398 OM THE FORMATION OF THE CHALK STRATA, 



in cavities previously existing in the matrix in which they are 

 contained. I suppose, then, that one of these cavities becomes 

 filled by some small aperture, with a liquid holding silex in 

 solution ; that siliceous crystals form all round the cavity, ex- 

 cept at the aperture, where there is nothing to attach them- 

 selves to ; and that an aperture in some other part, allows a 

 regular escape of the fluid, by which a correspondent supply is 

 demanded from the first mentioned aperture, until, after per- 

 haps the lapse of ages, the crystals fill up from the bottom 

 and sides to the aperture by which the fluid was admitted, and 

 form a solid crystallised mass from which the water is exclu- 

 ded. Nor does it even appear necessary for the formation of a 

 solid crystalline mass, that there should be more than one 

 opening, that which admits the fluid into the cavity; since the 

 calibre of the opening, bearing a proper proportion to the ca- 

 vity to be filled, it will remain open until the mass of crystal- 

 lization is completed ; it being reasonable to suppose, that the 

 fluid containing the silex would, in consequence of its superior 

 gravity, be continually supplying the place of that which, ha- 

 ving been deprived of its silex by crystallization, would of 

 course tend upwards ; and thus might the cavity become entire- 

 ly filled by crystallization, to the total exclusion of the sol- 

 vent." 



The surprise tiiat I have expressed at the conclusions of 

 Werxer and Kirwan, is perhaps more applicable to this cele- 

 brated fossilist ; for his works bespeak the labour of a lifetime 

 on the very soil we are now engaged in. He does not, indeed, 

 suppose a vacuum to have existed in any part of the strata, but 

 he supposes an infiltration percolating through hundreds of 

 these ranges of flints, which, to do its work, must have first pro- 

 ceeded to the lowest, and gone on gradually towards the top ; 



but, 



