/ 
H.C. Sargent—Carboniferous Cherts in Derbyshire. 277 
obviously impure, limestone. This feature may, perhaps, be due to 
a draining-off of carbonaceous matter in the limestone in the course 
of dehydration, and, if so, it is obvious that its migration would be 
arrested at the contact with the more rapidly consolidated. chert. 
CONCLUSION. 
It is believed that the foregoing evidence justifies the view, 
expressed at the commencement, that the cherts of Derbyshire are 
mainly of direct inorganic origin and contemporaneous with the 
associated limestone and clays. It is, however, possible that 
siliceous organisms have furnished a subsidiary source of supply, 
since dredgings show that organic silica is not absolutely insoluble 
insea-water. Itis thought, further, that the evidence shows that the 
chert is not a metasomatic replacement of the limestone, except to 
a very limited extent. 
The existence of large beds composed of siliceous organisms which 
have not been dissolved (p.272) is a point that cannot be ignored by 
advocates of the organic theory. 
Although the Derbyshire cherts alone have been discussed, it is 
believed that the same conclusions will be found to apply to other 
areas. I had an opportunity last summer of making a brief 
examination of the fine deposits of chert in North Flintshire, which 
are said to extend through a thickness of 350 feet, and have been 
erroneously described as being without intervention oflimestone.! At 
the Pentre Quarries, near Gronant, the chert occurs as tabular sheets 
in thin-bedded, siliceous, black limestone, the occurrence being 
similar to that of the beds described above at Ashford, and elsewhere 
in Derbyshire. After further field-work I hope, with the editor’s 
permission, to discuss these Flintshire cherts on another occasion. 
I have to thank Dr. H. H. Bemrose for kindly allowing me to 
examine his thin sections of chert and for much useful information 
as to exposures ; also Dr. H. H. Thomas for kindly permitting me 
to examine thin sections in the collection of the Geological Survey. 
The interest taken by Dr. H. F. Harwood in the colour-problem has 
been of great service. 
P.S.—Since the foregoing paper was written Mr. W. Alfred 
Richardson’s interesting article? on ‘‘The Relative Age of 
Concretions”’ has appeared. The arguments in that paper seem to 
be mutually destructive. In Mr. Richardson’s opinion, ‘‘ con- 
formity of stratification lines [round a concretion] is caused in every 
case by concretionary growth.’’3 In the case of chert-nodules he 
holds to the replacement theory, and he fixes the date of their 
formation as “‘ at the earliest later than the first compacting of the 
matrix ’’.2 In this case Lindgren’s law of equal volumes would 
? G. J. Hinde, “‘ Organic Origin of Chert’”’: Grox. Mac., 1887, p. 445. 
> Got Maa., March, 1921, pp. 114-24. 
& Tbid., p. 11st 
