The effect of igneous intrusions.—A special case of much importance arises when lavas are intruded into sediments that have previously been partially enriched in the ways above described. The igneous intrusion not only introduces new contact zones, and more or less fracturing, but it brings into play hot waters with their intensified solvent work, their more active circulation, and the reaction between waters of different temperatures. The special efficiency of these agencies is believed to be the determining factor in many cases.
The influence of rock walls.—The rock walls themselves are thought sometimes to be a factor in ore-precipitating reactions. By mass action, they may withdraw a constituent of the solution and destroy its equilibrium in such a way as to cause the precipitation of the metallic constituent. Once deposited on the walls ores aid, by mass action, the further accretion of ores.
The special forms which ores assume in deposition, as beds, veins, lodes, stockworks, disseminations, segregations, etc., are chiefly incidental to the local situation in which the essential chemical or physical change takes place.
CHAPTER VIII.
STRUCTURAL (GEOTECTONIC) GEOLOGY.
The structural phases which rocks assume.—In the previous chapters, the general method by which rocks are formed has been set forth, and many of their structural features have been touched upon incidentally. It remains to assemble the structural features already mentioned, and to consider certain additional structural phases which rocks assume.
STRUCTURAL FEATURES OF SEDIMENTARY ROCKS.
In the deposition of sediments in the sea, or in other bodies of standing water, the coarser portion of the material is usually deposited in the shallow water near the shore where the wave-action is strongest, and the less coarse of various grades is deposited at greater and greater distances from the land, while only extremely fine silt is usually carried out to abysmal depths (see [p. 380]). To this general law of distribution there are important exceptions. Fine sediments are sometimes deposited near the shore, and where currents, tidal agitation, or floating ice are effective, coarse deposits are occasionally carried far out from the shore.
Stratification.—Sedimentary rocks are usually arranged in more or less distinct layers; that is, they are stratified. The stratification consists primarily in the superposition of layers of different constitution or different compactness on one another. Layers of like constitution or compactness are often separated by films of different material which cause the partings between them. The bedded arrangement of stratified rocks is due to various causes, but primarily to the varying agitation of the waters in which the sediment was laid down. Where the depositing waters are agitated to the bottom, coarse sediment is likely to be deposited. Where the waters are quiet at the bottom, fine sediment is the rule. Since the agitation of the waters is subject to frequent change, it follows that coarser material succeeds finer, and finer coarser, in the same place. Hence arise beds, layers, and laminæ. The terms layer and bed are generally used as synonyms, while laminæ are thinner divisions of the same sort. The term stratum is sometimes applied to one layer and sometimes to all the consecutive layers of the same sort of rock. For the latter meaning the term formation is often used. Sometimes bedding seems to have been determined by strong currents which temporarily not only prevented deposition over a given area, but even cut away the loose surface of deposits already made, giving a firm surface from which succeeding deposits are distinct. This sequence of events is sometimes shown by the truncation of laminæ, and by other signs of erosion. The commoner sorts of bedded rock are limestones, shales, sandstones, and conglomerates.
The bedding of limestones is often caused by the introduction of thin films of clayey material which interrupt the continuity of the lime accumulation and cause natural partings. Sometimes, however, bedding arises from variations in the physical condition of the lime sediment itself. Lamination is not usually conspicuous in pure limestone, though it may be well developed in the shaly phases of this rock. Shales are normally laminated as well as bedded, and the lamination is often more notable than the thicker bedding. Bedding in shale may arise from the introduction of sandy laminæ, or by notable changes in the texture of the shale material. Similarly, sandstones are sometimes divided into beds by shaly (clayey) partings, but more often by variations in the coarseness of the sand itself, or by the presence of laminæ that are less coherent than those above and below. Sometimes the layers appear to be determined by the compacting of the surface of sand already accumulated before it was buried by later deposits. Sandstones may be thick- or thin-bedded, and their bedding passes insensibly into lamination.