Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/42

30 observable in the larger crystals. They are generally of a considerable external lustre, occasionally approaching to metallic; and are sometimes, though rarely, iridescent on the surface. In some of the most recent specimens afforded by Huel Gorland, the crystals exhibit a singular external brilliancy, occasioned, apparently, by a deposition, after the formation of the crystal, of small thin facets which cannot be detached by the knife, of the same form as the faces of the crystal itself The crystals of this substance are mostly aggregated, and frequently confusedly grouped: it is by no means common to find one in which all the solid angles are disengaged. A singular variety was found in small quantities in Huel Jewell mine, of a dark red colour and remarkable lustre, in hollow octohedrons, formed by very minute crystals, each arranged in the same direction, and attached at the solid angle. This variety is sometimes accompanied by uranium in small tabular crystals of a light green colour.

The compact red oxyd of copper, usually denominated tile ore, which by Brongniart is called Cuivre oxidulé ferrifère, has been found in several mines in Cornwall. In Huel Gorland, this red oxyd often occurred massive, sometimes with portions of native copper passing through it; and in this state, as well as when crystallized, passing into black oxyd. Crystals were frequently found deposited on native copper, the irregular crystallizations of which formed, as it were, the nuclei of the superimposed crystals; and these were generally varieties, the primitive form being rarely found so circumstanced. Crystals also occasionally enclose minute portions of native copper.

Specimens of this substance occur, consisting almost wholly of a pure mass of aggregated crystals; it is however usually accompanied by quartz, and occasionally by a compact and very hard substance, apparently composed of quartz, in intimate mixture with the Gossan before described. It has also been found with fluor spar in Huel