Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/389

Rh MINERALOGY 371 second iu some varieties of specular iron, of the last in chalcedony, opal, and other species. The ahove terms are indefinite, but of necessity, as there is every degree of fineness of structure in the mineral species, from perfectly impalpable, through all possible shades, to the coarsest granular. The term phanero-crystalline has been used for varieties iu which the grains are distinct, and crypto-crystalline for those in which they are not discernible without the aid of a lens. Granular minerals, when easily crumbled in the fingers, are said to be friable. The minute or crypto-crystalline minerals form aggregates some what similar to the above. When globular or oolitic, the minute crystals often appear to radiate from a centre, or form concentric crusts. These are often globular or nodular; as in dolomite. Some what similar are the stalactites and stalagmites, in which the mineral (especially rock-salt, calc-spar, malachite, hajmatite, limonite) has been deposited from a fluid dropping slowly from some overhanging body, or some rent in the roof of a cave. In this case there is generally found a long pendent cylinder or cone, the principal axis of which, generally hollow, is vertical, whilst the marginal parts are arrangt-d at right angles to it, except where they curve round the termination of the tube, when they become hemispherical. By far the largest masses of the mineral kingdom have, however, been produced under conditions in which a free development of their forms was excluded, and are termed amorphous. This has been the case with the greater portion of the minerals composing rocks or filling veins and dykes. The structure of these masses on the large scale belongs to geology, but some varieties of the textures, visible in hand specimens, may be noticed. The individual grains or masses have seldom any regular form, but appear round, long, or flat, according to circumstances, and as each has been more or less checked in the process of formation. Even then, however, a certain regularity in the position of the parts is often observable, as in graphic granite, where the axes of the skeleton crystals of quartz a-re parallel. The rock is termed massive when the grains which form it are small, or granular when they are longer and more dis tinct. Sometimes the rock becomes slaty, dividing into thin plates ; or concretionary, forming roundish masses ; at other times the interposition of some foreign substance (gas or vapour) has rendered it porous, cellular, or vesicular, giving rise to drusy cavities. These cavities are often empty, but have occasionally been more or less filled by products of change in the rock. It is named amygdaloidal when the cavities so filled have the form of an almond. Changes of Crystalline Structure. &quot; Pseudomorphs &quot; are minerals which appear under a form of crystallization which does not belong to the species. They may be recognized either by their having no cleavage, which is most usual, or by their cleavage being altogether different in direction from that of the mineral imitated. Generally they have rounded angles, rough and dull surfaces, and when broken show a granular structure. The faces of the crystal, moreover, are often covered with minute crystals of a form different from that of the mineral imitated, but which is that belonging to the substance now present. Occasionally the resemblance to real crystals is so perfect, from the perfect polish of the faces, that they are distinguished with difficulty. They may be frequently found still undergoing change. Pseudo- Pseudomorphs have been classed under four heads: morphs 1. Pscudomorplis by Alteration. Formed by a gradual change of classi- composition in a species. Of these there are two varieties: they fied. may be pseudomorphous by loss of an ingredient, or by addition of an ingredient ; change of augite to steatite is an example of the first, and of galena into anglesite is one of the second. 2. Pseudomorphs by Substitution. Those formed by the replace ment of a mineral which has been removed, or is gradually under going removal; e.g., galena takes the form of pyromorphite. 3. Pseudomorphs by Incrustation. Those formed through the incrustation of a crystal, which may be subsequently dissolved away. Often the cavity is afterwards filled by infiltration; e.g., change of fluor to quartz. 4. Pseudomorphs by Paramorphism. Those formed when a mineral passes from one dimorphous state to another ; e.g., change of aragonite to calcite. These different kinds of change are not always distinguishable. In some cases a change may take place through alteration of the surface, and then, this process ceasing, the interior may be dis solved out, leaving a pseudomorph like one of incrustation ; or a pseudomorph that appears to be a result of mere chemical altera tion may be wholly due to substitution simply. Again, changes of scapolite to a felspar, and of augite to uralite (hornblende), have been considered by Scheerer examples of paramorphism, scapolite being considered dimorphous with some felspars, and augite with hornblende. But, while such paramorphic changes undoubtedly take place with aragonite, their occurrence in these silicates which are common associates in the same rock, and must have been formed under like circumstances is hardly prob able. Where mineral bodies have taken the form of organisms, it is more a case of molecular replacement than of true pseudomorphism. Pseudomorphism should be understood, however, to consist, not simply in alteration of crystals, but in many instances of changes in beds of rock. Thus all serpentine, whether in mountain masses or in simple crystals, has been formed through a process of pseudo morphism or, iu more general language, of metamorphism of olivine and augite. The same is true of other magnesian rocks, as steatitic, talcose, and chlorite slates. The crystalline rocks often offer examples of a change similar in nature. The graphite of these rocks is probably but a metamorph of some vegetable organism. Thus the subject of metamorphism, as it bears on all crystalline rocks, and that of pseudomorphism, are but branches of one system of phenomena ; the chemistry of both is the same, and a knowledge of such changes is indispensable to a study of the older rock strata of the earth. The common change of pyrites, forming the main ingredient of the upper part of metallic lodes, to earthy red or brown iron ore, thus producing the &quot;gossan&quot; of miners, is one of many examples of these processes now in progress. Often the gossan contains dis seminated silver or gold, derived from the decomposed ores. This is a case of pseudomorphism, as truly as when a simple crystal of pyrites becomes limonite ; the mode of change and its laws are the same. Again, phosphates, vanadiates, and arseniates of lead, &c., as well as car bonates and sulphates, are among the surface species, or those that occupy the upper part of metallic lodes ; they are the results of altera tion within those depths to which atmospheric agencies penetrate. Pseudomorphs are always records of past existences, in some cases they may be the only evidence we possess of such prior existence. Figs. 238, 239 are pseudomorphs of quartz or hornstone after datholite ; the measured angles of these crystals show that the imitated crystal was datholite ; but that mineral does not now occur in crystals of either of these forms. The process of petrification of organic bodies is in reality a species of pseudomorphic Fig. 239. formation, and has been pro duced in all the above modes. External and internal casts of organic bodies are not uncom mon. In other cases the original substance has been replaced by some mineral which has preserved, not merely the external form, but even the minutest detail of internal structure, so that the different kinds of wood have been distinguished in their silici- fied trunks. The most common petrifying substances are silica and carbonate of lime. In encrinites, echinites, belemnites, and other fossils, the crystals of calc-spar often occur in very regular positions. In some varieties of petrified wood both the ligneous structure and the cleavage of the calc-spar are observable. Different from the above are mineralized bodies, in which the original structure is still retained, but their chemical nature partially changed. In these a complete series may be often traced, as from wood or peat, through the varieties of brown coal, common coal, anthracite, and graphite. Causes of Change. The causes of change are the Origin of simplest and most universal operations about us : (1) the P se &quot; d h g &quot; process of gradual alteration to which some substances are m liable on account of the presence of oxygen and carbonic acid in the atmosphere, and the reaction of substances thus formed on adjacent ingredients, aided or promoted by electrical currents or by heat; (2) the solvent power of ordinary waters, cold or hot, or of steam ; (3) reactions, in accordance with chemical principles, of the ingredients