Coloured Clays Enlarge
Dec. 1. 1807. Publish’d by Ja.s Sowerby London.
British Mineralogy
Quartzum argillaceum

Coloured Clays

  • Class 2. Earths.
  • Order 2. Mixed.
  • Gen. 2. Pulverulent Quartz.
  • Spec. 1. Mixed with Argilla.
  • Syn.
    • Potters Clay. Kirw. 1. 180.
    • Clay. Bab. 50.
    • Argile glaise. Haüy, 4. 442.
    • Loam, Potters Clay, Pipe Clay, and Variegated Clay. Jameson, 1. 301.

Oxide of Iron seems mostly the colouring ingredient in Clays as well as in most other substances. The upper specimen here figured is of a peculiarly delicate brown, with some greenish parts about it, and has a little of the fracture and appearance of Fuller’s Earth. It, however, is a very good Clay, readily plastic with water, which is sufficient to distinguish it from that substance. In this the green hue was stronger when first gathered. This came from the canal near the Kent road. It is not rare.

The middle specimen came from a well in Richmond Park*, with colours as beautiful as any of Seps Marbles. The lilac tints maybe ascribed to a portion of Manganese, the usual cause of crimson and purple tints. It may, however, be owing to a mixture of two different Oxides of Iron, viz. the reddish, and the blueish or grayish. The lower one displays also the bright Ochre red so mixed with the white and yellow, that we might almost call it Terra miraculosa or Wonderful Earth, if that name were not already engaged by a Lithomarga found at Klanertz, of which we shall not fall far short when we exhibit the Lithomarga from Ireland. In the middle figure, the upper surface is rather more polished than ordinary, and appears as if one mass had slipped over another. This is not uncommon in many fossils; for, when two substances pass over one another, they give each other a smoothness which is called Slicken-sides, as in Ores of Lead, &c.

  • * 390 feet below the surface.
  • † 408 feet from the surface.
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