Page:The American Cyclopædia (1879) Volume X.djvu/259

 LEAD 253 dolomite in fine particles, the result of the dis- integration of the rock, and infiltrated clay. The sides of these openings are often covered with large and beautiful crystals of galena. An exceptionally large cave in the vicinity of Du- buque was 123 ft. long, 40 to 50 ft. wide, and 20 to 30 ft. deep, and when first opened was found half filled with detritus. The decompo- sition and disintegration of the upper layer of the Galena limestone give* rise to surface ore, the galena being distributed in the clayey loam of the prairies. It is called float mineral, and generally indicates proximity of mineral-bear- ing crevices. Some openings are noteworthy for their extraordinary productiveness. The Longworthy crevice, which has been worked in different places along a line of nearly three fourths of a mile, has produced, it has been stated, about 10,000,000 Ibs. of ore. Several other cavities have produced from 2,000,000 to 4,000,000 Ibs. The origin of these crevices seems to be due to the same cause by which what are called joints by geologists have been formed in almost every variety of rock occurring in large homogeneous masses, and especially where they have a decidedly crystalline texture. In the dolomitic rocks of the lead region we have all the conditions which usually occur in the formation of a well developed jointed structure. It is a marked feature of the dis- trict that the fissures have approximately an E. and W. and N". and S. direction, a fact which is everywhere recognized by miners, and which is of great practical importance. All through the mining district the heaviest diggings, with but few exceptions, will be found in crevices varying but little from E. and W. in their general direction. The N. and S. fissures, on the other hand, are usually much less important, as they are generally in sheet form or their bodies wedged in close between walls of solid rocks, and do not extend into openings. It is probable that the E. and W. course of the principal crevices has been deter- mined by the fact that this is the direction of the axis of upheaval by which the whole lead region has been slightly elevated along the N. boundary of the district, and which will be seen to have determined the draining of the region. The axis of upheaval may have deter- mined the course of the main set of fissures, while the tendency of all masses of rock thus situated to the formation of a subordinate set nearly at right angles to the principal ones may not unreasonably be looked on as the ori- gin of the norths and souths. The disposition of ore in the fissures is due, according to Prof. Whitney, to precipitation of the lead from the oceanic waters, from which the rocks them- selves were thrown down, by means of organic matter, either directly, by the reduction of the sulphates to sulphides, or by the generation of sulphuretted hydrogen by the decomposition of the organic matter, and the subsequent con- version of the metal into sulphide. The theory of the injection of the mineral masses from be- low is untenable, owing to the entire absence of continuation of the crevices into the under- lying strata. The cause of the abundant min- eral deposition in the Galena limestone, while in the lower strata it is very slight, is doubtless due to the increase of animal and vegetable life during this period, as the fossiliferous character of the blue limestone abundantly proves. The galena from this region is of great purity, and contains but a trace of silver. The lead deposits of Missouri may be divided into three districts, the southwest, the middle, and the southeast. The latter, by far the most important, are embraced in an area 5 m. in width and 100 m. in length. They were dis- covered and first worked in 1720 by Renault and his mineralogist La Motte, who came out with a large party under authority of a patent granted by the French government to John Law's famous company. Mine La Motte and the Potosi lead mines were discovered and opened by them ; little however had been done up to Renault's return to France in 1742. The only smelting of the lead ores appears to have been done on log heaps, a wasteful process, much practised even of late years. In 1798, as stated by Schoolcraft in his "View of the Lead Mines of Missouri," p. 19, Moses Austin of Virginia, having obtained a grant of land from the Spanish government near Potosi, sunk the first regular shaft, and erected a re- verberatory furnace, and also a shot tower. According to the same authority, there were 45 mines in operation in Missouri in 1819, giving employment to 1,100 persons; in 1811 Mine Shibboleth produced 3,125,000 Ibs. of lead from 5,000,000 Ibs. of ore. From 1798 to 1816 Mine si Burton and the Potosi diggings were estimated to have produced over 500,000 Ibs. annually; and from 1834 to 1837 the pro- duction of Mine La Motte is rated at an aver- age of 1,035,820 Ibs. of lead per annum. For 14 years succeeding 1840 Dr. Litton in his state geological report makes the annual average of all the mines over 3,833,121 Ibs. The lead ore is mainly confined to the third magnesian limestone, which is nearly a pure dolomite. No workable lead deposits have been found either in the overlying or underlying strata. Beneath the limestone, throughout this region, porphyries are found which are older than the Silurian limestones, and belong, according to Pumpelly, to the azoic formation, of which they may be the youngest member in Missouri. They are the near equivalents, in point of age, to the great iron-bearing rocks of Lake Supe- rior, New Jersey, and Sweden. Various other ores are found associated with the galena, as the carbonate of lead, sulphuret and silicate of zinc, iron and copper pyrites, and at some of the mines, as Mine La Motte, ores of manganese, nickel, and cobalt. The surface of the country in the lead region is strewed with crystallized quartz derived from the lead-bearing rocks, and called by the miners u mineral blossom." The modes of occurrence of the lead ore are gener-