C— GEOLOGY. 99 



Silicified foraminiferal limestones and oolitic cherts such as those 

 from Bullslaughter Bay, Tenby, described by Dixon, ^^^ are obviously 

 cases of replacement. The oolitic cherts, which are devoid of spicules, 

 are lenticular intercalations in rocks of Zaphrentid phase, crowded with 

 highly spicular cherts, so that the average sea -water which soaked them 

 while they were still within its influence would probably carry some 

 dissolved organic silica. 



(b) We now come to the cherts occurring in bands or beds, sometimes 

 tabular, more often nodular, at various levels and horizons. Such cherts 

 are particularly characteristic of the D.^ beds of the Midlands and N. Wales, 

 and the equivalent level of Pembrokeshire and of Gower ('Black Lias' ); 

 and whatever be the method of formation of the cherts of the Midlands 

 and Flint, there can be no doubt that some of the Zaphrentid-phase 

 cherts of S. Wales resemble the standard-phase cherts in being due to 

 replacement. Of these rocks Dixon ^'•^* remarks that doubtless in all 

 cases they owe their silica largely to sponge spicules. Such spicules have 

 been observed in sections of the chert from many localities, e.g. the ' Black 

 Lias ' of Gower, also Chirk, Holywell and Prestatyn^-^ in Flint. G. J. 

 Hinde^"® refers to the cherts of N. Wales as ' remarkable and hitherto 

 unequalled sponge beds.' 



The presence of the sponge spicules has hitherto been as a rule claimed 

 as indicating the source of the associated chert. Sargent regards the 

 matter from a different light, his argument being that, if solutions capable 

 of dissolving organic silica were present to the extent necessitated by the 

 sponge-spicule theory, it is hardly likely that any spicules would be 

 preserved. This argument would, on the other hand, counter' that adduced 

 from the frequent absence of spicules in sections of Carboniferous cherts. 



Mr. Sargent's opinion as regards the bedded cherts of the Midlands and 

 Flint is that they are due to deposition contemporaneously with the 

 •country rock, and not, except to a limited extent, to any metasomatic 

 replacement thereof. The source of the silica is found in the immense 

 ■quantity which must be poured into the sea in solution in river-water. 



(c) Evidence of the presence of organisms is generally very clear in 

 the case of the laminated cherts of Dixon's lagoon-phase deposits. Thus 

 the radiolarian character of the Codden Hill cherts of Barnstaple has long 

 been familiar. Similar radiolarian cherts occur in the P beds (D,p) of 

 'Gower. There can be little doubt that the silica of these deposits is 

 largely, at any rate, derived from the associated organisms, and that 

 the cherts are contemporaneous in the sense that they are not due to the 

 :subsequent introduction of silica. It cannot be denied, however, that 

 the amount of silica present in these rocks is so much in excess of the 

 ladiolaria themselves as to suggest direct precipitation. 



The chief facts relative to cherts may be summarised as follows : — 

 The origin of the silica is (1) organic, and derived from siliceous 



organisms (radiolaria, diatoms and sponges); (2) inorganic, and derived 



from the silica in solution in sea-water. 



12S Pembroke and Tenby Memoir, p. 70. 



i2« Ibid., p. 70. 



i'-'6 G. H. Morton, Proc. Liverpool Biol. Soc, vol. i. (1887), p. 69. 



"0 Geol. Mag., Dec. iii., vol. iv. (1887), p. 444. 



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