Page:The American Cyclopædia (1879) Volume X.djvu/256

 250 LEAD sulphate, phosphate, and arseniate. In some localities these compounds are abundant, and form a considerable proportion of the ore raised. The carbonate of lead or cerussite crystallizes in the orthorhombic system, and contains when pure 77*52 per cent, of lead. It occurs both crystallized and earthy; in the latter condition it is white, if not contaminated by copper or other metals; it is the most abundant of the oxidized lead ores. The sul- phate of lead, or anglesite, occurs frequently in orthorhombic crystals of great size and beauty; it contains 68-31 per cent, of lead. The phosphate or pyromorphite is found fre- quently in the upper part of lead veins ; it occurs in beautiful green hexagonal crystals, which are composed of three molecules of phosphate and one of chloride of lead. The arseniate or mimetesite corresponds in compo- sition to the phosphate, containing three mole- cules of the arseniate to one of the chloride ; it occurs in yellowish crystals of the hexagonal system. All of the above oxidized compounds of lead, with the exception of the carbonate, have more of a mineralogical than metallurgical interest. Some mines are noted for the oc- currence of fine crystallized specimens of these and other lead compounds. The Wheatley mine, near Phoenixville, Pa., has been one of the most celebrated mines in the world in this respect. The number of minerals occurring there is referred to subsequently. There are, further, a number of minerals containing, be- sides lead, other metals, as antimony, copper, and silver, which are subject to metallurgical treatment, but which are not properly speak- ing lead ores. Galena is widely disseminated in nature ; it is found in crystalline and strat- ified rocks, and occurs in veins, beds, and irregularly distributed masses. Two classes of deposits are to be distinguished: 1, those in which the galena is associated with other metallic sulphides, as silver, copper, iron, and zinc, and often combinations of these sulphides with antimony and arsenic; and 2, those in which the galena is nearly or quite free from associations with other metals. To the first class belong many of the most celebrated silver mines, as those of Freiberg in Saxony, Claus- thal in the Hartz, Przibram in Bohemia, and many of those in the United States. The veins worked in these mines are generally in the older crystalline or metamorphic rocks, and usually belong to the class of true veins con- tinuous in depth containing the metallic de- posits and gangue minerals in regular bands or layers. Although the lead products of these mines may be relatively large compared with that of the other metals present, yet they would not be profitable were they worked for lead alone. The presence of lead is, however, advantageous in the extraction of the other metals, especially silver. The workable de- posits of galena free from other metals are not very numerous; they are chiefly confined to England, Spain, and the United States. They occur mainly in or are associated with lime- stone and dolomite, in pockets, layers, and gash veins, but rarely in true veins. The metals gold, silver, and tin are seldom found in workable veins except in the older and crystalline rocks, while lead is mostly found in unmetamorphosed and more recent rocks. In this respect it re- sembles zinc, with which it is often associated. "In Great Britain galena occurs in veins in palaaozoic rocks, namely : in the carboniferous or mountain limestone in Cumberland, Dur- ham, Northumberland, Yorkshire, Derbyshire, and Flintshire; in the Devonian in Devon and Cornwall ; and in the lower Silurian in Shropshire." (Percy.) The mountain lime- stone series is made up of beds of limestone which alternate with sandstones and shales, having a thickness altogether in the north- ern counties of 2,000 ft., and in Derbyshire of 1,500 ft. The miners distinguish three classes of deposits: rake veins, pipe veins, and flat veins. The rake veins correspond nearly with what are called true or transverse veins, show- ing a comby structure and " slickensides," or polished walls; they do not, however, always descend through the strata in a regular manner, but go down by a series of vertical and oblique portions, the change of inclination being coin- cident with a change in the character of the rock through which the vein passes. The pipe veins are quite irregular deposits of no great length, more like what have been desig- nated as gash veins. The flat veins are de- posits formed between the layers of two adja- cent beds. "As a general rule, the lodes of the limestone districts have a comparatively soft matrix ; large bodies of clay, locally called 'flucan' and 'dowk,' often occupy a great portion of the original vein fissure, and these in many localities are found to alternate with portions of exceedingly pure and solid galena ; while even the superficial clay, when a thick cap of that material overlies the back of the lodes, has often yielded large amounts of ore, as in the * hushes ' of the north and in Flint- shire, and as most remarkably exemplified in the 'diggings' of Missouri. When the lodes, on the other hand, exhibit less of mechanical detrital matter, and are filled chiefly by crys- talline deposits, the matrix consists most abun- dantly of calc spar, with which fluor spar and barytes are variously associated ; while zinc blende, or in its absence calamine, is a general concomitant, iron pyrites comparatively in- frequent, and spathic iron ore, quartz, and pearl spar, or occasionally witherite and cala- mine, are confined to certain districts and zones of depth. The general direction of the lodes is more or less east and west, although cases occur where some of the great ' cross courses,' or north and south veins, have in particular parts, as notably near Holy well in Flintshire, yielded large amounts of ore. The Devonian rocks ('killas') of Cornwall and Devonshire have for many years past been noted for the production of lead ores yielding a very large