130. 



90 - Dr. Hinde on the ]\[icroscoptr Structure 



spicules of the Malm are of smaller proportions than those of 

 other greensand areas in which true chert prevails. 



It is only within a very recent period that the part played by 

 sponges as rock-formers has been recognised. The organic nature 

 of limestone rocks has long been known ; indeed, solid beds filled 

 with the debris of moUuscan shells, corals, crinoids, and crusta- 

 ceans could not fail to be observed, and closer investigation also 

 recorded the fact that soft earthy limestones like our Upper 

 Chalk are largely composed of minute Poraminifera. On the 

 other hand, the organic nature of flint, chert, and other siliceous 

 rocks does not seem to have been suspected till very recently. 

 It has generally been supposed that these siliceous rocks of sedi- 

 mentary strata were due to the direct deposition of silex from 

 waters charged with this mineral in solution, and the recurrence 

 of the siliceous beds was supposed to arise from the prevalence 

 at intervals of currents of warmer water holding an abundance 

 of silex. This theory was even put forward so late as 1878 by 

 Prof. Huh, F.R.S.,^ the Director of the Geological Survey of 

 Ireland ; and this in face of the fact — subsequently ascertained 

 by Prof. Sollas^ — that some of the microscopic sections studied 

 by the author were filled with sponge-spicules. 



The smaller proportion which organic siliceous rocks bear to 

 organic calcareous rocks in the geological series may be due to 

 the fact that the organisms in nature secreting silica as the 

 material for their skeletons are few in comparison with those 

 whose skeletons are built up of carbonate of lime, and this in its 

 turn may arise from the lesser amount of silica in sea- or fresh- 

 water in comparison with lime. Thus the silica-secreting organ- 

 isms are limited to the three groups of Diatoms, Eadiolarians, 

 and Sponges. All microscopists are aware of the deposits due 

 to accumulations of Diatoms ; that at Eichmond, in Virginia, 

 for example, being 30 ft. in thickness ; but rocks formed by these 

 organisms have not been recognised earlier than the Tertiary 

 Epoch. Strata in which Eadiolarians constitute the principal 

 constituents were, until within this last three years, miknown, 

 but rocks largely made up of these organisms have been dis- 



2 " On the Nature and Origin of the Beds of Chert in the Upper Carbon- 

 iferous Limestone of Ireland," ' Scientific Transactions of the Eoyal Dublin 

 Society,' voh i. (n. s.), 1878, part 1, p. 71. Since the reading of this paper, 

 Prof. Hull has communicated to the Koyal Society a note on my paper, " On 

 Beds of Sponge-remains in the Lower and Upper Greensand of the South of 

 England," ' I'hilosophical Transactions,' 1S85, p. 403. In this note, which 

 has not yet been published, Prof. Hull still advocates the theory put forward 

 by him in 1878, that, as regards the chert of the Carboniferous rocks, the 

 silica is derived directly from the sea-water, and further that there is no 

 foundation for my suggestion that it has been derived from sponge-spicules 

 (G. .T. H., May, 1887). 



3 'Annals and Mag. Nat. Hist.,' vol. vii., 1881, p. 141. 



