Professor Epwarp [iutt—On the Nuture and Origin of Beds of Chert. 83 
formable to the limestone, or rather the chert beds which represent it. The change 
from the one formation to the other is remarkably abrupt in that district, as it is 
also in the north-western districts. In other places it is less so, and there appear to 
be passage beds—but these are just the results we should expect from a general 
elevation of the sea-bed unaccompanied by any considerable displacements of the 
strata themselves. The shales and sandstones of the Yoredale series which overlie 
the limestone are themselves rarely silicified, nor are the fossils they contain con- 
verted into chert. All these considerations lead us to the conclusion that the 
silicification of the limestone took place before the deposition of the shales of the 
Yoredale series, and was accompanied by a general elevation of the sea-bed, which 
then became covered by shallower water than previously. 
Manner of Silicification—The modus operandi is the most difficult of all the 
questions we have proposed to ourselves in this inquiry; but we think the con- 
siderations already adduced lead us gradually to tolerably clear notions on the 
subject. We have also the observations of chemists and naturalists regarding 
similar transmutations in the mineral and animal kingdoms, particularly those of 
Professor Bischof in the one case, and of Professor Martin Duncan in the other.* 
In the first place, the phenomena I have described go to show that the forma- 
tion of the chert has resulted from the replacement of calcareous matter by silici- 
ous—because the microscopic sections indicate more or less distinctly the presence 
of organic forms of animals which from observation we know to secrete only car- 
bonate of lime from the waters of the ocean——these are corals, crinoids, foramini- 
fera, polyzoa and molluscs. In the second place, it has been shown that there is 
evidence that the calcareous material to be acted upon was placed in a condition 
highly favourable to the transmutation process, namely, that it was soft and porous, 
and was overspread by waters generally shallow. Professor Duncan has shown 
that it is under such conditions as these that the corals of the miocene period were 
converted into silicious material in the West Indian area, and he points out that 
the prevalence of heat and sunlight conduce to favour the transmuation process, 
inasmuch as they favour chemical reactions. Now such conditions as those I have 
suggested would result during the carboniferous period, as at the present day, in 
the production of warm waters pervading the central British areas ; and if these 
waters happened to be charged with silica in solution, chemical reactions would at 
once be set up, favoured and promoted by tidal or other currents. As Bischof and 
other chemists have shown, either minerals or organic substances formed of carbon- 
ate of lime are always liable to replacement by silica when submerged in waters 
charged with this mineral; and we know also that the seas of the present day 
(and we may infer those of geological times) contain it in solution. As regards 
the actual chemical process, that is a discussion on which I have no intention of 
entering, preferring rather to leave the subject in the hands of my colleague, Mr. 
KE. T. Hardman, F.c.s., who is well qualified to offer an opinion on the subject, 
and has appended the results of his investigation (see Part II.). 
* Supra cit. 
