422 Rev. W. Buckland on the Paramoudra> and 



mass of strata which usually attains a thickness exceeding 500 feet 

 in most parts of the English chalk formation. 



With respect to the stratified arrangement of argillaceous 

 geodes and concretions, we have abundant examples of it in the 

 horizontal beds of septaria found in the Sheppy clay, in the Kim- 

 meridge clay, the lias, and indeed almost all secondary argillaceous 

 strata ; and we have beds of lenticular concretions both siliceous 

 and calcareous, equally abundant in many of our sandy strata, e. g. 

 in the green sand of the Isle of Wight, in the Stonesfield sand near 

 Woodstock, and the sand of the inferior oolite near Bath. 



As to such of these concretions (whether insulated or disposed 

 in strata) to which no extraneous nucleus can be discovered, it is 

 not easy to say what determined their centres of attraction. But it 

 does not appear possible that they could have been formed by infil- 

 tration into pre-existing cavities, like the irregularly disseminated 

 geodes of the trap rocks ; since this hypothesis in the case of chalk 

 would imply the anomaly of there having once existed, extending 

 uniformly over many hundred square miles, as many strata of air 

 bubbles as there are of flint alternating with the chalk ; and of 

 which air holes not one was left empty or partially filled up ;* 

 whilst on the other hand many of the nodules could not have been 

 formed in such air holes, as they entirely derive their shape from 

 some extraneous bodies affording a nucleus to the silex that has 

 incrusted them. 



* It is important to distinguish between cavities in the chalk itself, and those within 

 the body or shell of the siliceous concretions contained in chalk; and whilst I contend 

 that in the latter case the cavity of the flint has been sometimes filled in a manner ana- 

 logous to the infiltration of geodes in the trap rocks (see Note, .419) yet the pre- 

 sence of the organic body, to the decay of which alone these last named cavities in the 

 flints are indebted for their existence, proves that there was no cavity antecedent to 

 that decay, and that the silex was originally deposited in the form of a mould round the 

 organic body from which it derives its shape. 



