H. C. Sargent—Carboniferous Cherts in Derbyshire. 267 
the associated limestone. It varies from white to grey, blue and 
black, being as a rule, but no means always, darkest in the darkest 
limestone. The pigmenting agent appears, without doubt, to be 
generally organic carbon. The following partial analyses of five 
specimens of Derbyshire chert have been kindly prepared by 
Dr. H. F. Harwood and Mr. 8. Young, of the Imperial College of 
Science and Technology. Owing to the varying amounts of lime- 
stone inclusions and calcareous organisms in most cherts, complete 
analyses possess little value. 
Hi II Til IV Vv 
Fe (as Fe,03) Nn OcO2i i O:GOn i sOs0on Or 20226 
COg : : -Not est. — Notest. 0°40 1:66 
Org.C . : = 0:09 O32 0°54 0:33 
I. White massive chert, Ashford. 
II. Brown massive chert, Crich Hill. 
III. Black nodular chert, Dale Quarry, Wirksworth. 
IV. Black nodular chert, Klondyke Quarry, Crich. 
V. Pale-grey nodular chert, Intake Quarry, Middleton. 
These analyses show a consistent relationship between colour 
and percentage of carbonaceous matter, except in No. V, a pale- 
grey rock, in which the carbon is segregated into minute flakes 
visible under a pocket-lens. 
The percentage of iron does not show the same consistent relation- 
ship to colour except in the case of No. II, a brown chert nearly 
free from other impurity. It is not, however, uncommon to find 
black nodules with the weathered surface oxidized to a rusty brown. 
It is, therefore, possible that ferrous iron may sometimes be the 
colouring agent.t 
EvIDENCE OF CONTEMPORANEITY. 
All the tabular sheets of chert are conformably interstratified 
in the limestone with which they are associated. They are never 
transgressive. When the thickness of a bed varies, swelling or 
contracting, as is often the case, within a short distance, the 
bedding planes of the overlying rock always conform to the surface 
of the chert. Similarly, the rows of nodules are always aligned 
parallel to the bedding of the limestone. Isolated nodules often 
occur between the rows, but this is not a case of transgression. 
When lenticular or current bedding of the limestone has taken 
place, converging rows of nodules or thin sheets of chert may be 
seen following the bedding planes. Good examples of this feature 
occur in Intake Quarry near Middleton, in Wensley Dale, and in 
Bradwell Dale. 
The nodules usually occur in solid limestone away from partings. 
In vertical section their longer axes are almost always parallel to 
the bedding. When they occur in laminated, shaly limestone the 
ae C. S. Garnett has shown that the colour of ‘‘ Blue John’’ is due to 
bituminous matter (Trans. Chem. Soc., vol. exvii, 1920, pp. 621-3). 
