Page:The American Cyclopædia (1879) Volume XI.djvu/597

 MINERAL DEPOSITS 579 granitoid rocks of the Alleghanies, belong to this class and carry more or less gold. Iron pyrites is an almost constant associate of gold in segregated veins, sometimes being present in great quantities. Copper is also frequently contained in them, and in less quantity nickel. Though segregated veins have generally no great lateral or vertical extent, they sometimes attain a thickness of 20 or 30 ft, and have a length on the surface of a mile or more. The Pine Tree and Josephine lodes on the Mariposa estate in California have a thickness of 4 to 12 ft., and are said to be traceable for some miles. These are reported to be segregated veins lying in sheets of slate rock of Jurassic age. Segre- gated veins are generally of much more mod- est dimensions, and are seen to lie alternately or en echelon along the outcrops of the con- taining rocks. In Australia the gold-bearing segregated veins are commonly termed " quartz reefs," having doubtless derived their name from the fact that, being harder than the as- sociated rocks and yielding less readily to at- mospheric erosion, they are left in relief, some- times projecting in ridges above the surface. c. Fissure Veins. These are of indefinite ex- tent, laterally and vertically. They have been formed by volcanic or earthquake action, by which the rocks have been fractured and dis- placed. In all cases where an important crack or fissure is made by subterranean upheaval, either by the slipping in of wedges of rock or by the shifting of the sides of the fissure so that their irregularities fail to match, the walls are prevented from returning to their original positions, and an irregular open crevice is pro- duced. When subsequently filled by foreign matter containing metals or ores, such a fissure becomes a fissure vein. In some instances the fracture of the rocks has considerable regu- larity, and the fissure may be of uniform width for several hundred feet in either direction. More generally, and especially where a fracture FIG. 4. -Section of a Fissure Vein, showing banded struc- ture, a a. Country rock. bb. Calc spar. cc. Galena. d d. Heavy spar sulphate of baryta, e e. Comby quartz. is attended with displacement, the fissure is of very unequal width ; the vein matter has in places a thickness of many feet, while at other points, where the projecting walls approach or come in contact, the vein becomes very thin and may be quite pinched out. From their mode of formation fissure veins are without definite limits horizontally or vertically. They may frequently be traced for miles upon the surface, and their limits in depth are rarely reached. Hence they hold more extensive and continuous deposits of ore than any other kind of mineral veins, and constitute the most trust- worthy bases for mining operations. Fissure veins frequently exhibit a banded structure in the materials which compose them, and this forms one of their most striking characteris- tics. This feature is produced by the deposi- tion on their walls of successive layers of dif- ferent minerals, such as quartz, fluor spar, calc spar, copper or iron pyrites, blende, galena, and baryta. These layers often correspond on either side of the central line, showing that the deposition of the different sheets took place simultaneously on both walls. Some- times a fissure vein exhibits a double or triple FIG. 5. Double Fissure Vein, a a. Country rock, b b. Calc spar. cc,ee. Comby quartz, d. Heavy spar. A B. First and second fissures. series of bands, showing that after being filled with ores it was again opened and a new fis- sure formed, and then this was filled in the same way as the first. The quartz which con- stitutes a large part of the material composing fissure veins frequently shows a "comby" structure, due to the formation of crystals which shoot out from the walls and interlock where they meet. Another common feature in fissure veins is the " fluccan " or " selvege," a sheet of clay which lines either wall and causes the vein matter to cleave off readily. This fluccan seems to be due partly to the attrition of the sides when moved with im- mense force upon each other, and partly to the action on the walls of chemical solutions filling the fissure. The. sides and sometimes the in- terior of fissure veins generally show polished and vertically striated surfaces (" slickensides"). These are undoubtedly produced by the fric- tion of the walls on each other or on the ma- terial composing the vein. As will be inferred from what has been said of their mode of formation, fissure veins cut indiscriminately through all kinds of rock. They frequently traverse stratified rocks across their lines of