VII - THE GRAMMAR OF SILICA (not hitherto published)

Summary

FLINT

1. Flint is an impure, but quite definite and distinct condition of silica, collected or secreted out of chalk rocks or the earthy beds connected with them; sometimes in large flat beds or masses, but for the most part in irregular knots and lumps of which the secretion seems in many instances to have been originally provoked by some organic substance.

2. Flint is characteristically black, pale, dull yellow, or grey; the paler kinds opaque, the dark feebly translucent, and these are to be considered the true substance of flint (the paler varieties containing some admixture of clay).

3. The material of flint is so delicate in grain or texture that it can take casts of the most delicate organic structures, but the modes of its secretion and deposition are hitherto undetermined.

4. Its substance is entirely singular among minerals in being so intensely tough in its coherence, though brittle, that it strikes fire easily against steel, and when violently broken it shows no flaws in its mass, and the fractures have curved and zoned surfaces, which from their resemblance to shells are called “conchoidal.” Beautiful examples of entirely conic fracture of this kind will be found in the British Museum. I. F. 9 at Sheffield is entirely illustrative.