H. C. Sargent—Carboniferous Cherts in Derbyshire. 275 
for precipitation, owing to differential denudation. This feature 
would be difficult to explain under the sponge-spicule theory. 
Davis ! has shown experimentally that when a suspension of finely 
divided clay was mixed with a solution of sodium silicate, the addition 
of ammonium carbonate caused flocculation of silicic acid and 
formation of gelatinous silica. The silica and clay were segregated 
into alternate layers, and it is interesting to note that the experiment 
produced many of the features of the bedding of chert, such as 
nodules, irregular thinning and thickening of bands and lens-like 
terminations. 
The massive cherts, referred to above, which are of purely local 
occurrence, now require consideration. These rocks, so far as existing 
exposures show, occur solely in the neighbourhood of former volcanic 
activity. Near Bakewell they are closely associated with the 
Cracknowl (or Crackendale) vent, and they overlie the lava of that 
district. At Crich there is a bed of lava in the hill, at some distance 
below the chert. Wherever they occur they are seen on one 
horizon only, and are not repeated again and again through a 
considerable thickness of limestone, as is the case with the main 
chert beds. It would appear that the supply of silica was continuous 
till it was exhausted. 
The absence or paucity of organic colloids (see Analyses I and IT) 
is a noteworthy feature, distinguishing these rocks from the main 
chert beds. The result appears to be that, in colour, they show no 
relationship to the associated limestone. White is the prevailing 
colour, but grey beds are often interspersed. The Crich rock is 
brown, owing to admixture of iron. Not infrequently the rock has 
the appearance of a sinter, resembling the present-day deposits of 
hot springs. Fluorite is frequently associated with these cherts, 
as it is with quartz-rock. It occurs as the infilling of cavities and 
veins, and in minute grains scattered through the rock and in the 
associated limestone. At Ashford, a specimen of limestone from a 
parting in the chert contained 0°36 per cent of insoluble residue, 
which consisted of amorphous fluorite with a little cryptocrystalline 
silica. 
Fossils are rare in these cherts. That organisms were sometimes 
entangled in the silica is rendered probable by the occasional 
occurrence of flakes and small concretions of carbonaceous matter, 
but it is rarely that any trace of the organism remains. 
As with the main cherts, so, too, with those under consideration, 
simultaneous precipitation of silica and calcium carbonate has 
often taken place. Streaks and patches of limestone are common 
in thin sections, their margins usually showing signs of replacement 
(PI. IV, Fig. 4), and short discontinuous bands, 3 or 4 inches 
thick, may be seen in the body of the rock. 
1 Op. cit., pp. 399-401. 
