tained in the above table but probably even more clearly portrayed in the graph (fig. 5). They are the transitions brought about by the changing character of the ores. Prior to 1870 lead was derived from simple ores by a comparatively simple procedure, but after that date, which marks the connecting of the East and West by rail, the field of industrial opportunity was greatly enlarged, bringing about increased demands, and subsequently the realization of the limits of the ores then being used to meet the requirements. The result was that ores wholly different in character had to be used, and the industry, therefore, was compelled to progress from simple direct methods to complex ones, requiring, more and more, the highest order of industrial coordination. As a matter of fact, few industries in the United States have escaped this transition, for it is a characteristic of industrial progress and runs parallel with an advancing civilization.

Survey.

LEAD ORES: OCCURRENCE, MINING, AND TREATMENT.

Although no two deposits of the same type of lead ores occur exactly alike, there is a similarity among types found in the same general sections of country. Thus, in the Mississippi Valley where the land is generally flat, the ore bodies occur in horizontal or gently rolling rock strata, but in the Rocky Mountains they may occur in all sorts of positions of rock strata from horizontal to vertical. In the southeastern Missouri district the ore bodies lie in pitching troughs and in a number of different ways: (1) Disseminations in limestone and shale, (2) horizontal sheets along bedding planes, (3) filling the walls of joints, and (4) in cavities in the country rock. The first is of the greatest importance commercially. They are found from 150 to 800 feet beneath the surface and vary in size from 5 to 100 feet in thickness; 25 to 500 feet in width, and have been traced for upwards of 2 miles. Being of such enormous extent and con

tained in strong rock strata, the ores can be and are mined in great rooms or "stopes" where, by the aid of mechanical devices both in breaking down the ore and in handling it, large tonnages are obtained. These large scale mining operations are an economic necessity, for the reason that hardly more than 70 pounds of lead are obtained from each 2,000 pounds of ore brought to the surface, so that only by extensive and efficient mining operations can the low price of lead be maintained.

The formation of these vast ore bodies required millions of years for the accomplishment. They had their beginning at a time when the greater portion of the middle section of the United States was an inland sea banked by land composed of rock formations of plutonic origin and containing galena as well as other minerals. During successive periods of erosion of these land areas by surface and underground waters in circulation, the metallic compounds resulting were carried by these waters into the sea, where they became incorporated in the sediments then forming. At a subsequent geologic period the inland sea receded southward, leaving these sediments, which were gradually consolidated into the rock masses of limestone and shales that to-day form the land areas of the central portion of the United States. The galena in these rocks was at first widely dispersed in several successive beds, but, due to the activities, in part of waters descending, and in part ascending through them, the galena was again taken into solution and was moved about in the fractures in the rock beds through which these waters traveled. Finally, however, on coming in contact with reducing agents, such as organic matter in the limestone or shale, the galena was precipitated from the solutions resulting in the accumulation of the masses of ore as they are found to-day.

The lead deposits of the Rocky Mountain States occur in limestones, sandstones, shales, and also in rock formations of volcanic origin, all of which, due to orogenic activity, have been twisted, folded, cracked, and faulted. The ore bodies are irregular in dimensions and extent and, literally speaking, one day will find the miners picking in ore and the next day in barren rock. The combination of these conditions requires, first, extensive timbering of the mine to prevent the falling and sliding of rock both when in the ore and when in barren rock, and, second, conducting mining operations on a small scale with a preponderance of hand labor. The cost of this method of mining is considerably higher than that in the Mississippi Valley and would be reflected in the price of lead except for the fact that the presence of silver, copper, or zinc in these ores enhances their value to such an extent that the lead can be marketed in competition with that from other sources.

As to the origin of these various deposits, they are all associated directly with plutonic activity, hot waters originating deep down in the earth and holding lead, silver, copper, gold, and zinc in solution, ascending and depositing their metallic burden as they encountered the right conditions of temperature, pressure, and reducing agencies. The Coeur d'Alene district of Idaho, for instance, is an area composed originally of metamorphosed sandstones and shales of one of the oldest geologic periods, which have been intruded by masses of molten rock of varying compositions and then solidified. Accompanying these intrusions were water solutions of subterranean origin carrying metallic compounds from which the metals were later precipitated, forming fillings in open spaces but largely replacing the surrounding rocks along zones of fissuring or shearing. The deposits, which have a vertical range of 4,000 feet, were probably formed beneath several thousand feet of overburden, which has since been removed by erosion, bringing the ore bodies to light.

