Decomposing calcareous Pudding-stone Enlarge
Dec.r 1. 1804. Publiſhed by Ja.s Sowerby. London.
British Mineralogy
CIII
Quartzum calcareum: Var. decomponens

Decomposing calcareous Pudding-stone

  • Class 2. Earths.
  • Order 3. Aggregate.
  • Gen. 1. Quartzum.
  • Spec. Calcareum.
  • Gen. Char. Quartz aggregated by the help of some cement.
  • Spec. Char. By the help of carbonate of lime.

It often happens that siliceous aggregates decompose, especially when their cement is calcareous; which is the case with the present specimen. It is perhaps now first spoken of, and gives us reason to suppose that the iron or colouring substance is disengaged from the calcareous cement by some agent capable of penetrating the inmost recesses of the stones. This cement filled the cavities of the mass, leaving them hollow, or with the pourous remains of the pebbles only filling a part of the old cavity. It has been asked whether these stones are not rather forming than decomposing. We answer, that the cavities are shaped as if each had been filled by a whole stone, and the whole mass seems to be falling to pieces. In these masses some of the stones are whole, and in part soft, and may be scratched by a knife, or even by the nail, while the other part is as hard as a common flint pebble. Some of the pebbles are formed almost wholly of Carbonate of Lime, some of Silex, others partly of Oxide of Iron. I have some of the second kind from a well dug in Richmond-park, found among clay at the depth of 365 feet, taken notice of by my friend Mr. J. Murray, then gardener to Mr. Addington. I first received specimens of this nature from Warwick by chance; and the appearance of it was new to me, and to all whom I consulted about it. Lady M. Thynne, who was going to Warwick, kindly proposed to send me any mineralogical subject found in the neighbourhood. I requested her Ladyship to pay some attention to this, and was soon favoured with many curious specimens of decomposing rock; among which was that here figured. It is a piece of rock, chiefly quartz and carbonate of lime, inclining to be somewhat spongy and reddish with the oxide of iron: some of the cavities are empty, others have some remains of the pebbles, and others are nearly whole. The lower figure has the appearance of having been a common pebble, the colouring part of which has suffered oxygenization so as to become a loose ferruginous ochre, and the earthy parts of the stone are nearly separated. Some of the best mill-stones are of this nature on a larger scale, the quartz being left very porous; and these are preferred for grinding of corn.

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