Page:The American Cyclopædia (1879) Volume X.djvu/257

LEAD 251 proportion of silver, and the structure of the lodes themselves offers many points of high interest. They are invariably found at a greater distance from the granite bosses which form so striking a feature of these counties than the tin and copper-bearing veins, and usually in killas of a softer character. The majority of the lodes are cross courses, traceable in some in- stances for miles, although it seldom happens that the richly lead-bearing part has been found to extend for more than a few hundred feet, or at most fathoms, in length. The associated minerals are principally fluor spar, quartz, some- times in a loosely granular state, often as a white hornstone, bitter-spar, zinc blende, and more rarely spathic iron ore, fahlerz, and bournonite, while the intersections with east and west veins have been marked by the oc- currence of various ores of silver." (Smyth.) Other localities are the Silurian slates of Car- diganshire and Montgomeryshire, and near Shelve on the south of Shrewsbury, where the veins run east and west, and in the isle of Man, which yields a highly argentiferous galena. In the southern part of Spain, in the mountain range of the Sierra Nevada, there are exten- sive deposits of lead ores, which have been mined by the Phoenicians, Carthaginians, Ro- mans, and Moors, and are still productive at the present day ; and the smelting of the im- mense masses of rich slags left by the Romans has been a profitable industry in recent times. The rocks of the Sierra Nevada are mainly mica, talcose, and clay schists, enclosing strata of dolomite and sometimes gypsum. The prin- cipal lead mines occur in the spurs of the Sierra Nevada, namely, the Sierra Cartagena, Sierra Almagrera, and the Sierra de Gador. "The former is composed of sedimentary rocks of the ancient transition strata, argillaceous schists, slate, mica schists forming the base of the fis- sured black crystalline limestones, sandstones, and conglomerates. The erupted rocks are greenish and porphyric, and they are par ex- cellence the rocks accompanying nearly all the metalliferous deposits of this locality, which for the most part are in contact with schists and limestones, and extend over a large area. Although numerous veins . of ore occur, yet more generally the ore exists in intercalated beds and irregular pockets in the limestones. In the districts of Almazarron the erupted rocks are traversed at many points by veins of galena, which would indicate that the ore is of later date than those rocks. There are two classes of ore, carbonates and galenas, of which the former predominate. The galenas delivered to the smelters vary in produce from 15 to 60 per cent, of lead, containing from 32 to 121 oz. of silver per ton of lead produced. The deficiency of water throughout the entire met- alliferous district of the south of Spain, added to the absence of all motive power, will ever render the dressing of the ores difficult if not impossible." (Petitgand, quoted by Percy.) The Sierra Almagrera consists of argillaceous schists, passing into mica schists, without any eruptive rocks, but traversed by a number of ore veins, the largest of which is called the Jaroso. This vein is about 18 ft. wide at a depth of 180 ft., narrowing to 4^ ft. at a depth of 420 ft. It is filled principally with brown hematite, throughout which is disseminated argentiferous galena, with some blende and copper pyrites. The Sierra de Gador is the most famous of all the Spanish lead-producing localities. The plateau on its summit is lit- erally riddled with shafts of greater or less depth, and shallow excavations. The ore, mainly galena unassociated with other metallic sulphides, occurs in pockets of variable size and extent, sometimes isolated, sometimes con- tinuous and corresponding to the stratification of the rocks. The ore is occasionally found in dolomite, but is then not very abundant. It is most productive in a yellowish clayey mass which contains small pieces of dolomite ore, and associated with masses of galena ; or the whole deposit may consist of fragments of ore and rock enveloped in clay. The Sierra de Gador was the scene of extraordinary ac- tivity during the third decade of this century, the production amounting in 1827 to 42,000 tons. This enormous production overstocked the market, and the price of lead fell to a point below the cost of production. The ore was here largely obtained from shallow exca- vations which were soon exhausted. Mining is still carried on, but the production is at present small. These mines seem not to have been worked by the Romans, owing to the small amount of silver in the galena. In the United States the most important lead deposits are found in the Mississippi valley. There are two prominent localities, known as the upper and lower mines, the latter in Mis- souri, the former included within the border- ing states of Wisconsin, Illinois, and Iowa. The upper mines have been thoroughly inves- tigated by Prof. J. D. Whitney in connection with the geological surveys of Iowa, Wisconsin, and Illinois, and the results of his studies are to be found in the published geological re- ports of these states. The following descrip- tion is taken from these reports. The extent of this lead-producing region is about 4,000 sq. m., of which about 2,200 sq. m. is in the state of Wisconsin. The most productive por- tion of the region is that which lies between Dubuque, Galena, and Shullsburg, so that Iowa and Illinois raise more lead in proportion to the area over which mining operations have been conducted than Wisconsin does. A circle of 4 m. radius, with its centre a little N. E. of Galena, would include nearly all the produc- tive diggings, with the exception of those at Apple river and in the vicinity of Elizabeth. It is probable that these mines were not work- ed by the aborigines. Fragments of galena have been found in the ancient mounds of the northwest, but no metallic lead. The discov- ery of lead ore in this region is attributed to