3. Tabular.—Some granites, when they are traversed by parallel seams, appear divided into tables. These tables vary in extent from a few inches to several fa thorns. They appear in some cases to be mere varie ties of the stratified structure.
4. Columnar—When the seams are arranged in di rections parallel to several planes, the granite is divided into columnar masses, which resemble the columnar structure of trap and porphyry rocks. We many years ago, as mentioned in our account of the Hebrides, ob served this columnar structure in the granite of Mull ; and xince that time Humboldt has described it as occur ring in the granite of Caraccas, as it does in the granite rocks of Carlsbad.
5. Stratified structure, or Siratification.-: Granite is sometimes disposed in great beds in gneiss and other rocks, and occasionally these beds appear divided into strata. In other instances, in granite mountains, we ob serve, besides the tabular, globular, and other struc tures, also the stratified ; but this latter is, in general, less perfect than what is observed in gneiss, and other similar rocks.
6. Beds in Granite.—Granite does not afford so many different beds and veins as occur in gneiss, mica-slate, and other similar rocks. In Scotland, it sometimes eon tains beds of quartz and of felspar. In Switzerland, beds of quartz in granite have large drusy cavities, the walls of which are lined with magnificent crystals, and groups of rock-crystal. At Zinnwald, in Bohemia, the tin is worked in a quartz bed, situated in the middle of the granite. Beds of limestone are also met with in granite mountains, as in the Pyrenees ; and some of them of great extent, having been traced by that excellent observer, Charpentier the younger, for four leagues, and with a thickness of ninety feet. We need not speak of the beds of gneiss, mica-slate, clay-slate, porphyry, trap. &c. upon which it often rests, and with which it frequently alternates.
7. Metals in Granite —On a general view this rock contains hewer and less extensive metalliferous veins and beds than the slaty rocks of the primitive class. Tin, of all the metals, is that which is most peculiar to granite. Tin-stone occurs in the granite of Cornwall, Saxony, Limoges, and in these countries is generally associated with walfram.
Iron is frequent in granite. The mines of Traver sella, in Piedmont, are situated in a granite which is subordinate to mica-slate. The mines of brown iron ore at Taurynia, and of Fillolo in the eastern Pyrennees, are in granite. Iron pyrites is frequently found disse minated through granite ; and galena, or lead-glance, graphite, molybdena, bismuth, gold, silver, copper, zinc, manganese, cobalt, are among the metals sometimes met with in this rock.
8. Formations of Granite.—Granite occurs in masses, often many mites in extent, surrounded by gneiss, mica slate, and clay-slate, and so connected with these rocks, that the whole may be considered as the result of one grand process of crystallization ; that is, the granite is of cotemporaneous formation with the gneiss, as the gneiss is with the superimposed mica-slate; and the mica-slate, again, with the clay-slate which rests upon it. —In other instances, the granite alternates in beds, often of enormous magnitude, with gneiss, mica-slate, clay-slate, and other primitive rocks, or it traverses these in the form of veins.
9. Decomposition of Granite.—Some granites resist, for ages, the destroying effects of the weather ; while others are resolved into sand or clay in a comparatively short period. The obelisk which is at present in the place of Saint Jean de Latran, at Rome, and which was quarried at Syena, under the reign of Zetus, King of Thebes, thirteen hundred years before the Christian era ; and that which is in the place of Saint Pierre, also at Rome, and which a son of Sesostris consecrated to the sun, have resisted the effects of the weather for three thousand years.
On the other hand, there are granites, as those in some districts in Scotland, which are speedily disinte grated into gravel or sand. But between these two ex tremes, of extreme durability and rapid decay, there are numerous intermediate degrees. In the same moun tain, or even in the same hillock, granites of different qualities will sometimes be met with. One portion will be excessively obdurate, and resist long the gnawing effects of the weather ; another variety, immediately be side it, will be of a very decomposable nature ; and while a third, associated with the two former, will pos sess an intermediate degree of durability. Granites vary in their mode of decomposition. Some assume the globular form ; others that of rhomboidal or irregu lar masses. These are further disintegrated, and then the constituent parts fall asunder ; when a kind of gra vel, or sand, depending on the size of the grains, is formed. The felspar in this gravel is further altered, and easily changed into a clay, which is carried into hollows or plains, and forms beds of clay ; the quartz grains, by attrition, arc reduced in size, rounded in form, and give rise to beds of sand ; and, when mixed with the matter of felspar, to sandy clays. The mica is fur ther broken down, and becomes mixed with the clays and sands formed from the felspar and quartz.