Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/414

From Wikisource
Jump to navigation Jump to search
This page has been validated.
Dr. Mac Culloch on the Geology of

accompanied by similar nodules of calcareous spar, often also penetrated by green earth (chlorite baldogée.) This amygdaloid accompanies the larger masses of green chalcedony, and seldom extends many inches beyond them. The great masses are either in the form of irregular nodules, reaching from the size of an egg to that of a man's head, or they resemble short veins, of which the opposing sides are flat, and somewhat parallel, and which might perhaps more properly be called angular nodules. Their colour is as various as their other external characters, but the most frequent varieties are comprehended in the following list.

  1. White chalcedony? approaching to quartz, and sometimes to hornstone, in its aspect, fracture, and other qualities.
  2. Pure chalcedony of a more or less milky hue, constituting the chalcedony and the white carnelian of lapidaries.
  3. The first variety of chalcedony, of a brown colour with narrow parallel stripes of green.
  4. The same, irregularly mottled with green, white, and brown.
  5. The opaque, or first variety of chalcedony, of an uniform dark sap green, with the dull fracture and aspect of wax, and like that substance, translucent on the thin edges.
  6. The translucent variety, similarly coloured, but with a more conchoidal fracture, and of a glassy aspect.
  7. Varieties which in one specimen exhibit a perfect gradation from pure white or colourless chalcedony, to the deepest green.

All these varieties contain imbedded spherical bodies, varying from the size of the minutest poppy seed, to that of a mustard seed. These are frequently formed of pyrites, and although they are often entirely decomposed, yet they now and then exhibit the remains of a crystal of pyrite within them, and their external surfaces retain a ridgy and crystallized aspect, the remains of the