192 Dr. Wallicli on the Origin of the 



of some shell or other foreign substance that happened to 

 come in the wav and became thus accidentally enveloped 

 either partially or wholly. According to Mr. Graham, " a 

 dominating quality of colloids is the tendency of their particles 

 to adhere^ aggregate, and contract. In the jelly itself the spe- 

 cific contraction or syneeresis still proceeds, causing separation 

 of water ivith a division into a clot and serum, and ending in 

 the j^'i'odxiction of a hard stony mass of vitreoiis structure " 

 (Graham, " On Silicic Acid and other Colloids," Proc. Roy. 

 Soc. vol. xiii. no. 65, June 1864, p. 336). 



Here, then, is the key to my hypothesis, and, as I conceive, 

 proof that the characteristic features, including the stratifica- 

 tion and nodulation of the flints, are due to the inherent 

 properties of the double colloid formed by the intense dispo- 

 sition of the colloidal protoplasm to enter into mechanical 

 union (as in the case of glycerine and water) with the organic 

 silica of the sponge-spicules and network — this tendency 

 dating, however, only from the period when they ceased to be 

 integral portions of a living structure and had already become 

 only its residuary substances. 



Having now explained, but still too cursorily to admit of 

 my producing all the evidence that could be adduced in sup- 

 port of my view, the processes whereby the nodules and tabular 

 layers of flint and the cherty varieties are formed, from tlie 

 earliest to the latest stage of their nascent condition, it remains 

 for me to connect these with my hypothesis in such a manner 

 as to show : — firstly, the adequacy of the hypothesis to 

 account not for one, or two, but for all the distinguishing- 

 features of the flint-formation as it now presents itself in 

 the Chalk; and, secondly, in what respects the replace- 

 ment and other theories that have been proposed must be 

 considered faulty and insuflficient to account for any thing 

 more than the formation at the bed of the ocean of an impure 

 flint, and the silicification of certain calcareous-shelled creatures 

 which are entombed in the chalk and flint. I cannot secure 

 this end more readily and, under existing circumstances, more 

 appropriately than by quoting such portions of my former 

 paper as bear on my hypothesis. But for the reasons assigned 

 I should, of course, have been content to append references to 

 the pages in question. 



Referring to the insufiiciency of the hypothesis previously 

 offered in explanation of tlie i'lint-formation, I asked : — 



" "Whence, then, did all the sihca come? Why is it almost 

 invariably found existing in layers parallel to the stratifica- 

 tion of the Chalk ? And what has really been its history, 

 from first to last ? 



