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 natui'al consequences of the phsenomena here contem- 

 plated. 



II. Chalk Flints. — I now proceed to the consideration of the 

 organic structures, and microscopical fossil bodies, observable in 

 chalk flints. It was 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 floating in its waters, had become enve- 

 loped and saturated by the fluid silex ; and it was also inferred 

 that these organisms had served as points 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 in the flints, was adduced as ad- 

 ditional confirmation of this opinion ; 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, investigated this subject with his wonted ability 

 and caution, and many interesting observations on fossil sponges, 

 illustrated by excellent figures, will be found in the second volume 

 of the ^ Organic Kemains 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 flint, 

 proves the abundance of these organisms in the cretaceous seas. 

 The microscopical examination of flint corroborates this inference, 

 for the brown reticulated tissue, 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 



