H. 0. Sargent—Carboniferous Cherts vm Derbyshire. 2 
further action by the solutions. Other evidence leading to the 
conclusion that the chert was consolidated more rapidly than the 
limestone has already been given, and it is clear that rapid 
dehydration and consolidation would quickly limit replacement. 
Professor A. H. Church,! in his well-known experiment, found 
that gelatinization of silica stopped the reaction between silicic acid 
and calcium carbonate if the concentration of silica in the solution 
exceeded 0°15 per cent. 
The occurrence of chert in the clays at the top of the limestone 
shows that replacement is not an essential feature of its formation. 
The invariable conformity of the limestone with the sheets of 
tabular chert, even to small details of undulating surfaces, appears 
to be quite inconsistent with the view that the chert is a 
pseudomorph. 
EVIDENCE AGAINST THE SPONGE-SPICULE THEORY. 
The theory of the solution and redeposition of organic silica, 
although nearly unanimously rejected by recent American 
investigators, still finds support among English geologists. It is 
proposed to examine the conflicting evidence. 
It seems clear that these extensive contemporaneous beds of 
chert were rapidly formed, and it is admitted by supporters of the 
theory that an agent capable of effecting the solution, and likely 
to be present in the sea, is unknown. 
Remains of unsilicified calcareous organisms, as already noted, 
are abundant in the chert, and hence it is clear that acids generated 
by decaying organic matter were of no great importance as solvents. 
The alkaline reaction of sea-water above a calcareous ooze is 
believed by some? to be capable of dissolving organic silica. This 
may, perhaps, be the explanation of the replacement of sponge 
spicules, more or less completely, by calcite in limestone, but never 
in a cherty matrix. Itseems certain, however, that a considerable 
period of time would be required for the process. The solution 
actually observed in sponge-spicules from modern dredgings is 
limited to a widening of the axial canal and the disappearance or 
thinning of more delicate processes.° 
Spicules, of which the amorphous silica has become crystalline, 
are abundant in many thin sections of both chert and limestone, 
and it seems obvious that if solutions capable of dissolving organic 
silica were present to the extent necessitated by the theory, it is 
hardly likely that any spicules would be preserved. 
Tinta a on Silica’’: Journ. Chem. Soc. Lond., vol. xv, 1862, 
: 2 Cf. E. E. L. Dixon, Abs. Proc. Geol. Soc. Lond., No. 1,051, 1920, p. 49. 
3 Cf. E. F. Davis, ‘‘ The Radiolarian Cherts of the Franciscan Group ”’: 
Univ. Cal. Pub. Bull. Dep. Geol., vol. xi, 1918, p. 366. 
4 Cf. G. J. Hinde, ‘‘ On Beds of Sponge Remains in the Lower and Upper 
Greensand of the South of England’’: Phil. Trans., pt. 11, 1885, p. 431. 
_ ® Murray and Irvine, ‘‘ On Silica and the Siliceous Remains of Organisms 
in Modern Seas’’: Proc. Roy. Soc. Edin., vol. xviii, 1891, p. 231. 
