Red Calcedony or Carnelian
Believing the formation that affords these specimens is very little known, and as far as 1 know, not hitherto spoken of by any author, I feel much satisfied with a sort of discovery that is very interesting. The specimen, fig. 1, is from Llanrhaiadr, in North Wales, as mentioned above; the Limestone containing it is characterised by the remains of a shell, formerly called an Anomia, but now distinguished by the generic name Produetus (Min. Con. v. 1, p. 103.) The specimen f have chosen includes this shell, but little changed in substance, and tilled with Limestone like the rest of the rock, curiously wrapped up as it were, in the red siliceous mass, which is more or less perfect Cornelian, as the lapidaries would call it, resembling that imported in the rough from the East Indies, &c.* but which has not been assigned to any particular rock, being found, according to the best information I can get, loose upon the surface in various parts. Great Britain affords Carnelians also among the loose gravels of her shores, (see tab. 83.) long worn and disengaged from their primitive situation, very probably from such a rock as that I now figure, by its decay and this rock remains in some places as a type to assist us in reasoning upon a subject hitherto obscure. The Cornelian appears to he congenial with the Limestone rock, and to occur in various shapes, passing into Calcedony in little round drops, mamillæ, &c. as the figure shews: it has little inverse rhombs of reddish and yellowish carbonate of Lime about it (similar to those of tab. 4, from Carnarvonshire); may they not all belong to such Limestone rocks, as when more exposed to decay, by their situation near the sea, have furnished the Cornelians found among the flinty gravel that so often guards the coast in a very providential manner? Some other specimens shew a nearer approach to flints, such as the Cornelians of the coasts are found among; may they not also have a similar origin? The lighter Caleedony and flints of the Downs of Wiltshire, &c. seem more allied to those of the flinty chalk, as are those of the ochracious gravel, perhaps, for the flints being washed out, rolled, and exposed, would, when mixed with the ochracious loam, absorb more or less of its colouring matter; this opinion is strengthened by comparing many of the organic remains, found in gravel, with those common in the flinty chalk, such as casts of Echini, Terebratulæ, what are now called Alcionii, from which the chalk figured at the top of tab. 7, may be considered to have derived its form,† &c.
The Limestone on the east coast of Ireland contains flints that I understand become redder as they approach the trap which lies over it; the under figure is taken from one of these from Larne, it has Analcimo crystalled in a hollow upon it, a substance generally understood to be formed in trap rocks only.
- * Some say that many are coloured by art, I should have thought this an unnecessary cheat.
- † Tab. 215 shews a conjuration depending on other parts of these curious animal remains.