276 H.C. Sargent—Carboniferous Cherts vv Derbyshire. 
The lava-flows underlying these beds have suffered intense 
décomposition, which must have liberated large quantities of silica. 
It has been shown elsewhere ! that the orthoclase and albite molecules 
were conspicuously present. Lindgren? states that in the great 
geyser regions of Yellowstone National Park, New Zealand, and 
Iceland the predominating springs “ are essentially sodium chloride 
waters with large amounts of silica, believed to exist in part as 
sodium silicate, a large quantity of free carbon dioxide, and a little 
hydrogen sulphide”. “The waters are always hot and usually 
ascend through volcanic rocks.” 
The features of the massive cherts of Derbyshire, which distinguish 
them from the main cherts, suggest a different origin, and it 
is believed that pneumatolytic discharges from the cooling lava- 
flows, or juvenile waters, the expiring manifestations of volcanic 
activity, may have carried similar salts and been the direct source 
of the silica of which they are composed.’ It is suggested that these 
rocks may, for distinction, be conveniently named “‘ magmatic¢ 
cherts ”’. 
BANDING OF THE CHERTS. 
In all the cherts a banding, consisting of alternate light and dark 
layers, is not infrequently seen. In the nodules the bands are 
generally concentric or parallel with the outer surface. In the 
tabular cherts they are usually parallel with the bedding pkanes. 
This feature appears to be the result of a segregation or rhythmic 
precipitation of impurities in the course of dehydration. The light 
bands often consist of clear silica, apparently free from impurities. 
“Alternating segregation of silica and calcite,’ as noted by 
Mr. Dixon,* has not been observed by me in the Derbyshire cherts. 
A feature that may often be noticed in thin sections is the presence 
of a thin bituminous band, or string, following the line of contact 
between chert and limestone (Pl: IV, Fig. 3). Similar strings 
sometimes occur in the body of the limestone itself. Dr. Sorby ° 
called attention to this feature in limestone from Stony Middleton. 
So far as can be judged from thin sections and hand-specimens, 
the bituminous material belongs to the limestone and not to the 
chert. Sometimes, when there is no definite band, the limestone 
shows a distinctly greater depth of colour in the neighbourhood 
of the contact. Moreover, the band sometimes occurs at the contact 
between white chert, free from carbonaceous matter, and dark, 
1 H. C. Sargent, ‘‘ On a Spilitic Facies of Lower Carboniferous Lava Flows 
in Derbyshire’’: Quart. Journ. Geol. Soc., vol. Ixxiii, 1918, pp. 11-23. 
2 Mineral Deposits, 1913, p. 52. : 
3 Cf. Dewey and Flett, ‘On some British Pillow-lavas and the Rocks 
associated with them ’’: Grou. Mac., 1911, p. 245. Also Flett, ‘‘ The Iron-cre 
Deposits of Nobblehouse and Garron Point and their Genesis’’ (Edin. Geol. 
Soc.) Abstract in Grou. Mac., Vol. LVI, 1919, p. 92. 
Sy Wocwicibisspso0} 
° “ On the Application of Quantitative Methods to the Study of the Structure 
and History of Rocks’’: Quart. Journ. Geol. Soc., vol. lxiv, 1908, p. 225. 
