of deposition from solution in heated waters. This work was perhaps the leading authority on the subject of ore deposits during the mining litigation of the last quarter of the nineteenth century, and is frequently mentioned in the reports of the courts. in mining cases. Consequently it is still of much value, as showing the conceptions of ore deposits and kindred subjects that were in the minds of the experts who testified in these cases and thus influenced the courts in their decisions of cases involving geological conceptions. For example, in the Eureka case the court says: "The definition of a lode is, that of a fissure in the earth's crust filled with mineral matter, or more accurately as aggregations of mineral matter containing ores in fissures. See Von Cotta, Treatise on Ore Deposits,' Prime's translation, p. 26.” Another theory, especially advocated about 1873 by Sandberger (1826-1898), was that of "lateral secretion." It assumes that the vein minerals were originally contained in the wall rock, and were leached out by waters circulating from the walls into the fissures, where the solutions were relieved of their dissolved minerals as the result of different physical and chemical conditions there prevailing. Le Conte's (1823-1901) theory belongs to this period. His argument was that the contents of mineral veins seem to have been deposited from hot alkaline solutions ascending through fissures. As to the vein matter, he states that deposition from solution is proved by the fact that vein quartz has a specific gravity of 2.6, which is true only of quartz formed in the "humid" way. Silica produced in the "dry" way has only a specific gravity of 2.2. The waters are heated because they come from fissures which extend to great depths where the rocks are hot. The heat and the pressure greatly increase the solvent power of the water. But, if the "vein-stuffs" have been deposited from solution, then the mineral ores which are so intimately associated with the other vein contents must have been deposited by the same agency. He does not make an explicit statement concerning the source of the water, but it may be fairly presumed that he supposes it to be of meteoric origin; so that, in brief, his theory is: deposition in fissures, etc., from solution in alkaline meteoric waters heated during descent in fissures, etc., to great depths in the earth's crust by the regular increase of heat that occurs as depth is gained. Each of these, as well as other less notable theories of the cause of ore deposition, contained some germ of truth except, perhaps, the "wholly irrational" ideas of Werner; but want of space forbids any detailed discussion of them. The valuable parts of each are embodied in the theories of ore deposits now held by modern geologists as the result of a century and a half of investigation. Except as to certain subordinate details, they are matters substantially agreed upon by scientific investigators of such subjects, and are entitled to rank with other scientific principles as representing truthfully the causal and other relations existing in nature. They furnish, likewise, valuable aid to the practical operations of mining. During the latter quarter of the past century the development of the science of geology was greatly promoted by the geological surveys in the United States, both by the individual States and by the National Government as well as by similar surveys maintained by many foreign governments. By these agencies large numbers of facts regarding ore deposits have gradually accumulated; and these have greatly contributed to the creation of theories of ore deposits resting on reasonable and scientific grounds. Near the close of the century (1893) Pŏsepnéy (?-1895), professor of mining geology in the School of Mines at Pribram, Bohemia, in a paper read before the American Institute of Mining Engineers at their meeting at the Columbian Exposition at Chicago, proposed a theory of great value, which stimulated renewed discussion and investigation of the subject, advancing our knowledge greatly. His fundamental conception was the division of the part of the earth's crust that contains water, which has descended into it from meteoric sources, into two zones: (1) The vadose region, where the water is in active circulation through cracks, fissures, joints, permeable planes in bedded rocks, interstitial spaces, etc. (2) The ground-water region, where the water contained in the rocks is comparatively quiet. He believed minerals to be brought up in solution by the ground waters, from the depths, or barysphere, and deposited above in fissures, etc., as veins. Here during geological mutations they were oxidized, altered, rearranged, concentrated, etc., by the action of the waters of the upper or vadose circulation. He was of opinion that by the latter agency a large part, perhaps the larger part, of the workable ore deposits have been concentrated from bodies, which, as originally precipitated from ascending ground waters, were too lean to be profitable. This latter part of his theory has withstood all the discussion and investigation it has since aroused; and it now forms an integral part of modern theories of ore deposits. In the same year Vogt, of Norway, presented the idea of magmatic segregation to account for the direct formation of titaniferous iron ores and certain other ore-bodies from molten rockmagmas. He also accounted for a large part of the remaining kind of ore-deposits as being formed by the