On the Flint Nodules of the Trimmingham Chalk. 437 



LVIII. — On the Flint Nodules of the Trimmingham Chalk. 

 By W. J. Sollas, M.A., F.R.S.E., F.G.S., Professor of 

 Geology in University College, Bristol. 



[Continued from p. 395.] 



The formation of the Flints. — If it were possible to give a 

 satisfactory explanation of the formation of the flints, a difficult 

 and interesting problem would have been solved ; and though 

 a complete solution has not yet been attained, it is neverthe- 

 less certain that the observations of the past ten years have 

 brought us remarkably near to it. 



In this part of the paper (which was not read at Swansea) 

 I propose to offer a brief general discussion of the subject, 

 arranging our inquiries under the four following heads : — 

 (i) The source of the silica of which flints consist; (ii) its 

 accumulation, chiefly as sponge-spicules, in the flint-bearing 

 deposits ; (iii) the solution of the accumulated silica ; and 

 (iv) its redeposition as flint and other forms of mineral 

 silica. 



(i) The proximate source of the Silica. — Two opposite 

 opinions are held with regard to this : — one to the effect that 

 the silica was introduced into the flint-bearing bed, subse- 

 quent to its formation, in solution from without ; the other, 

 that it was deposited contemporaneously, and subsequently 

 dissolved in situ. According to the first view, which is 

 advocated by Hull and Hardman in explanation of the 

 Carboniferous chert of Ireland, and by Renard for the Car- 

 boniferous Phthanites of Belgium, a shallow sea became 

 charged with an unusual amount of silica derived from the 

 siliceous rocks of surrounding lands. The siliceous waters 

 permeating the calcareous sediments below brought about a 

 replacement by which they were converted into flint. The 

 second was the opinion of Ehrenberg and Lyell, and is sup- 

 ported by W. Thomson, Wallich, and quite recently by 

 Alexis A. Julien. According to it the silica has been de- 

 rived from siliceous organisms, either collected into distinct 

 layers or scattered through some other deposit like the sili- 

 ceous remains now found dispersed in the Atlantic ooze. 



For some years past I have regarded this latter proposition 

 as an almost self-evident truth. 



1. In a discussion on Dr. Wallich 's paper on "the Phy- 

 sical History of the Cretaceous Flints," Dr. Sorby stated, in 

 terms admirably terse, the general argument which has long 

 been advanced in its favour. He says *, " Though deep- 



* Quart. Journ. Geol. Soc. xxxvi. p. 91. 



