Page:The American Cyclopædia (1879) Volume X.djvu/258

252 LEAD Le Sueur, who made a voyage up the Missis- sippi in 1700 and 1701, for the purpose of dis- covering ores. The first mining seems to have been done by Julien Dubuque, a half-breed In- dian, who in 1788 settled on the site of the city which bears his name, and continued until his death in 1809. The land occupied by him was relinquished to the United States by the In- dians in 1832. Leases were first issued by the government in 1822 (under the act of March 3, 1807), but mining did not become general till 1826-'7. Owing to the difficulty of collecting rents, a resolution was adopted in the house of representatives in February, 1839, providing for the survey and valuation of these lands, with a view to their sale. This survey was made by Dr. D. D. Owen, with the aid of 139 assistants, in the autumn of the same year, and the report was transmitted to the land office in April, 1840. In 1847 the mineral lands were thrown open for entry and purchase. From records kept at Galena by Capt. Beebe and others, it appears that the amount of lead an- nually produced by the mines of this region increased from 5,000 to 10,000 tons (of 2,240 Ibs.) between 1829 and 1839 ; after that it rose rapidly, and attained its maximum from 1845 to 1847,' when it reached nearly 25,000 tons. Since that time the production has materially declined, and no trustworthy record of its amount is obtainable. A notable feature of this lead region is the entire absence of drift, although the country surrounding it on all sides is covered by gravel, pebbles, and bowlders, showing that the surface has not been covered by water since the earlier geological period, and certainly not during the quarternary peri- od or that of the drift. Geologically, this dis- trict belongs to the lower Silurian. Resting upon a floor of crystalline slaty quartzose and granite rocks is the lower sandstone, the equiv- alent of the Potsdam of New York, from 300 to 500 ft. thick. Above this sandstone, and passing into it by alternating beds of silicious and dolomitic material, is the lower magnesian limestone, the equivalent of the calciferous sandstone of New York. It is nearly pure do- lomite, heavy and compact, with a thickness of from 250 to 300 ft. It is slightly metalliferous, but no profitable mining for galena has ever been carried on in it. The next layer is the upper or St. Peter's sandstone, a purely silicious rock of but slight coherence ; its thickness is 80 to 100 ft. Next in order come the buff and blue limestones. The former, allied to the birds- eye and Black river limestones, is a dolomite from 15 to 25 ft. in thickness, containing 10 to 25 per cent, of insoluble matter. The lat- ter, the equivalent of the Trenton limestone, occurs in beds 20 ft. thick, and is the first group of strata in the series, as developed in the northwest, in which there are any purely calcareous beds, one of the divisions being a pure limestone with but a trace of magnesia ; this blue limestone is decidedly metalliferous. The next formation containing the deposits of galena was formerly called the upper mag- nesian limestone ; but as the term originated in a misapprehension of the geological structure of the district, it has been superseded by the more appropriate name of the Galena lime- stone. This formation is a thick-bedded, light gray or yellowish gray dolomite, nearly chem- ically pure, distinctly crystalline in its tex- ture, and usually rather granular, although oc- casionally quite compact ; its maximum thick- ness is from 250 to 275 ft. ; the middle por- tion is marked by an abundance of flints ar- ranged in parallel layers. Next above the Galena limestone is the Cincinnati group, com- posed chiefly of argillaceous and silico-argil- laceous shales, with a small amount of calca- reous and magnesian carbonates ; its thickness is from 60 to 100 ft. Above the Cincinnati group follows everywhere in the lead region and its vicinity the Niagara limestone, a heavy mass of pure dolomite, the third in order, con- taining like the Galena limestone silicious no- dules or flints, especially in its middle and lower portion, arranged in layers parallel to the stratification. The thickness of the forma- tion is undetermined ; 350 ft. is given as an approximation. The outcrop of the Niagara limestone forms a marked feature in the topog- raphy of the lead region, since, owing to the peculiar denudation which has taken place in the district occupied by these rocks, it is left in abrupt and picturesque bluffs or cliffs along the principal streams. It also caps the mounds or outliers of rock which, severed from their ori- ginal connection, stand like sentinels posted for observation. The lead occurs in the Gale- na limestone in the form of sulphuret or gale- na, filling either partially or entirely fissures and cavities in the rock. The fissures are con- fined to the one formation, and are hence not true veins, but are distinguished by the name of gash veins. The principal forms of occur- rence are the vertical and flat crevices or sheets, and the expansion of these crevices into pock- ets, openings, and caves. The crevice openings and cave openings are mostly confined to the upper portion of the Galena limestone, and the flat sheets and flat openings to the lower por- tion. The latter is the only form of deposit which is found to any extent in the blue lime- stone. The vertical sheets are generally solid masses of ore, unaccompanied by gangue or veinstone, while the flat sheets are generally associated with blende, calamine, and pyrites. The longitudinal extension of a vertical sheet of ore varies from a few yards to 100 yards, and its vertical extension from 20 to 40 ft. The greater the length and width of the sheet, the more likely it is to pass into some other form, and lose its simple sheet character. One instance has been noticed of a continuous sheet of ore 140 ft. in depth. It is from the open- ings or caves that by far the greater part of the ore is raised. These are not generally filled with solid ore, but are partially filled with a mass of loose material, consisting of ore and