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Porci I

porphyry, felspar, quartz, horn-stone, crystals, base, porches and pitch-stone

PORCI I. (from the Latin. porticos) a kind of vestibule, supported by columns, much used at the entrance of the ancient temples, halls, churches, and many other buildings. See ATRIUM.

In the ancient architecture, a porch was a vestibule, or a disposition of insulated columns, usually crowned with a pediment, forming a covert place before the principal door of a temple, or court ofjustice.

\\Then it had flair columns in front. it was called a tetra style ; when six, hexastyte; when eight, °dos! yle ; when tell, decastyle, &c. \'itruvius calls it pronas ; Pollux rpodotoc, prodomos : when finer than ordinary, the ancients call it, also, yvopyleum.

Porches are almost universal in churches, and are usually on the south side, and in the second bay from the west, but the position is frequently determined by the circumstances of the locality.

Norman porches are frequently of large dimension, and highly enriched with the ornament peculiar to that period ; many such have been preserved in buildings which in other respects have been entirely rebuilt in it later style. Many fine porches of this kind are to be found at Malinesbury, Sherbourne, and Southwell, the last having a room over it ; there is also a very fine one at the Temple Church, London. Early English specimens occur at Wells, Salisbury, Lincoln, and Westminster ; and the examples in this as well as in the Norman style, are almost invariably of stone. Decorated and Perpendicular porches are frequently of wood, sometimes entirely closed at the sides, but more often of open work, and with ornamental barge boards at the end of the roof; many very beautiful specimens are to be found of both styles.

A few Galilee porches exist in seine of our cathedrals. See GALILEE, and CATHEDRAL. also CHURCH.

POlt PHYlt Y, (from TropqSvpoc, purple) a denomination that distinguishes a large class of primitive rocks, composed of one substance, in the form of grains, or crystals, imbedded in another, consisting most commonly of a compact paste, as its basis. The base is clay-stone, horn-stone, corn pact felspar, pitch-stone, pearl-stone, or obsidian ; the implanted grains, or crystals, are of quartz, or felspar. Of porphyry there appear to be two formations ; the most ancient consists prin cipally of horn-stone and felspar porphyry, and the most recent are of clay, pitch-stone, pearl-stone, and obsidian porphyry. The porphyritic formation is not veQ distinctly separated from the other rock formations which accompany it, nor is its rank among the primitive mountains, with regard to very clearly ascertained. The mountains of

porphyry are not stratified, and never enclose beds of other substances. Its texture is commonly compact, but it oeca sionally occurs ill schist stone. It is not very rich in mineral veins ; the clay porphyry is the most rich. The mines of Schweitz, in Hungary, which are of this description, are found in this species of rock.

Some writers have reckoned live species of rocks belong ing to the proper porphyritic formation, which are as follows : viz., 1, Horn-stone porphyry, the base of which, being horn stone, is generally red or green, with a conchoidal, or splin tery fracture ; and enclosing crystals of quartz and felspar. This is also distinguished. says Kirwan, by its hardness, slight transparency, and want of lustre ; it is fusible without difficulty. Sometimes the felspar is decayed, and sometimes also the horn-stone, whilst the quartz and hornblende remain entire: the whole thus acquires the appearance of indurated volcanic ashes, though the quartz might prove the contrary : if the felspar alone be decayed, the horn-stone will appear porous, and may be taken fbr lava. Its transitions are into granite and sand-stone. 2. Felspar porphyry, the base of which is commonly red compact felspar, enclosing crystals of felspar and quartz. 3. Sienitic porphyrq, which differs from the preceding in containing crystals of hornblende in addition to the other ingredients. 4. Pitch-stone porphyry, the base of which is pitch-stone, either red, green, brown, gray, black, or yellow, of various shades, having generally many colours at once in the same specimen. According to Kirwan, this porphyry has the following characters : lustre, greasy, 2.1 : transparency, 2.1 : fracture, imperfectly con choidal : hardness, S, 9, 10 : the felspar often blue : the fracture of some is slaty and colour yellowish-gray : lustre, scarcely 1 ; transparency, 1 ; hardness, scarcely 9; specific gravity, 2.452. 5. Clay porphyry, the base of which is indurated clay, passing into horn-stone ; generally of a reddish colour, and containing crystals of quartz and felspar. The colour of this porphyry, belonging to Kirwan's argillaceous porphyries, is generally some shade of gray, or greenish-gray, or brown, or blackish or reddish-brown, or isabella-yellow. Lustre and transparency, 0 ; fracture, earthy ; hardness, from 5 to 7; sometimes adhering to the tongue.