igneous rocks have characteristically a highly complex chemical composition, and although frequently containing the metallic element, etc., which are of economic importance, these are widely disseminated, and in nearly all cases in chemical combinations, as the minor ingredients of siliceous minerals. Although the igneous rocks sometimes contain valuable ores, they are in many, if not all instances, due to secondary enrichment and are not a result of primary crystallization from fusion. As all the material of the earth's crust was at one stage in the series of changes it has experienced consolidated from fusion, it follows that the ores and minerals now of economic value did not then exist, or were widely diffused and have since been formed or concentrated.
The processes of concentration referred to are carried on in various ways through the agency of mechanical, chemical, vital, molecular, and electrical forces, acting singly or in association. For example, concentration through the action of mechanical agencies is illustrated by the manner in which rocks are reduced to fragments in the every-day process of denudation and the resulting débris removed by streams and redeposited. In this process an assorting in reference to size, specific gravity, etc., takes place, and certain substances, as sand, for instance, is accumulated in one locality, and certain other substances, as clay, deposited in another locality. During this process gold, platinum, etc., owing to their high specific gravity, may be concentrated in stream channels. The accumulation of mineral matter through the action mainly of chemical agencies, occurs when the waters percolating through rocks dissolves certain substances, as calcium carbonate, for instance, and on coming to the surface as springs, or dripping from the roofs of caverns, deposit calcareous tufa, stalactites, etc. Silica, iron, manganese, and other substances are frequently concentrated in a similar manner.
Concentration of previously widely disseminated substances principally through the agency of vital forces, is illustrated by the manner in which molluscs and polyps
obtain calcium carbonate from water and deposit it in their shells or skeletons. The part played by plants in this same connection is shown by the way in which they eliminate carbon dioxide from the air or from water, and concentrate the carbon in their tissues. From the carbon accumulated in this manner, under certain conditions, deposits of peat, lignite, coal, graphite, etc., have resulted.
What may provisionally at least be termed molecular concentration occurs when similar molecules are brought together largely by water and crystallized to form mineral species. In order to simplify this brief discussion as much as practicable, this phase of concentration will be included under the chemical processes referred to above.
The three principal methods by which mineral substances are concentrated, namely, the mechanical, chemical, and vital, have in the main different fields of action. The mechanical and vital agencies operate at the surface of the lithosphere, although organic products, principally certain acids, descend into the earth in solution in water and play an important part in deep-seated chemical changes, as in the formation of mineral veins. The chemical agencies bring about the concentration of mineral substances both at or near the surface and at a depth.
The intensity with which the several agencies just referred to operate varies according to conditions. The mechanical agencies, for example, acting mainly through the aid of flowing water, are in general most potent in humid regions and where the land is high above sea-level. Vital agencies depend largely on climate and are most active in warm humid regions. The chemical agencies are influenced largely by heat, the presence of water, and by pressure.
It is interesting to note that a high degree of heat leads to the dissipation and wide distribution of substances previously concentrated; fusion, for example, permitting of the intimate mingling or recombination of substances, previously segregated, although during the dying stages of volcanic activity minerals like sulphur, cinnabar, etc., may be directly condensed and thus concentrated from a vaporous condition.
During the formation of the three main classes of rocks composing the earth's crust, the agencies leading to the concentration of various substances now of economic importance have to a great extent been different, and hence in a marked way the stones, ores, fuels, gems, etc., to be expected in each of the three classes of rocks, respectively, are distinct. Certain exceptions to this broad conclusion, however, arise from the fact that rocks belonging to each of the classes referred to may have been brought within the influence of the same or similar concentrating agencies and like results produced in each class.