450 Mr. W. J. Sollas on the Flint 



ing as the white layer of its surface. Some of the silica com- 

 bined with the iron, alumina, and alkalies present in the ooze, 

 and so gave rise to the associated glauconitic grains. 



The last question which remains for discussion is the origin 

 of the various external forms assumed by flint. 



A good deal of misconception appears to have arisen on this 

 subject through a too exclusive attention to one particular 

 form of flint arbitrarily selected as the type of all others. For 

 this (generally the irregular nodular form) a theory is framed, 

 which is then made to account for the rest. Thus, when Dr. 

 Bowerbank attempted to show that flints are silicified horny 

 sponges, he accounted for the flint-veins of the chalk by sup- 

 posing them to be silicified horny sponges which had grown 

 over the sides of an open fissure at the cretaceous sea-bottom ; 

 and Dr. Wallich, after giving an explanation of flint nodules 

 and layers, speaks of the veins as formed by a "sluggish 

 overflow " of silica-saturated protoplasm " into fissures in the 

 chalk." There does not appear much to choose between 

 these rival explanations of the veins : both are attempts to 

 square a preconceived hypothesis with an obnoxious fact. 



The forms of flint are chiefly four — those following the 

 outline of some enclosed sponge-skeleton, irregular nodular 

 masses, tabular sheets, and veins. 



1. The tabular sheets, as offering the simplest case, may be 

 taken first ; they have in all probability been formed by the 

 solution and redeposition in situ of an extensive bed of 

 sponge-spicules. All the flint layers which I have exa- 

 mined exhibit abundant casts of various kinds of sponge- 

 spicules confusedly mixed together. The chert beds of the 

 Devonshire greensand, analogues of the chalk-flint layers, 

 also contain numerous casts of spicules ; and in the same for- 

 mation deposits of loose spicules occur several feet in 

 thickness. 



2. The Flint veins. — Of these more than one explanation 

 is possible ; but we select the following as the most likely. 

 We may fairly assume that the chalk traversed by the veins 

 was permeated by a solution of silica derived from siliceous 

 remains, and this at a time so far subsequent to its formation, 

 that it had already become compact enough to be broken by 

 fissures ; whether organic matter, as Dr. Wallich understands 

 it, would have endured so long as this, is uncertain but not 

 probable. The solution of silica was bounded on one side by 

 a free surface, that of the fissure ; and free surfaces are emi- 

 nently determinative of deposition, not only of silica but 

 of calcite and many other minerals as well ; we see this in 

 geodes and in the mineral deposits formed within shells. It 



