262 Dr. Wallich on Siliceous Sponge-growth. 



Interior of a single Flintstone from Horstead in Norfolk, 

 which had just previously been published at Munich. 



In his concluding remarks the author, while referring to 

 my paper on the chalk flints (Quart. Journ. Geol. Soc, Feb. 

 1880), expresses the opinion that "the contents of the flint 

 from Horstead, and of those from the Xorth of Ireland, prove " 

 what I only " assumed, namely that in its original condition 

 the cretaceous ooze was, like that of the Atlantic deep-sea 

 mud, filled with the spicular skeletons of sponges .... the 

 contents of both the Irish and Horstead flints showing that 

 the sponge-spicules are as much intermingled with Foramini- 

 fera and other calcareous organisms as in the Atlantic ooze, 

 and that therefore both these animal types flourished con- 

 temporaneously " {op. cit. pp. 80, 81). 



Without in any wise detracting from the value of the con- 

 cuiTcnt testimony thus furnished as to the great exuberance 

 and variety of the sponge-life which existed at the bottom of 

 the ancient cretaceous sea-bed, the Horstead nodule, per se, 

 would increase rather than diminish the force of the objec- 

 tions that have been raised by some writers to any analogy 

 between the mineral composition of the ancient and the recent 

 calcareous deposits. From a cursory remark made at the 

 close of Mr. Hinde's observation, he appears ready to admit 

 that the mass of the material in the flint-cavity under notice 

 could not have presented its unconsolidated character had it, 

 at the time of its inclusion, formed part of the deposit at any 

 depth below the immediate surface-layer of the sea-bed. This 

 is precisely what I have maintained and explained, in the 

 only way it seems possible to explain it, by showing how the 

 whole of the siliceous matter was continuously eliminated as 

 the deposit was being formed, and was retained in a compa- 

 ratively thin colloidal stratum that constituted, as it were, a 

 floating layer overlying and resting upon large areas of the 

 calcareous sea-bed. 



It is obvious, therefore, that without probing the deposits to 

 a much greater depth below their surface than we are likely 

 ever to succeed in doing, the analogy that exists between the 

 constituents of the ancient and recent calcareous deposits must 

 continue to be based on assumption. Mr. Hinde's observa- 

 tions tend only to enhance the necessity of this assumption. 

 They do not in the least degree assist us in " proving," as 

 claimed, that "in its original condition the cretaceous ooze 

 was, like that of the Atlantic deep-sea mud, filled with the 

 spicular skeletons of sponges." Indeed, it would be well to 

 bear constantly in mind that, so far as all the evidence goes 

 that has heretofore been obtained, there is no warrant for 



