Nodules of (he Trimmingham Chalk. 453 



sodic carbonate and silicic acid, which would combine at once 

 with the organic matter of the sponge to form a silicate with 

 it. Now sodic silicate is a crystalloid body, and would easily 

 find its way into the interior of the fibres and sarcode of the 

 sponge ; but if decomposed there into silicic acid and sodic 

 carbonate, the silicic acid would be entrapped in the organism, 

 since it is colloidal and could not diffuse out. In the same 

 way, when once a coating of colloidal silica had been formed 

 round any body, while sodic silicate could easily pass through 

 it, yet, when decomposed in the interior into silicic acid, as 

 before it would be unable to return outwards, since the col- 

 loidal silica coating the organism would act as a dialyzer and 

 would prevent it. 



" Immediately the organism had grasped and extricated 

 from the water a molecule of silicic acid, a difference of 

 specific gravity would be set up between the spot where the 

 silicic acid had disappeared from solution and the surrounding 

 water. This difference would be rapidly equalized by diffu- 

 sion, in which way the water which had yielded its silica to 

 the sponge would be replaced by fresh supplies, the silica 

 of which would again be removed and combined with the 

 substance of the organism, throughout which this process 

 would be actively going on, until in time it had combined 

 with all the silicic acid which it had power to fix. In this 

 way we produce that circulation of water which is absolutely 

 necessary to any theory of fossilization, and explain how with 

 merely molecular currents the sufficient supplies of silicic 

 acid would be brought within reach of the organism under- 

 going fossilization. While the sponge was exposed to the 

 direct action of the ocean-water, if it ever was so exposed, mo- 

 lecular currents might expedite the process ; but when covered 

 up by the fine sediment, in which it is afterwards found im- 

 bedded, it could only derive its mineralized water in this 

 molecular way, by the well-known action of diffusion and 

 without involving any of the transcendental mysteries of an 

 undiscovered attraction. An observation of Petzholdt's shows 

 that in certain cases the process of fossilization really has 

 continued in an organism after it has been silted up. Petz- 

 holdt found that in dolomite occurring immediately around a 

 flint, there existed but 2*31 per cent, of silica, while that a 

 little further removed contained a little more than twice as 

 much ; the precise figures are 4'73 per cent. This may be 

 explained on the hypothesis that animal matter entered into 

 combination with the surrounding silicic acid and continued 

 to do so after it had been overwhelmed in silt, and until its 

 affinities were satisfied. 



