Page:The American Cyclopædia (1879) Volume XI.djvu/598

580 MINERAL DEPOSITS deposition and outcrop, and are then called " cross-cut " veins, to distinguish them from those that are more or less accordant with the stratification. The materials composing fissure be cf e FIG. 6.~Fi88ure Vein with Cavity or "Vug" at Centre. a a. Country rock, b b. Heavy spar, c c. Calc spar, d d. Blende, e e. Comby quartz. /. Vug. veins are very varied ; indeed, it may be said that nearly all the minerals known are found in them. Quartz is a conspicuous ingredient in fissure veins, but sulphate of baryta, calc spar, and fluor spar sometimes form almost the entire mass of the veinstone. The ores which are contained in fissure veins, like the earthy minerals, are widely varied. Silver, copper, lead, tin, zinc, antimony, iron, and more rarely many other metals are found in them. Gold is less common in fissure than in segregated veins, and it is almost never the sole object of search in their exploitation ; but it is a recog- nized constituent in the veins worked in Corn- wall, and in the silver ores obtained from some of the mines in Idaho, Nevada, Mexico, and South America. Silver may be regarded as the most valuable constituent of fissure veins, and all the great silver mines of the world are worked in veins of this character. The Comstock lode, near Virginia City in Nevada, is a true fissure vein, and perhaps the largest and richest known. It has been traced on the surface for several miles, and has been worked to the depth of 1,600 ft. Its normal width is perhaps 200 ft., but in places it expands to 800 ft., though here divided into several veins by great wedges or " horses," split off from the walls. It cuts through syenite and propylite, and evidently marks the line of a great fissure opened by volcanic action. All the silver mines of Nevada belong to the same class with the Comstock, though the other veins are generally of much more moderate dimensions. They are worked in fis- sure veins which traverse all the varieties of rock found in that region, such as granite, por- phyry, trachyte, slate, and limestone. The num- ber of these veins, the disturbed and broken condition of the strata, and the abundance of volcanic rocks in the district, all prove that it has been long the theatre of intense volcanic and earthquake action. In Nevada and Utah some very rich mines have been worked in deposits of ore, of which the true character has been imperfectly understood and very much misjudged. These are the chambers or pockets of ore so characteristic of the White Pine district, Nevada, and that of Little Cot- tonwood cafion in Utah, the latter including the famous Emma mine. These districts have been the centre of intense mining excitement and scenes of the wildest speculation ; of the most unparalleled successes and sudden and complete failures. A large part of this history has been consequent on the peculiar nature of these mineral deposits. In both these districts the ore occurs in limestone, and often in cham- bers frequently of considerable size. These when first opened were supposed to hold in- calculable wealth, but they proved to be of limited extent and were soon worked out. The relations of the ore chambers of White Pine and Little Cottonwood to the silver- bearing fissure veins of Nevada and Utah are not at first sight apparent, and yet they are unquestionably products of the same general cause. The true theory of their deposit is probably this. The " country rock," i. ., the rock enclosing the deposits of ore, unlike that of most of the mining districts of the west, is limestone. Many limestones are soluble in atmospheric water which holds carbonic acid in solution. In some limestone countries the underlying rock is honeycombed by caves and subterranean galleries, forming a system of underground drainage. The table land of Ken- tucky affords a typical example of such a re- gion, and the Mammoth cave is only one of an immense system of natural sewers, by which the drainage of the surface is effected. If now the Kentucky table land were much dis- turbed by an earthquake and fissures were opened through the limestone, and these fis- sures were filled to form mineral veins, then, wherever these fissures communicated with the subterranean chambers and galleries, these would also be filled with vein matter and ore, and a condition of things would be produced similar to what we now find in Utah and Nevada, though on a much grander scale. With this explanation the western pockets and chambers of silver ore are seen to be natural offshoots and appendages of the fissure veins so common in the region where they occur. The Filling of Mineral Veins. The manner in which the materials composing mineral veins have been deposited has been a matter of much discussion among geologists, and one about which there has been and still is considerable diversity of opinion. The theories which have been advanced to account for the phenomena are briefly as follows, a. The Theory of In- jection. This was proposed by the Plutonists, who were prone to ascribe all the great changes on the earth's surface to the action of heat. There are few mineral veins, however, com- posed of materials which can be regarded as even the possible product of fusion, and most