H. C. Sargent—Carboniferous Cherts in Derbyshire. 269 
noted by me, crystalline calcite is associated with the silica in the 
fissure. 
PETROLOGY OF THE SILICA. 
It is generally recognized that the silica composing chert was 
originally in colloidal form. It has subsequently passed over 
mainly into crypto- or microcrystalline material, but a more 
coarsely crystalline development (quartz or chalcedony) is also of 
common occurrence. 
Lindgren?! points out that the work of Heim, Leitmeyer, and 
others leads to the conclusion that chalcedony is in all cases com- 
posed of quartz fibres, and that it always results from crystallization 
of gelatinous silica, which may, in becoming crystalline, either turn 
into granular quartz or into fibrous quartz, i.e. chalcedony. 
Impurities in the silica appear to have had a marked effect on its 
crystallization. In the dark-coloured or black cherts the develop- 
ment is mainly erypto- or microcrystalline, but small areas, of 
varying shapes, may often be seen in thin sections from which the 
impurities appear to have been drawn off by segregation, or which 
have, perhaps, been left clear by differential absorption. In the 
clear central portion of such areas granular quartz has formed. On 
the outer margin of the quartz there is usually a narrow zone of 
fibrous quartz (chalcedony), often in the form of radial aggregates. 
Its refractive index is always a little lower than that of the granular 
quartz (Becke’s test). Outside this zone, in the more highly 
pigmented matrix, the silica is crypto- or microcrystalline. 
When viewed in thin section by transmitted light, the chalcedony 
is frequently, but not always, of a vale-brown colour. Pirsson ? 
has shown that this colouration, which often occurs in felspar- 
spherulites in acid lavas, may be an optical phenomenon due to 
absorption of the blue rays in passing through angular fibres or 
scales of a colourless mineral ; the finer the fibres the greater being 
the absorption. This is, perhaps, often the case in the rocks under- 
discussion, but the chalcedony is also frequently coloured by a 
ferruginous or bituminous pigment, which is occasionally concen- 
trated in concentric bands (Pl. IV, Fig. 1). 
It is noteworthy that in the purer cherts, nearly or quite free from 
colouring matter, although some of the silica may remain in the 
ceryptocrystalline state, there is no chalcedony, but an extensive 
development of granular quartz. 
Liesegang? has shown that the presence of iron hinders 
crystallization in agates, and the foregoing considerations seem to 
justify the same conclusion in regard to impurities in chert. 
1 ‘‘ Processes of Mineralization and Enrichment in the Tintic Mining 
District ’’’: Hcon. Geol., vol. x, 1915, p. 233. 
2“ On an Artificial Lava-flow and its Spherulitic Crystallization ’’: Amer. 
Journ. Sci., vol. xxx, 1910, pp. 101-2. 
3 Geologische Diffusionen, 1913, p. 100. 
