== Sou == 
chalk itself, which has there been metamorphosed into a hard 
limestone, no traces of sponges remain, and nearly all recog- 
nizable fragments even of the calcareous organisms have been 
obliterated. It is possible that a small portion of the silica 
of the flints may be derived from other organisms such as 
the Radiolaria; but up to the present the only two or three 
forms of this group known to exist in the Chalk formation 
have been discovered by Prof. Zittel in strata in Westphalia 
teeming with unaltered sponge spicules; and so far as known 
at present the Radiolarians had but a_ slight development in 
the cretaceous ocean. 
The hypothesis of the derivation of the flints in the chalk 
from the silica of sponge skeletons is by no means a new 
one, but it has not hitherto been generally accepted from the 
absence of froof of the existence of sponges in sufficient 
numbers to furnish silica for the great mass of flints with 
which the chalk is filled, and from the difficulty of explaining 
the arrangement of the flints in nodular layers. Even in the 
last edition of the Students Elements of Geology published 
in 1871, Sir Charles Lyell attributed the chalk flints to the 
dissolved shells of diatoms which at certain periods swarmed 
in the cretaceous ocean and were alternately replaced by the 
foramenifera which supplied the material for the chalk. Within 
this present year the origin of the cretaceous flints has been 
made the subject of a paper by Dr. Wallich. (Quart. Jour. 
Geo. Soc. 1880, p. 68). This able investigator brought no 
evidence to show the general distribution of the sponges in 
the cretaceous ocean, but merely based his argument that the 
flints were derived from sponge skeletons, on the fact, that 
siliceous sponges are found abundantly in the deep sea dredg- 
ings of the Atlantic, and that on the assumption of their 
being present in similar numbers in the Chalk ocean, they 
would have been able to furnish the silica for the flints. The 
contents of this flint from Horstead and of those from the 
North of Ireland, prove, what Dr. Wallich assumed, that in 
