274 HH. O. Sargent—Carboniferous Cherts in Derbyshire, 
of the main chert-bearing limestone, a large supply of dissolved 
silica was available for the formation of chert. The variations in 
the horizon at which the chert-bearing limestone sets in, as noted 
above, may be attributed to differential movement on the sea- 
floor. 
There is abundant evidence of the simultaneous precipitation of 
silica and calcium carbonate. At Knivetonwood, about three miles 
north of Ashbourne, nodules of black chert, in clays at the top of the 
limestone, have a brown rind, from + to 3in. thick, consisting of 
an intimate admixture of calcium carbonate and cryptocrystalline 
silica, the latter being present in one specimen examined to the 
extent of 14 per cent. The limestone associated with chert in the 
same quarry weathers brown like a sandstone. In a specimen of 
this rock the insoluble residue, consisting largely of eryptocrystalline 
silica, amounted to 39°5 per cent. In the Black Marble Quarry, at 
Ashford, there is a strong development cf black chert in thin sheets, 
on which there is a black calcareous rind similar to that on the 
Knivetonwood nodules. 
The colour-relationship between chert and limestone, resulting 
from similar impurities, and the nearly simultaneous setting in of 
the chert with the change of character of the limestone, at whatever 
horizon the change takes place, are further indications of the close 
connexion between land-denudation and chert-formation. 
A feature very often seen in thin sections of chert is the presence, 
often in great numbers, of minute rhombs of dolomite or chalybite, 
the latter often oxidized to opacity. The absorptive property of 
colloids affords a simple explanation of the presence in the colloidal 
silica of metalliferous salts, the products of contemporaneous 
chemical denudation, which have subsequently crystallized out 
as dolomite or chalybite. It is difficult to understand how this 
feature can be reconciled with the sponge-spicule theory. Further, 
the chert in which the rhombs occur is sometimes embedded in 
limestone, which is not dolomitized, and contains no rhombs. This 
feature seems quite inconsistent with the theory that the chert is 
a pseudomorph. 
The occurrence of chert in nodules, of very varying shape and 
size, and often with patches of limestone enclosed, points to 
segregation subsequent to precipitation. Twenhofel! observes that 
the dominating factor in nodule-growth was the attraction exercised 
on the dissolved silica by the growing nodule. The occurrence of 
a calcareo-siliceous rind on nodules or sheets suggests that there 
was a heavy simultaneous precipitation of calcium carbonate, and 
that diffusion of the silica was arrested by dehydration and 
consolidation. ; 
The variation in vertical spacing between different rows of nodules 
or sheets, ranging from a few inches to several feet, suggests variation 
in the time required to effect a sufficient concentration of silica 
1 Op. cit., p. 427. 
