78 Dr. G. A. Mantell on the Animalculites of the 
into furrows and channels; and funnels (sand-pipes) would be 
formed by the gyratory action of eddies, or whirlpools, induced 
by opposing currents ; effects in every respect analogous to those 
observable on the mud-banks of a delta, during the recession of 
the tide. The beds of loose, unrolled, and but slightly abraded 
flints, the smooth rounded contour of the gently swelling hills 
and undulated coombs and valleys of chalk districts, appear to me 
to be the natural consequences of the phenomena here contem- 
lated. 
Il. Chalk Flints—I now proceed to the consideration of the 
organic structures, and microscopical fossil bodies, observable m 
chalk flints. Itwas a current opinion with Parkinson, Towns- 
end, M. Guettard, and other early observers, that the external 
forms of a large proportion of the flint nodules had been derived 
from various kinds of sponges and alcyonia, which, while growing 
in their native sea, or floatmg in its waters, had become enve.. 
loped and saturated by the fluid silex ; and it was also inferred 
that these organisms had served as pomts of attraction for the 
siliceous matter, and were in a great measure the cause of the 
irregular nodular character, and mode of distribution, of the flints 
of the cretaceous formation. The occurrence of minute shells, 
corals, and other organic remains im the flints, was adduced as ad- 
ditional confirmation of this opimion ; since a similar entangle- 
ment of foreign bodies in the hollows and meshes of recent 
sponges is constantly observable. My much-valued friend the late 
Mr. Parkinson, mvestigated this subject with his wonted ability 
and caution, and many interesting observations on fossil sponges, 
illustrated by excellent figures, will be found im the second volume 
of the ‘Organic Remains of a Former World.’ That various 
kinds of Porifera or Amorphozoa have formed the nuclei of im- 
mense numbers of the flint nodules, will be readily admitted by 
all who have paid but a moderate share of attention to the sub- 
ject ; and the prevalence of spicula of sponges in chalk and flmt, 
proves the abundance of these organisms in the cretaceous seas. 
The microscopical examination of flint corroborates this inference, 
for the brown reticulated éisswe, so general in siliceous nodules, is 
unquestionably referable to certain kinds of sponges. This fact 
Mr. Bowerbank has satisfactorily established in the valuable me- 
moir previously cited; a memoir which presents so admirable an 
illustration of the nature of the spongeous structure observable in 
chalk flints, and in the agates of Oberstein, and in the green jasper 
of India, and such clear and ample directions for the successful 
investigation of these organic remains, as to render but few re- 
marks on the subject necessary. I will only state that my own 
observations confirm those recorded by Mr. Bowerbank in every 
essential particular ; namely, in the frequency of the reticulated 
