150 THREE CRUISES OF THE “ BLAKE.” 
acid, in regions where no rapid oxidation takes place, either 
from currents, or waves, or from other atmospheric influences. 
An immense amount of silica must find its way into the sea, 
and be immediately dissolved by the excess of carbonic acid 
found near the bottom, while only a portion of the calcareous 
mud can be taken up in solution. Hence, this silica is at once 
placed under the most favorable conditions for resorption by 
organisms living upon the layer of protoplasmic substance 
which covers the bottom of the ocean, into which the silica has 
been received. As Wallich and others have most distinctly 
proved, this protoplasmic layer, where it exists, is the product 
of the organic life, and not its source. 
Wallich gives some most excellent reasons for supposing that 
the silex nodules found in the chalk owe their origin to sponges. 
This is additional evidence in favor of the opinion, which is 
gradually gaining ground, that the deep-sea calcareous mud is 
but a chalk deposited in the present epoch, and in every way 
to be compared with the chalks of the cretaceous period ; and 
further still, that this deposition of chalk has been going on 
uninterruptedly from cretaceous times, and that we may be said, 
as far as that special deposit is concerned, to be living in the 
period of the chalk, for no lithological distinction of any value 
has been established between the chalk proper and the calca- 
reous mud of the Atlantic. 
The Atlantic ooze is mainly made up of the calcareous casts 
of globigerinz, and their sarcode must have contributed in no 
small degree to the protoplasmic mass of the bottom. It must 
have been an important addition to the siliea supplied by 
sponges in the seas where siliceous foraminifera are abundant.’ 
As an additional source of supply of silica we have the exten- 
sive deposits of diatoms, which fall to the bottom after death. 
A similar condition of things probably existed durmg creta- 
ceous times, and the chalk flints were derived from the same 
groups of animals which now supply the silica and the globi- 
gerina ooze of the Atlantic. 
1 Sir Henry De la Beche states, that ter, but if allowed to remain too long in 
siliceous particles are generally dissemi- suspension the silex will become aggre- 
nated in water if mixed with clayey mat- gated into small lumps. 
