Page:The American Cyclopædia (1879) Volume XI.djvu/596

578 MINERAL DEPOSITS bed and of the trap and sandstone in the Lake Superior series. 4. Impregnations. In certain cases metalliferous minerals are found diffused irregularly through rocky masses, the deposits of ore having no definite boundaries or any regularity of structure, and appearing as though the rock had soaked up or absorbed the min- erals, as water saturates a sponge. Such ac- cumulations of ore are called impregnations. The deposits of mercury exhibit this character in a marked degree. 5. Fahlbands. This name has been given to a peculiar kind of deposit where the ore is sparingly diffused through certain layers, which are prone to disintegrate and are more fahl (i. e., foul or rotten) than the associated strata. Typical examples of this kind of deposit may be seen in the silver mines of Kongsberg, Norway, but they are not common elsewhere. Usually the fahlbands are only rich enough for working where cut by veins. 6. Stockwork. Where the masses of metalliferous rock are penetrated in every direction by threads or strings of ore, so that the whole must be taken out together, it is called a stockwork. Such deposits occur locally in many mines, but rarely to such a degree as to give character to the mining operations. The copper mines of Lake Superior, and the silver mines of Norway, Freiberg (Saxony), and Nevada, all furnish examples of stockwork. 7. Mineral Veins. These are usually sheets of mineral matter, of greater or less lateral and vertical extent. They have been divided into three principal varieties, which are generally well marked, but which sometimes blend in such a way as not to be easily separated. These varieties of mineral veins are known as gash veins, segregated veins, and fissure veins. a. Gash Veins. These are such as are confined to a single stratum or formation, and hence Fie. 1. Gash Veins filled with Lead Ore In Galena Lime- stone, a. Crevice opening. &, c. Crevices with pocket openings. are of limited extent both laterally and verti- cally. The best examples of gash veins are seen in the lead mines of the upper Missis- sippi. Here the ore is found in a single for- mation, the Galena limestone, a member of the Trenton group of the lower Silurian. It usually occurs in vertical fissures at no great depth, sometimes very narrow, sometimes FIG. 2. Horizontal Gash Veins or Floors of Lead in Galena Limestone, a. Crevice with pocket opening, b. Crevice- openings. opening into caves or chambers lined with " mineral." These gash veins have apparently been formed by the shrinkage of the Galena limestone after its deposition. Subsequently the shrinkage cracks were enlarged by the dissolving away of their walls, and were lined with galena, deposited from a solution which exuded from the adjacent rocks. Similar veins also containing galena occur in Missouri, but in a formation somewhat more ancient, the magnesian limestone, supposed to be the equiv- alent of the calciferous sand rock of New York. The fact that these gash veins are limited to a single formation has been amply proved by numerous shafts sunk in the hope of finding the ore at a greater depth, all of which have been failures, b. Segregated Veins. FIG. 8. Segregated Veins of Auriferous Quartz in Gneiss. These are usually lenticular sheets of ore-bear- ing mineral, which are conformable to the bedding of the associated rocks, that is, are interposed between the layers of such rocks. Segregated veins always occur in metamorphic rocks, and are usually inclined at a high angle with the horizon. They are called segregated veins because they are supposed to have been formed in the process of metamorphism by the separation or withdrawal of the materials which compose them from the adjacent strata, and their concentration along certain lines. Segregated veins are limited both laterally and vertically. They rarely exhibit anything of the banded structure which characterizes fis- sure veins, are chiefly composed of quartz, and form the great repositories of gold. All the quartz veins, which are so common in the