308 On the Formation of the Flints of the Upper Chalk, 



as suggested in my former paper, namely, by the rapid solidifi- 

 cation of a fluid — induced by special circumstances — combined 

 with its own molecular attraction; those special circumstances 

 being, generally, an affinity for an organic nucleus* ; occasionally, 

 mere mechanical action, such as motion taking place where the 

 sohition happened to be present. The point before obvious as a 

 fact, but difficult of explanation, of the penetration of soft parts, 

 has now also, 1 trust, received some elucidation. 



The second point to which I would call attention relates, not to 

 the formation of the nodular flints, in which the bodies just consi- 

 dered are usually found, but to a no less interesting and confirma- 

 tory fact as to the suggested origin of the tabular flints. I sug- 

 gested, at p. 16 of my former paper, that "where there was a layer 

 of minute organic bodies, there would be fewer centres of attraction, 

 and tabular flints would be formed.^^ This has been confirmed 

 by facts of subsequent detailed observation. The tabular flints, 

 which extend unbroken over vast spaces, are not found as de- 

 scribed by Mr. Bowerbank, ' Geol. Trans."* vol. vi. p. 184, but are 

 formed of two plates united in few places. Between the plates, 

 and totally unconnected with the chalk above or below, is a 

 layer of pulverulent substance, not chalk, but which, on submis- 

 sion to the microscope, is found to be made up of most minute 

 organisms — so minute as only to be at all discernible under 

 a quarter-inch achromatic, and only clear under an eighth. 

 These are of a character and belong to genera totally distinct 

 and difierent from the ordinary microscopic forms abounding 

 in the beds of chalk immediately above and below the several 

 tables, while the latter forms are extremely sparing in this 

 most interesting and remarkable layer. The principal forms 

 in this are extremely numerous and minute Gallionellse, a mi- 

 nute Navicula, and several other forms. These forms make up 

 the whole mass, or very nearly so. These facts point to phseno- 

 mena, as to the time and manner of the deposit and depth of 

 the water, which require careful and separate consideration, and 

 on which therefore I forbear now to enter. Their important bear- 

 ing on our immediate subject cannot fail to be recognised. Not 

 a trace is found on the under side of these tabular flints of the 

 nature described by Mr. Bowerbank (p. 254) as appertaining to 

 flat sponges, while the remarks made by me on p. 5 are most 

 fully borne out^ I have had an opportunity of examining a large 



* As to the formation of the nodular flints by " separate centres in one 

 mass " (p. IG of my former paper), this will surely be understood by any one 

 who has watched two or more globules of water or mercury approach and 

 unite. Had they contained organic nuclei, instead of forming fresh and 

 larger perfect globules they would have formed irregular nodules like the 

 flints. 



