Shell-formed Calcedony Enlarge
April 1 1809 Publish’d by Jas Sowerby London
British Mineralogy
Silex Quartzum, var.

Shell-formed Calcedony

  • Div. 2. Imitative.
  • Var. Shell-like.

I am obliged to the generosity of Mr. Milne of Fonthill for this specimen. That place and the neighbourhood often afford Cornua Ammonia of very extraordinary dimensions, I was told of one that was at Tisbury nearly as big as the large wheel of a coach, I have one of 21 inches in diameter, and my friend Mr. W. Cunnington has one larger. They are often of Sandy Limestone, or Limestone with crystals within, and now and then there are found the remains of similar shells in Flint with the chambered divisions partly calcedonized, if 1 may use the expression, and containing Quartz crystals; but the most curious specimen ever seen is the present, except one as nearly like it as possible, which I am told is in the possession of Sir Edward Hulse of Salisbury. It is perhaps as extraordinary a geological specimen as any that have been seen. The mass is chiefly Flint: but the shell has been lined, and having subsequently decayed, left the Calcedony of the most delicate and exquisite structure. The peculiarly regular folding manner of the divisions, and their uniformity, are no where disturbed. The Calcedony is placed distinctly around where the shell originally was. There are other minute shells about the specimen, and also some Carbonate of Lime, both crystallized and amorphous.

The lower figure exhibits one of the partitions, showing the tube which passes round the shell, different from what is usual, as not being a continued channel, but divided into short tubes, each of which communicates by its openings with two chambers.

Geologists, as they contemplate the changes which have here taken place, may well join with David, Linneus, and all true philosophers, in exclaiming, “Oh! Jehovah, great and manifold are thy works; in wisdom hast thou made them all; the Earth is full of thy riches.”

Sir H. C. Englefield showed me a beautiful specimen of a spiral shell with the end passing from Calcedony to Opal.

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