Amorphous Quartz or Flint
Flints that lie upon the Chalk about Salisbury plain, &c. in Wiltshire show much variety, and have mostly been noticed as being casts of various animal remains. I believe the appearance represented in the upper figure has not been before mentioned. The marking cannot be called dendritrical, or described by any other term used in Mineralogy that I know of; and in addition to oilier unaccountable circumstances attending the formation of Flints, which seem to have been in different states of fluidity capable of taking the nicest casts or impressions, we here find an after substance taking up more or less of the space or vacancies that were apparently formed in the flint by a hot and dry process cracking it. This substance distinguishes itself in the form of a pure white opaque Calcedony, and has in this instance adhered to one side of the crack In such a manner as often to allow the flint to divide again and show the partial passage of the Calcedony which has formed in globules like moisture, and running in various directions confined by the narrow space, has become flattened; and while the effect excites curiosity, it serves to explain the sort of fluidity that succeeded that of the Hint. These flints have a marly brown outside.
The lower specimen is a fractured flint from an upper series below Highgate hill towards Barney and the whiter markings appear to be in consequence of some parts being more susceptible of change by exposure to weather or bleaching than others. It is thus I would account for it; and as most flints have apparently enveloped various matters, the parts within may be in a state of union, yet sufficiently irregular to be acted upon partially; these mixtures may also account for the variety of colours, transparency, spots, &c. in flints: this indeed is distinctly seen in some that have enclosed Madrepores, which are replaced transparently, while the rest is more opaque and has the usual gray appearance.