ORE DRESSING OR CONCENTRATION.

The mined ore, no matter what its origin, on being brought to the surface, is treated to separate the dirt from the valuable minerals and further to separate the several minerals from each other. This treatment is called ore dressing; the structure in which it is conducted being called the mill. The treatment is almost wholly mechanical and involves "crushing" to unlock the waste and valuable portions of the ore; "screening" to grade the broken ore; and "concentrating" to separate the valuable from the waste portions of ore. Mills constitute the surface improvement of a mine, located there for the reason that the lead content in a ton of ore is very small and very heavy, and to transport such heavy ores just as they are taken from the mines, directly to the smelters, would involve an expense greater than the value of the ore would warrant. By first eliminating the waste portions, however, and concentrating the valuable minerals to such a degree that at least one-half the weight represents recoverable lead, shipping to the smelters becomes feasible.

SMELTING AND REFINING.

Smelting is the name applied to the practice of extracting a metal from its ore by the aid of a flux and heat. Were the ores of lead absolutely pure, smelting would be a simple operation, but the presence of impurities makes it an intricate and highly specialized art. From the smelters' point of view there are two classes of lead ores in the United States-high-grade nonsilver ores, such as those of southeastern Missouri, and low-grade ores carrying silver, zinc, gold, and copper in addition to lead, such as are found elsewhere. The

former are the nearest approach to pure ores and the smelting of them involves roasting, that is, heating at a high temperature in the presence of air, sufficient to oxidize and drive off the sulphur from the galena and free a great proportion of the lead. The furnace used for smelting this class of ores is known as the Scotch hearth and while in the early days, when the lead industry centered entirely in Missouri, it was the chief form of smelting furnace, its use to-day is greatly restricted, due to the increasing complexity of the ores and their decreasing purity.

The ores of the second class, comprising almost two-thirds of the lead ores mined, have other valuable minerals intimately associated with them as well as nonmetallic compounds which are even more refractory, so that heating to a very high temperature and in the presence of a flux is required to separate the lead from the associated substances. Blast furnaces are used for smelting this class of ores and the flux generally used is a mixture of sand, iron ore, and limestone. The nonmetallic substances in the ore have a greater affinity for the silica in the sand than they have for the lead so that in the course of smelting new relationships are formed, the lead is freed and flows out of the furnace pure, except for the other metals contained. These are eliminated by refining. Smelting is a delicate operation requiring an intricate manipulation of the mixture of fluxes and ores composing the furnace "charge," so that after the mixture has passed from top to bottom of the furnace all of the impurities have parted company with the valuable metals and gathered together as a "slag" while the lead and other metals have joined together as a separate body. The "slag" being lighter floats on the molten metals with the result that when the furnace is "tapped" the "slag" flows off first in one direction and the molten metals follow in another.

Silver, copper, antimony, arsenic, and bismuth are often present in lead in small yet sufficient quantities to be objectionable, and refining comprises the removal of these metals, so far as practicable. Silver and copper are eliminated by the use of zinc, the zinc being melted into the lead in a large iron pot and the mass made red hot. In this state the copper and silver combine with the zinc forming a new compound which rises to the top of the molten mass as a scum. The scum is then distilled in a retort which process eliminates the zinc and leaves a residue from which the copper and silver are recovered. Antimony, arsenic, bismuth, and the minor nonmetallic impurities are eliminated after the removal of the silver and copper by making use of the fact that, at a red heat and in the presence of air, they are oxidized and converted into new compounds which separate from the lead. The practical application of this truth is the refining process generally known as "poling," deriving its name

IN A LEAD MINE.

MINERS ABOUT TO ASCEND. FROM A TRANSPARENCY IN THE DIVISION OF MINERAL TECHNOLOGY, UNITED STATES NATIONAL MUSEUM.