270 H.C. Sargent—Carboniferous Cherts in Derbyshire. 
EVIDENCE AGAINST METASOMATISM. 
_ Much of the evidence already given in favour of contemporaneity 
is also obviously opposed to the theory that the chert is a 
pseudomorph. There are other considerations that tend in the same 
direction, although the microscope shows that a limited amount - 
of replacement has generally taken place at the contact between 
chert and limestone. It is obvious, however, in thin sections that 
impurities have not only hindered crystallization, but also, to a large 
extent, replacement. 
In the dark or black cherts the replacement is usually limited to 
a thin band of clear crystalline silica at the contact between chert 
and limestone or around calcareous organisms (Pl. IV, Fig. 2). 
The calcite of small organisms, perhaps as a result of freedom from 
impurity, is sometimes entirely replaced, but unsilicified calcareous 
fossils are very abundant. Dr. G. J. Hinde? called attention to 
these features as disproving the pseudomorph theory. 
The contact between chert and limestone is usually quite sharp 
and often serrated (Pl. IV, Fig. 3). The nodules, when black, 
often break away from the enclosing rock with a clean fracture, 
making it difficult to obtain a hand-specimen showing the contact. 
In the purer white cherts, on the other hand, there is usually no 
definite line of contact, but a broader transition zone in which the 
replacement reaction appears to have been active, and in which 
crystalline silica and calcium carbonate are intimately intermingled. 
Wherever this partial replacement has taken place, the replacing 
material is clear crystalline silica, probably always in the form cf 
minutely granular quartz. The impurities appear to be always 
excluded from the contact films of the solutions that take part in the 
reaction.” 
Itis significant that in the qaartz-rock described by Dr. Bemrose,* 
which, as he states, is clearly a replacement of limestone, there is 
no amorphous or cryptocrystalline silica, no chalcedony, and the 
replacing silica is all in the form of clear granular quartz. 
The conclusion seems to be justified that the impure silica of these 
cherts, which remains in a crypto- or microcrystalline state, has taken 
no part in a replacement reaction with calcium carbonate. 
If the pseudomorph theory be true, thin partings in laminated 
limestone, consisting of detrital terrigenous material (Fig. 1), should 
be repeated in the chert, since they would obviously not be replaced 
by the metasomatic reaction. There is no such repetition. 
Minute grains of calcite may often be seen associated with the 
quartz in the partial replacement zone. This probably indicates 
that the consolidation of the silica took place before the calcite 
grains were completely replaced, and they were thus preserved from 
1“ Organic Origin of Chert’’?: Grou. Maa., 1887, p. 445. 
2 Cf. Lindgren, Mineral Deposits, 1913, p. 671; also Tarr, op. cit., p. 415. 
3 “On a Quartz-rock in the Carboniferous Limestone of Derbyshire ’’: 
Quart. Journ. Geol. Soc., vol. liv, pp. 169-83. 
