natural history, a kind of stone of a plain uniform mass, spotted with separate concretions, of great hardness, giving fire with steel, not fermenting with acids, and very slowly and difficultly calcining in a strong fire.

Porphyry is of several sorts; as, 1. The porphyry of the ancients, which is a most elegant mass of an extremely firm and compact structure, remarkably heavy, and of a fine strong purple, variegated more or less with pale red, and white; its purple is of all degrees, from the clay-colour to that of the violet; and its variegations are rarely disposed in veins, but spots, sometimes very small, and at others running into large blotches. It is less fine than many of the ordinary marbles; but it excels them all in hardness, and is capable of a most elegant polish. It is still found in immense strata in Egypt. 2. The hard red lead-coloured porphyry, variegated with black, white, green. This is a most beautiful and valuable substance. It has the hardness, and all the other characters of the oriental porphyry; and even greatly excels it in brightness, and in the beauty and variegation of its colours. It is found in great plenty in the island of Minorca; and is extremely worth importing, for it is greatly superior to all the Italian marbles. 3. The hard pale-red porphyry, variegated with black, white, and green. This is of a pale flesh-colour; often approaching to white. It is variegated in blotches from half an inch to an inch broad. It takes a high polish, and emulates all the qualities of the oriental porphyry. It is found in immense strata in Arabia Petraea, and in the Upper Egypt; and in separate nodules in Germany, England, and Ireland.

Porphyry-shell. See Murex.