape, structure, and position. 
.But no such connexion can be traced. The masses are dis- 
tributed in layers, and these do not present themselves equally 
throughout the Chalk formation. I am aware it has been 
asserted that the siliceous beds of Diatomacese of the South of 
Europe and the North of Africa are of later formation than 
the Chalk. But the Chalk itself yields sufficient evidence of 
the presence of diatomaceous and other minute siliceous 
organisms in the waters of that period. The occurrence in a 
fossil state in the Chalk, and, as I have shown, in the flint 
itself, of forms living at the present time, indicates that the 
sea abounded with the same kind of siliceous organisms in 
those times as in the present. And, on the other hand, there 
is no evidence to prove that the waters of the Cretaceous period 
were more profusely charged with silex in a state of solution 
than the waters of the existing seas. So that the simple 
molecular affinity of the siliceous particles for each other, 
although operating " under certain circumstances," cannot be 
accepted as the principal agency by means of which they have 
been aggregated together, through vast undisturbed spaces, 
in the irregular manner in which they are discovered in the 
Chalk formation. 
In the guanos, although on a very limited scale, this 
aggregative tendency may be detected. In this kind of 
deposit masses may often be found composed wholly of 
cohering frustules of Diatomacese. The Rhizoselenise occur 
thus impacted together, forming small bundles, and possessing 
considerable cohesive force. It is worthy of notice that these 
