266 H.C. Sargent—Carbonrferous Cherts im Derbyshire. 
GENERAL DESCRIPTION. 
The chert of Derbyshire occurs mainly in the upper beds of the 
Carboniferous Limestone throughout the entire outcrop, and also 
in the clays and thin limestones passing up into the Limestone 
Shales. The chert-bearing limestone is usually thin-bedded, dark 
or black in colour, and belongs to subzones D, and D, of Vaughan’s 
classification. Sporadically, feeble developments are found in the 
massive, white or pale D, limestone at a considerable distance 
below the main beds. 
Sometimes, especially in the northern part of the outcrop, the 
main chert-bearing limestone is pale-grey or white, and loses its 
thin-bedded character. 
The chert occurs. either as rows of nodules of very varying size 
and shape, or as tabular sheets up to 8 or 10 inches thick and of 
considerable horizontal extension. Rarely it may be seen filling 
vertical fissures or joints in the limestone. 
A feature that has an important bearing on origin is the frequent 
change of horizon at which the chert-bearing limestone sets in. 
In the neighbourhood of Matlock, Cromford, and Crich it is only 
from 50 to 60 feet thick, but near Middleton Moor, about 2 miles 
south-west of Cromford, it sets in 150 feet lower, replacing pale, 
massive, chertless limestone. At Ashover the thickness is about 
150 feet and on the western side of the massif it may attain 
400 feet.2 Between Longstone and Buxton the chert-bearing 
beds are 450 feet thick.? 
The transition from the pale, massive limestone to the overlying 
dark, thin-bedded, chert-bearing rock is usually extremely abrupt. 
In addition te the main chert deposits described above, there are 
other occurrences distinctly local in distribution which require 
separate mention. They are found mainly in the neighbourhood 
of Bakewell and to the west and north-west of that town, at 
- Ashford and near Longstone Edge. These cherts consist of massive 
beds sometimes 8 or 9 feet thick, and they are usually white or 
grey without any relation to the colour of the associated limestone. 
Similarly, on the north-west of Crich Hill there is a development 
of massive, brown chert of which 6 feet in thickness are exposed, 
but the base is not seen. It can be traced round the north side of 
the hill, and reappears on the east near the Old End Mine. 
It is believed that these beds of massive chert owe their silica 
to a different source from that of the main chert beds. 
In colour the cherts, other than the massive beds just 
referred to, generally bear a close relationship to that of 
1 Cf. ‘‘ The Geology of the Northern Part of the Derbyshire Coalfield” : 
Mem. Geol. Surv., 1918, pp. 13-14. 
2 Most of these details of thickness are taken from ‘‘ The Geology of the 
Northern Part of the Derbyshire Coalfield ’’: Mem. Geol. Surv., 1913, p. 20. 
3T. F. Sibly, ‘‘ The Faunal Succession in the Carboniferous Limestone of 
the Midland Area’’: Quart. Journ. Geol. Soc., vol. lxiv, 1908, p. 38. 