combined action of water and gases following an igneous eruption, this action being FIG. 4. Example of magmatic segregation. Section of termed "pneumatolysis." His conception differs from de Beaumont's in that, instead of considering the molten masses to be drawn from a molten interior, he thinks they are derived from the solid crust and are the result of localized eruptive action within that crust. The conclusion follows because terrestrial physics has disproved the idea of a mobile, molten interior of the earth. This brings us to the views held by geologists at the present time, according to which ore deposits are believed to have been formed in the following ways: I. During the cooling of fused masses, or magmas, a process of segregation of the different minerals composing them has gone on by which concentrations rich enough in metals to be classed as ore-bodies have been formed, usually in the exterior part of such magmas. The examples of this class are titaniferous iron. ores, chromite, and probably certain copper and nickel ores. There is general agreement among geologists as to the fact that these deposits have been formed by segregation from fused rock masses, but there is still much discussion of a highly technical character as to what natural forces have operated to produce such segregation. These, however, are not of practical importance, and I shall not attempt to state the various arguments. FIG. 5. Relation of level of ground water to topography and to surface drainage. Lines with arrows are lines of flow. From Van Hise's treatise on metamorphism; professional paper No. 47, U. S. G. S. II. Another mode of formation has been by deposition from water as members of a sedimentary series, either (1) as chemical precipitates, such as the Clinton iron ores of the eastern United States, or (2) mechanical deposits, such as the placer gravels containing metallic gold or cassiterite. III. A further method, which accounts for the formation of most of the economically important deposits, has been by the solution of the mineral in water and its subsequent redeposition either (1) in preëxisting cavities in the rocks (fissures, interstitial spaces between the grains, contact planes, caverns from solution, etc.) or (2) by a replacement or chemical exchange by which the rock in contact with the mineral solution was dissolved and carried away by the water, while in its place some mineral or minerals were deposited from the solution. There is substantial agreement among all modern geologists as to water being the agent concerned in the production of this class of ore deposits; but there is still a marked divergence of opinion as to the source of this water and the conditions under which it has done the work. Two theories are held. One is, that such part of the meteoric, or rain, water, as sinks into the rocks is the agent which accomplishes the work of making ore deposits. This water is believed to descend to great depths and to be in slow but continual movement or circulation through the mass of the rock itself as well as along the cracks, crevices, and faults which exist in all rocks. This water dissolves the minerals out of the rocks with or without the aid of heat, ascends along fissures, etc., to the surface, and deposits the dissolved minerals in veins and other forms of ore-bodies. In Chamberlin & Salisburys' recent text-book on geology, the theory of ore concentration by the circulation of meteoric waters is upheld. They say:* "There are other occasional methods, but the chief process of concentration, immeasurably surpassing all others, consists in leaching out the ores disseminated through the country rock, and their redeposition in segregated form, as an incident of the recognized system of water circulation." In the form here stated, this theory is made highly improbable, by the fact that, though all the materials necessary for vein formation silica, iron, calcium carbonate, sulphur compounds, etc. are everywhere present, as well as fissures in the rocks through which the water circulates, still it is only in exceptional cases and in limited areas that vein formation and the creation of ore-bodies has occurred. These exceptional cases and limited areas are nearly always related to igneous activity and its products. The most prominent advocate of this theory is C. R. van Hise. His fundamental conception is that the earth's crust is divided into three zones: (1) A zone of fracture extending to a depth of 1625 to 32,500 ft., according to the strength of the different kinds. of rocks in which such fractures may be formed. If fractures are produced in this zone by dynamic causes they will remain open and form passageways for mineral solutions. (2) A zone of combined fracture and flowage. (3) A zone of flowage where the pressure is such that no open fractures can exist; for the rock will flow under the dynamic forces instead of fracturing. Consequently, water cannot penetrate into this zone, but can only enter the first and second. He holds that meteoric waters from the surface descend into the fractures of the first and second zones, percolate through and dissolve out the minerals from the rocks, and, ascending along other fissures toward the surface, deposit these dissolved minerals as veins and other ore-bodies. He admits that this circulation may, in some instances, be accelerated by the heat derived from igneous intrusions, but he seems to consider that the greater part of mineral deposits has been 4 Vol. i, p. 453. |