454 Mr. W. J. Sollas on the Flint 



u Bischoff says that in the Infusorial beds of Rolt and 

 Geishingheim Ehrenberg found that no empty shells occur, 

 since all the smaller species are filled with siliceous matter — a 

 fact that meets with its explanation on this combination-and- 

 concentration hypothesis. The same author mentions the 

 occurrence of a Belemnite in which the calcareous rostrum 

 was all replaced by barytic carbonate, whilst the more organic 

 phragmocone was replaced by silica. 



" It is generally assumed that the casts of Echinoderms in 

 flint required for their formation the intermediate agency of 

 sponges which inhabited their interior. No doubt sufficient 

 evidence has been adduced to prove that this has certainly been 

 the case with some of these casts ; but one may just point out 

 that it need not have been so with all of them ; for it is pos- 

 sible that in a good many instances the animal matter of the 

 decomposing Echinoderm itself may have sufficed to separate 

 the silica from the surrounding medium without requiring 

 invariably the assistance of indwelling sponges. So, too, in 

 regard to the teeth of Mosasaurus found by Mr. Charlesworth 

 to be injected with silica, we are not reduced to supposing 

 with Dr. Bowerbank that the presence of this silica required 

 for its explanation the preexistence of a sponge extending 

 throughout the tubules of the tooth. This need by no means 

 have been so, since the animal matter which we know once 

 was present there is of itself sufficient explanation of the 

 presence of the silica. In all these and similar cases the silica 

 concentrated by the dissipation of the animal matter, which 

 served in the first place to imprison it from solution, might 

 remain in the crystalloid or the colloid state ; at this distance 

 of time we cannot determine. The silica of flint is generally 

 found in a cryptocrystalline condition : no tendency to a crys- 

 talline appearance is seen in the general mass of the nodule ; 

 but, at the same time, it acts feebly on polarized light. This, 

 however, proves nothing concerning its original condition, 

 whether it was colloidal or crystalline ; for I have lately 

 succeeded in determining as a fact what has long been held 

 as a hypothesis, viz. that as glass, when kept at a moderately 

 high temperature for a long while, becomes devitrified, i. e. 

 crystalline, so flint, in the course of ages, may have lost its 

 originally colloidal properties and settled down into the static 

 state of crystalline silica. The way in which I have deter- 

 mined this leads me to the subject of the well-known silicified 

 shells of Blackdown. No one who has seen the silica filling 

 these shells could for a moment assent to Dr. Bowerbank's 

 extraordinary hypothesis of its spongious origin ; it is evidently 

 derived from the separation of silica from the siliceous waters 



