21 



That of the earliest geologists who assumed the deposit of chalk 

 and flint by chemical affinity from calcareous and silicious solu- 

 tions in hot springs pouring into the colder water of the ocean. 

 Then in 1840 Mr. Bowerbank started a new theory. Finding, 

 from careful and extensive examination of nodular and tabular 

 flints in thin sections under the microscope, apjiearances of 

 structures resembling that of recent sponges, he became con- 

 vinced that all flints were in fact silicified sponges. These 

 observations have been so often quoted and the theory so warmly 

 advocated b}' many observers, the reputation of their author as 

 a microscopical observer, and his knowledge of sponge, were so 

 great, that Mr. Dowker gave more than a passing notice to his 

 memoii's. Traces of the substantial forms of bodies resembling 

 sponges, with spiculte, xanthidiae, foramiuifera, and other 

 detained fragments were found in all flints, nodular, tabulas, or 

 infiltrated, as in Echini and fossils. Even where no traces of 

 sj)ongo structure could be determined, the mode in which the 

 spiculie and other extraneous matter are dispersed equally in all 

 parts and not j)recipitated in a lump, indicates that tlie organic 

 tissue which enclosed them retained its form sufficiently to allow 

 of their fossilization in their original places. Further, by a 

 cruder examination of the surfaces of the nodular flints resem- 

 blances to sponge were found. In the perpendicular and oblique 

 veins of flints, when occasional fissures fiUed with chalk occurred 

 between the lamince, it was conceived that the sponge had grown 

 fi-om the two sides of the crevice but had not coalesced. In the 

 cherts of the greeusand the sponge fibre was of a coarser texture 

 than that of a chalk flint. The animal matter of the sponge was 

 the active agent in determining the deposit of flint, and operated 

 equally through the whole of the body, the analagous action, 

 being found in the attractive influence of animal and vegetable 

 substances in forming certain pyritous fossils in London, Kim- 

 meridge, and Oxford clays. Moss-agates and Jasper when 

 examined gave similar evidence of organic origin 



Mr. G. W. Hawkins Johnson in a paper read before the 

 Geological Association, 1874, advocates the organic nature of 

 flint ; without committing himself to defining the character of 

 the organisms, he believed them to have abounded in proto-plasm 

 penueated by innumerable branching and inosculating canals. 

 This animal tissue having an affinity for Silica parted with its 

 carbon which was replaced by Silicon, the process commencing 

 immediately after the death of the organism. By a particular 

 process fully described by Mr. Dowker, flints and other bodies 

 in which Mr. Johnson found his basic organism, as the Septaria 

 of Clays, Coprolites of the green sand, and P}Titic nodules of 

 the chalk, were so disintegrated as to leave in relief the arbores- 



