1887.] 



on Sponge-remains in the Greenland. 



305 



directly from sea-water; and similar opinions as to the origin of the 

 chert bands in the Upper Carboniferous limestones of Ireland have 

 been put forward by Messrs. Hull and Hardman, and by M. Renard,* 

 with respect to the phthanites in rocks of the same age in Belgium. It 

 is a significant fact, however, in connexion with the chert-beds of the 

 Irish Upper Carboniferous strata that some have been discovered filled 

 with sponge- spicules like the chert of the English Greensand, and this 

 indicates a similar origin for the silica, and negatives the supposition 

 of Professor Hull that it was deposited " from warm shallow water 

 charged with silica in solution, in which chemical reactions would 

 be at once set up, favoured and promoted by tidal and other 

 cur rents, "f 



I have taken pains to quote the entire passage in Dr. Hinde's paper 

 in order to avoid the possibility of misrepresentation ; and I must 

 confess my inability to understand the reasoning of the author. He 

 regards the sponge-spicules as "the source " of the silica, and by their 

 decomposition in the presence of sea-water as having given origin to 

 the beds of chert ; but the question arises, from what source did the 

 sponge-skeletons themselves derive the silica from which they were 

 formed ? This could not have been from repeated solution and re- 

 construction, because by this process the supply of silica would have 

 been used up. The statement, therefore, that " the beds and irregular 

 masses of chert have been derived solely from the silica of the sponge- 

 remains instead of from that held in solution by the sea- waters them- 

 selves" is altogether unintelligible. The real "source "of the silica 

 is that small amount of this mineral which is always present in ocean 

 waters ; from this source the sponge-structures have been derived by 

 organic agency, and without that agency the silica would seldom be 

 solidified. Sponge siliceous skeletons are in reality the result of the 

 presence of silica in sea-water — not its cause. If there had been no 

 soluble silica there had been no siliceous sponges. But I am only 

 here concerned with a defence of the views arrived at, after full in- 



* Dr. Hinde, in referring to Prof. Eenard's Memoir (' Bulletin de l'Academie 

 Koyale de Belgique,' vol. 46, 1878, p. 471), goes so far as to question the author's 

 determination of the nature of the " circular sections " shown in one of the 

 figures (fig. 2) accompanying the paper. The author identifies them as crinoid 

 stems, Dr. Hinde suggests that they are really sponge-spicules ; a view that no one 

 so well acquainted with the Carboniferous Limestone as Professor Eenard will for a 

 moment admit. 



f Dr. Hinde does not mention his authority for the statement of the abundance 

 of sponge-spicules in the Carboniferous Limestone of Ireland, and I suspect that he 

 has in this case, as in that of the phthanites of Belgium, mistaken the sections of 

 crinoids for those of sponge-structures. There is no doubt some difficulty in dis- 

 tinguishing sections of sponge-spicules from ill-preserved segments of crinoid stems 

 such as occur in chert ; so that their identity must be determined by the forms 

 which are prevalent in the ordinary limestones. 



