278 H.C. Sargent—Carboniferous Cherts in Derbyshire. 
apply,! and since the matrix must have been removed as fast as the 
“ growing nodule ”’ grew, it is clear there would be no deformation 
of the surrounding beds ! 
Mr. H. L. Hawkins? has effectively disposed of the argument that 
contemporaneous concretions are never surrounded by conformable 
stratification lines, and it is only necessary to recall the high angle 
of rest, often exceeding 50 degrees, of rock-fragments on the 
seaward face of a coral reef, to realize that, whatever may be the 
results of laboratory experiments, nature does not support 
Mr. Richardson’s view of gravitational effects on the sea-floor. 
In attacking the chert-problem it is useless to deal with the nodules 
separately from the extensive tabular sheets which are often 
extremely well-bedded; a similar origin and relative age must 
undoubtedly be ascribed to both. Moreover, the “nodules”’ 
themselves, as seen on a vertical face of limestone, are very often 
merely transverse sections of branching ramifications of a large 
mass of chert in the interior of the rock, and not individual con- 
cretions, either spherical or ellipsoidal. 
I owe Mr. Richardson many thanks for much strenuous criticism, 
which has been very helpful in the formation of my own views. 
EXPLANATION OF PLATE IV. 
(The figures are all taken from slides in the author’s collection.) 
FIG. 
1.—Blue Nodular Chert, Nimble Jack, Via Gellia. Ordinary light, x 25 diams. 
(slide 565d). The central, white area, of irregular shape and free from 
impurity, consists of granular quartz. Outside the quartz fibrous 
chalcedony, in spherulitic aggregates, is developed in a zone of silica 
apparently coloured by impurity. Outside this zone the silica is crypto- 
or microcrystalline. 
2.—Black Nodular Chert, Nutseats Quarry, near Haddon Hall. Nicols 
crossed, x 25 diams. (slide577a). The thin, white bands are microgranular 
quartz, enclosing unreplaced calcareous material. In the lower left-hand 
portion of the figure part of the section of a coral is seen, the wall and 
septa being unreplaced. Granular quartz fills the spaces between the 
septa. The groundmass is mainly crypto- and microcrystalline silica. 
Oxidized crystals of a ferriferous dolomite or chalybite are seen in the 
figure. 
3.—Black Nodular Chert, Klondyke Quarry, Crich. Ordinary light, x 20 diams. 
(slide 439). The black portion of the figure is limestone. The contact 
between chert and limestone is marked by a thin, bituminous black band. 
4.—Magmatic Chert, Pretoria Mine, Bakewell. Nicols crossed, x 25 diams. 
(slide 573). All the silica in the section is in the form of granular quartz. 
The mottled grey material is unreplaced limestone. 
1 “The Nature of Replacement ’’: Heon, Geol., vol. vii, 1912, p. 529. 
* Grou. Maa., April, 1921, p. 192, 
