The felspar in granite has usually a vitreous lustre, and perfect foliated fracture, yet in some varieties it passes into earthy, with the loss of its lustre and hard ness; in short, it has passed into porcelain earth. This appearance is sometimes produced by the weathering of the felspar; sometimes it appears to be its original state. When veins containing pyrites traverse granite, the felspar and mica in their vicinity are converted into a kind of steatitical matter, by the action of the sulphu ric acid formed during the decomposition of the pyrites. The mica sometimes also decomposes by exposure to the atmosphere ; but the quartz is never altered. G ran ite, with earthy felspar, is found in Cornwall.
Sometimes the constituent parts of granite are regu larly crystallized ; principally, however, the felspar and quartz. The mica sometimes occurs in nests, unmixed with the other constituent parts, and these have been confounded with fragments. Sometimes the constituent parts are so arranged, that when the specimen is cut, its surface has a kind of resemblance to written charac ters : hence this variety has been denominated graphic stone. This particular variety is found at Portnoy, and in the island of Arran; also in the forest of Thuringia in Germany, in Corsica, in the mountain of Odontsche long, in the Uralian range, and also in France. Granite, with regular crystallized felspar, occurs in the island of Arran, in many places on the continent of Europe, and also in South America. At Nlount St. Gothard, all the three constituent parts are found crystallized together. In the island of Arran and the Saxon Erzgebirge, there is a remarkable variety of granite, in which the felspar, quartz, and mica, have a diverging radiated structure.
Several other varieties of granite have been described by geognosts. Of these, the two most important are protogin2 and sieni:e. We shall now describe them.
Protogine.
In this variety, talc, either in the lamellar compact, or steatitic form, or as chlorite, takes the place of the mica. Mont Blanc, and the surrounding mountains, are formed of protoginc. It is named protogine, (primxvi,) because Jurine, the author of the name, considers it as of very old formation. Daubuisson proposes to name it simply talcose, or steatitic granite.
Sienite.
Sienite is a granite, in which the mica is generally replaced by hornblende ; not always, as some varieties contain also mica. Werner says it is a granular aggre gated rock, composed of felspar and hornblende, with occasional grains of quartz and scales of mica. It is
named from Syene, in Upper Egypt, where the an cients quarried it in blocks of great magnitude.
Werner having remarked, that this rock was associa ted with the porphyries of Saxony, arranged it along with them, separated it from granite, and described it as a distinct species. On the Wernerian view, therefore, it is distinguished front granite by its hornblende and its situation. Our observations in Scotland oblige us to consider sienite as a variety of granite, as it occurs in the same beds with that rock, and exhibits every va riety of geognostical position hitherto observed in gra nite.
?ccidental mixed parts.—Besides felspar, quartz, and mica, the essential constituent parts of granite, and those minerals that take the place of the mica, viz. hornblende and chlorite, others sometimes occur in it. These, how ever, are to be viewed as accidental. Of these schorl is the most Frequent, and the next are garnet and tin stone. The following also occasionally occur, either im bedded in the rock, or in veins that traverse it, viz. rock-crystal, adularia, chlorite, pinite, actynolite, com mon opal, topaz, corundum, fluor-spar, beryl, diallage, epidote, apatite, magnetical iron ore, and iron pyrites.
Granite, besides the granular, exhibit various other kinds of structure, such as porphyritic,globular, tabular, columnar, and strated.
I. Porphyritic.—When large crystals of felspar occur imbedded in a basis of smaller granular granite, the porphyt-itic variety is formed. These imbedded crys tals are sometimes upwards of a foot in magnitude, as is the case with some granites in Saxony, and in other countries.
2. Globular.—Some granites are disposed in roundish halls or concretions, which are from a foot to several fathoms in diameter. These balls are sometimes com posed of curved lamellar concretions, which always in elude a harder central mass or nucleus. The spaces between the concretions are filled with granite of a soft er nature, which decays readily, and thus leaves the harder central masses heaped on each other, or strewed about. Such heaps, or tumuli, have been erroneously described as rolled masses brought from a distance to their present situation, by the agency of currents that formerly swept the surface of the earth. Examples of this kind of structure occur in the island of Arran, Bohemia, the Hartz, the Fichtelgebirge, and in other countries.