Outlines of mineralogy / by J. Kidd.

  • Kidd, John, 1775-1851.
Date:
1809
Catalogue details

Licence: Public Domain Mark

Credit: Outlines of mineralogy / by J. Kidd. Source: Wellcome Collection.

Provider: This material has been provided by King’s College London. The original may be consulted at King’s College London.

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    A very singular stalactitic form of calcedony oc- curred some years since in one of the Cornish mines : it is nearly opaque and colourless; and from its close resemblance to an agglutinated mass of the cylindri- cal, bones of small birds it has been denominated skeleton calcedony. But although calcedony appears to have been se- parated from water in as great a state of purity as any mineral; and often to have been placed under circum- stances as favourable as possible to crystallization ; it never shews the least tendency to an internal crystal- line arrangement of its particles : its fracture is always perfectly compact. Crystals, indeed, the substance of which is calcedony are often met with; but they arc always pseudomorphous ; and very frequently they are not solid, which is a strong argument in favour of their stalagmitic origin. Their surface is rarely smooth. The derivation of calcedony from the common black flint may be traced by very delicate gradations, and sometimes in the same specimen : for upon breaking a nodule of the flint a cavity is not unfrequently dis- covered, the surface of which consists of irregularly blistered calcedony ; the substance of the flint insen- sibly passing into that of calcedony. Calcedony is frequently contained in rocks of the basaltic genus, in nodular masses. These masses,' which are in general of a light bluish grey colour, are often hollow, and set with amethystine quartz crystals.