178 Dr. Wallicli on the Origin of the 



integral portions of the flint structure, so must the cherty rind 

 or crust of tlie nodular flints, and the white outer coating of 

 the tabular layers, both of which are due merely to the 

 accidental entanglement in the still viscid colloidal mass of 

 silica of minute calcareous organisms and their debris, " the 

 imperfectly silicijied layer of chalk remaining as the white layer 

 of its surface," as it is very properly described by SoUas, be 

 also thus regarded — a conclusion that would obviously be 

 absurd. 



Were the replacement theory applicable except in the case 

 of the cherty varieties, there would be no such thing as pure 

 flint ; but we should have in lieu of it a composite mass, not 

 homogeneous and, at times, almost translucent, but a substance 

 identical in every respect with the cherty core that occasionally 

 occupies what was, in the nascent state of the nodule, a portion 

 of calcareous mud around which the colloidal mass of silica and 

 protoplasm combined had closed in so as to form an internal 

 chamber or cavity, the outer surface of the never absolutely 

 silicified contents passing transition ally, though sometimes 

 somewhat rapidly, from perfectly pronounced chert to perfectly 

 pronounced flint. 



It is true that Mr. Sollas seems to have such unlimited faith 

 in the silicifying powers of his hypothesis that he sees no 

 difficulty in supposing that " concentration of the silica " from 

 the " silicate of animal matter," formed by the combination 

 of silicic acid with animal matter of various kinds, may take 

 place by the extrication of the organic part of the compound '^^ 

 though he admits that this supposition is a " pure assump- 

 tion which agrees very well with other well known facts in 

 chemistry " (Joe. cit. p. 456). At page 454 he says, " In all 

 these and similar cases the silica, concentrated by the dissi- 

 pation of the animal matter, which seemed 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." 

 But even this extreme and ambiguously expressed view of the 

 potentialities of colloid matter would hardly be tenable in these 

 days, as explaining the only practicable way in which the an- 

 nihilation — for it must be that or nothing — of the basal organic 

 substance could be brought about which enters into the con- 

 stitution of every shell and spicule, and which contains one 

 elementary body that is certainly not an ingredient of pure 

 flint, and could be got rid of only by entering into chemical union 

 with another of the released elements to form carbonic acid. 

 How comes it, then, that the constituent elements of the basal 

 organic matter of the Foraminiferal and other calcareous, and, 

 indeed, of all siliceous-shelled organisms, including the sponges 



