436 Dr. O. J. Sinde — Organic Origin of Chert. 



and the chemical process was accelerated by the warm surface waters 

 of a shallow sea. The origin of the Chert from the siliceous skeletons 

 of Diatomacese, Polycystinse, and the spicules of sponges is distinctly 

 denied, and it is affirmed that it can only be considered as a secondary 

 product due to the replacement of lime-carbonate by silica. 



Mr. Hardman gave, in his part of the paper, the results of numerous 

 analyses of the rock, showing that it consisted principally of 

 anhydrous silica, reaching up to 95"5 per cent., with a varying 

 admixture of carbonate of lime, iron, and a few other minerals. 

 Judging from its chemical constitution, Mr. Hardman considered 

 that the formation of Chert can only be accounted for by a process 

 of pseudomorphism in limestone, and " supposing that the ocean in 

 which the limestone was being formed contained an unusually large 

 percentage of silica, it is but natural to suppose that the water would 

 elect to dissolve small portions of the more easily soluble carbonate 

 of lime and in their place to deposit equivalent portions of the less 

 soluble silica in the gelatinous state" {op. cit. p. 92). 



Shortly after the appearance of the paper just referred to, M. 

 Alphonse Eenard, of Brussels, published, in a paper entitled ^ 

 " Eecherches lithologiques sur les Phthanites du Calcaire Carbonifere 

 de Belgique," the results of an examination of the same kind of 

 rock in the Carboniferous Limestones of Belgium. This author 

 very carefully studied the macroscopic and physical characters of the 

 Chert, and he reached nearly the same conclusion respecting its 

 origin as that of Messrs. Hull and Hardman, viz. that the phthanites 

 (as the Chert beds are technically termed) have been formed by the 

 silicification of the organic and inorganic calcareous elements which 

 composed the limestones, and that this more or less complete pseudo- 

 morphosis has taken place at an epoch when the sediments, although 

 retaining a certain amount of plasticity, already possessed the 

 structure which may be seen in the normal Carboniferous Limestone. 

 The author further adds, that it is impossible to explain the formation 

 of phthanites by the accumulation of organisms with siliceous 

 skeletons, and there ^ was nothing to prove that the silica which had 

 infiltrated into the limestone was derived from the decomposition of 

 the spicules of sponges or the frustules of diatoms. 



I may just remark that the observations of M. Eenard on the 

 characters of the Chert or phthanites appear to have been very 

 carefully and thoroughly worked out, and it is not to these but 

 to the theoretical conclusions as to the inorganic origin of the 

 silica of the Chert that I venture to take objection. 



Since 1878, no special memoir on the nature of the Carboniferous 

 Chert has appeared, but a curious commentary on the statement 

 of Prof. Hull that this Chert was not derived from siliceous organisms 

 such as the spicules of sponges, was afforded by the discovery by 

 Prof. Sollas,^ that some of the very sections described and figured 

 in Prof. Hull's paper were so constituted of sponge-spicules that 



^ Bulletin de I'Academie Eoyale de Belgique, 2 s. t. 46, pp. 471-498, pi. L. 



2 Op. cit. p. 498. 



3 Ann. and Mag. Nat. Hist. vol. vii. (1881) p. 141. 



