Summary: (1) Cretaceous flint from the sea-beaches of England was shown by X-ray powder photography to consist of quartz of which much was very fine-grained. Opal or cristobalite, if present, were in amounts too small for detection. The flint had the microscopical characteristics of chalcedony.
(2) Included calcareous matter, of which some was the undoubted remains of fossils, had been partially replaced by silica. This was well displayed by an aggregate of eumorphic carbonate, with Fe, Mg, and Ca, of which part had been converted to pseudomorphs of quartz.
(3) A chemical analysis and a continuous weight-loss versus temperature curve are presented for the flint. The obstinate retention of water at elevated temperatures does not in itself imply the presence of opal; adsorption and occlusion in a fine-grained system could suffice to give this effect.
(4) In the raw state the flint was blackish by incident light. Prolonged treatment with hydrogen peroxide caused a fading of this colour. After adequate calcination the flint was white by incident light, but brown by transmitted light even at thicknesses of 30–40 microns.
(5) Calcination caused also a decrease in density and refractive index. The density-temperature curve declined steeply from 300° to 600° C. and then much more slowly to 1000° C. A further sharp fall succeeded from 1100° to 1400° C. This second fall was caused by the formation of cristobalite; below 1200° C. quartz alone was detected by X-ray powder photography.
(6) The density-temperature and refractive index-temperature curves showed a general similarity of shape.
(7) The cristobalite formed at 1400° C. was of anomalously low density and refractive index.
(8) Anomalously low densities and refractive indices were shown by flint calcined at various temperatures irrespective of whether quartz or cristobalite was present. These properties were attributed to the presence of minute pores.
(9) To the pores was ascribed also the brown colour by transmitted light which accompanied the anomalous properties mentioned and appeared in both quartz-rich and cristobalite-rich preparations. Recrystallization of the cristobalite prepared at 1400° C. increased the density and refractive index and discharged the brown colour; these effects are explicable by the removal of minute pores. Comparable examples of the production of brownish colours in dispersions are enumerated; some are of geological interest.