8. Uses.—It is cut and polished, and used as an orna mental stone.
Euphotide.—Hauy.
Euphotide.—Jameson. Gabbro of the Italians. Diallage Rock of others.
Constituent parts.—It is a compound of saussurite and diallage, and these two minerals are sometimes intermix ed with talc, hornblende, actynolite, garnets, grains of pyrites, 8r.c.
Geographical distribution.—It occurs along with ser pentine in the Shetland islands and in Cornwall. Von Buch found it in great abundance in Norway, even as far north as the North Cape, and it has been met with in Germany, Switzerland, and Italy.
Uses.—When cut and polished, it presents a very' beautiful surface, hence it is much esteemed in some countries, as Italy, as an ornamental stone. The nerd di Prato, verde di prato, granito di gabbro, of the Italians, ale either varieties of euphotide, or serpentines with dis seminated metalloidal diallage.
IX. Porphyry.
Porphir.— Werner.
Porphery.--.Kirwan.
Porphyry.—Jameson.
Par phyre.—Brochant and Daubuisson.
1. Xime —The Grecian word from which the name porphyry is derived, signifies red ; hence the name of the formation is borrowed from that kind, which is de nominated Antique Red Porphyry. It is worthy of re mark, that red, or colours bordering on it, or passing into it, prevail in rocks belonging to the porphyry for mation.
2. Composition and varieties —It is a compound rock, having a basis, in which the other cotemporaneous con stituent parts arc imbedded, either in the form of grains or crystals. Neither the base nor the imbedded parts are always of the same kind. On the differences of the first, depends the distinction of the different kinds of porphyry. The base is sometimes claystune, forming claystone porphyry, hornstone forming hornstone por phyry, compact felspar forming felspar porphyry, pitch stone, when it is named pitchstone porphyry ; and if it contains much hornblende, it has been named sienitic porphyry. The imbedded parts are most commonly felspar and quartz, which are usually more or less per fectly crystallized. The quartz is usually crystallized, and in double six-sided pyramids. The felspar crystals
are broad six-sided prisms, but usually very indistinct. The felspar is more or less fresh, sometimes even glassy, sometimes completely disintegrated and earthy, or only in white specks. The frequency and magnitude of these mixed parts, of quartz and felspar, modify the ap pearance of the different kinds of porphyry very much. Sometimes one, sometimes the other, but more fre quently both occur together, and along with several other minerals which are less frequent, as crystals of hornblende and mica. The basis and the mixed parts of the porphyry also differ in colour and several other properties. It sometimes contains chalcedony and agate, which arc in massive pieces, or in small layers or plates. Further, there sometimes occur balls of a greater or less size ; in clay-porphyry the centre of these balls is chalce dony, but their exterior is hornstone porphyry ; but in pitchstone-porphyry, these balls are composed of a par ticular kind of conchoidal hornstone, but the centre is quartz. It sometimes also contains precious and com mon opal, and these are either disseminated through it, or traverse it in the form of very small veins.
3. Structure.--Porphyry is seldom stratified, and when it is stratified, the strata are very indistinct. It is usually either massive, and merely traversed by numerous ac cidental rents, or disposed in distinct concretions, which are tabular and columnar, or they are globular, and these again are composed of concentric lamellar concretions.
4. Foreign beds.—It contains few foreign beds, with exception of granite, gneiss, and greenstone, which are intermixed with it. One of the best examples of por phyry with foreign beds is that given by Beudant, who describes three different sorts of beds in the sienitic porphyry, in the environs of Schemnitz in Hungary. These are, 1. Small beds of mica-slate that alternate with small granular sienite. 2. Beds of quartz. 3. Beds of compact limestone impregnated with steatite, and in termixed with serpentine.