176 Dr. Wallich on the Origin of the 



lime by silica — siliceous chalky but not flinty being the result ; 

 whilst the second stage commenced with the fiUing-up of the 

 chambers of the Foraminifera and the interstices of the chalk 

 by a simple deposition of silex, and ended when the siliceous 

 chalk became converted into black flint, an incompletely 

 silicified layer of chalk remaining as the white layer of its 

 surface. 



The replacement-theory as taught by Prof. Rupert Jones* 

 is undoubtedly applicable to the flints, so far as it goes. Mr. 

 Sollas's version robs it of this attribute. For how and why 

 the replacement by silica, admitted to have extended, during 

 the first stage, to the ooze and some small and many large 

 shells, should not, without let or hindrance, have, at the same 

 time, entered the chambers of the Foraminifera, which, though 

 small, present no peculiarity of structure that could interfere 

 with the penetration into their chambers of the "simple" 

 siliceous solution — how or why this solution should have 

 reached the calcareous particles of the ooze and certain 

 shells without gaining access to them through the inter- 

 stices existing amongst the oozy particles themselves, and 

 should not in tlie first instead of the second stage have 

 silicified these interstices — and, above all, how or why tlie 

 siliceous solution, which, from the commencement of the first 

 to the final completion of the second stage, must necessarily 

 have gained access to the interior of the mass of ooze by per- 

 meating its boundary-walls, should have failed throughout to 

 silicify these, and should have left them in the shape of an 

 " incompletely silicified layer of chalk remaining as the white 

 layer of its surface," are problems far too subtle for ordinary 

 understandings to grapple with, although Mr. Sollas appears, 

 to have long ago solved them to his own satisfaction, as the 

 following remark, at p. 452 of his paper, somewhat personally 

 attests : — 



" As I have already shown, in an earlier part of this paper, 

 that flints originate as silicified chalk, we need not spend time 

 on a formal confutation of Dr. Wallich's hypothesis "! 



These details may appear wearisome, and, could they be 

 taken apart from Mr. Sollas's conclusions, might with advan- 

 tage be ignored. They become important, however, when 



• Prof. Rupert Jones qualifies the theory by adding : — "As this mineral 

 (silica) rarely succeeds calcite as a true pseudomorph, it is only the amor- 

 phous, or detrital, carbonate of lime of the organisms constituting the 

 limestone that has been replaced by silica (as flint &c.), and not the 



crystallized material of Echmodermatal spines &c the guards of 



Belemnites, nor the shells of Inoceramus, Ostrea, Terehratula, &c." {loc. 

 cit. antb, p. 447). 



