VEINS.

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1a, more frequently traverse it, and may thus be distinguished frombeds, which are always parallel to the contiguous strata. It has beenremarked, that those veins, which occur in the vicinity of each otiierand contain the same minerals, are usually parallel; the fissures hav-ing probably been produced at the same lime, and by the same cause.

39. In most cases, veins are much inclined to the horizon, andsometimes are almost vertical. They frequently dip in the directionof the declivity of the mountain.—It is an important character ofveins, that they are often divided into several branches, which some-times terminate in the contiguous rocks, and sometimes wind andreturn into the principal vein.

40. The 'walls of a vein are the rocks, contiguous to its two sides orsaalbandes; and in many cases are undoubtedly the sides of the orig-inal fissure. The upper and lower sides of inclined veins are alsocalled the roof and floor of the vein, or the hanging and lying sides.

The sides of veins, especially of metallic veins, are in generalvery determinate, being marked by a delicate seam, or by a layer ofsome indurated, argillaceous substance. In many cases, the walls ofa vein appear to have undergone some alteration, and differ more orless from the same rock at a distance from the vein. Sometimes in-deed the substance of the vein is intermixed with that of the walls.

41. The breadth of veins, more particularly of metallic veins,usually' lies between a few inches and a few feet. But, in some instan-ces, veins of calcareous spar, &c. have been observed more than 100feet in breadth.

The length of veins, which, according to Jameson, seldom exceeds6000 ft. may, however, vary from a few feet to several miles.

Few veins have been explored to so great a depth as that atKuttenberg , in Bohemia , where a shaft lias been sunk 3000 feet. Inthe metallic veins of Cornwall are two or three mines about 1300 ft.deep.—Many veins cease to be metalliferous at the depth of 1200 or1500 feet.

42. The different minerals, which fill some veins, are irregularly-aggregated. But it is often the case, that veins have a regular struc-ture, being composed of a greater or less number of layers of differ-ent substances parallel to the walls. The same order or alternationof layers exists from both walls to the middle, like the leaves of abook, so opened as to form an acute angle. Thus, if quartz be con-tiguous to the wall on one side, it will have the same situation on theother; if sulphuret of lead be the second layer on one side, it will alsobe the second on the other, and so on. And further, each succeedinglayer, estimating from the walls, is impressed by the crystals of eachpreceding layer.