One or more of these form veins of considerable thickness, or beds of irregular shape, at Ilmenau, Ilfeld, Kleinlinden, Warwickshire, &c. 103. RED ZINC-ORE. ROTHZINKERZ. (Germ.) MINERAI ROUGE DE ZINC (FRANKLINITE). (Fr.) Combined with Franklinite forms a bed of very considerable thickness at Franklin, in New Jersey. 104. GALENA. BLEIGLANZ. (Germ.) GALÈNE. (Fr.) Usually associated with blende and sulphurets; forms veins of considerable extent and thickness, and occurs otherwise in separate beds. 105. ANTIMONY-GLANCE. ANTIMONGLANZ. (Germ.) Forms veins of considerable thickness, e. g. at Magurka, in Hungary. Usually associated with other sulphurets; occurs in separate formations of considerable thickness. 107. MARCASITE, or HYDROUS PYRITES. MARKASIT oder WASSERKIES. (Germ.) MARCASSITE. (Fr.) Forms subordinate layers imbedded in other rocks, e. g. in the Browncoal formation at Littmitz, in Bohemia. 108. PYRITES. SCHWEFELKIES, PYRIT. (Germ.) Usually associated with some chalcopyrite; forms beds, veins, or irregular masses of considerable size, e.g. at Domokos in Transylvania, Rio Tinto in Spain, Schmöllnitz in Hungary, Goslar at the Hartz, Fahlun in Sweden, Agordo in the Alps. 109. CINNABAR. ZINNOBER. (Germ.) CINABRE. (Fr.) Occurs but rarely in beds of considerable size or thickness, e.g. at Almaden in Spain, Idria, California. 110. SULPHUR. SCHWEFEL. (Germ.) Forms rounded concretions and layers, in marl formations, e.g. at Radoboj in Croatia, Sicily, Perticara in Umbria. PART III. OBSERVATIONS ON THE PROCESSES OF ROCK FORMATION IN NATURE. THE NATURAL PROCESSES by which rocks have been formed and are still in course of formation are partly indicated in the foregoing pages. The following are those known to us from actual observation: 1. CONSOLIDATION OF SUBSTANCES FROM A STATE OF IGNEOUS FUSION BY PROCESS OF COOLING. This is the process which all lavas undergo, and by which, probably, all igneous rocks have been formed. We must assume that a first crust of the earth was likewise so formed, but we cannot with certainty point to any of the rocks remaining to us at the present day as representing this primeval formation. 2. DEPOSIT OF SUBSTANCES FROM A STATE OF SUSPENSION IN WATER, AND OF SUBSTANCES FALLEN THROUGH THE AIR. Thus are formed the sedimentary rocks, under which general designation every kind of deposit is included. They may be divided as follows: (a) Mechanical deposits (actual sediments).-To this class belong deposits of mud, sand, and pebbles of every kind, which by process of condensation and cementation produce argillaceous shale, clay-slate, limestone, sandstone, conglomerate, and other similar rocks. From the atmosphere are deposited particles of dust and sand. These are frequently held in a state of suspension for a considerable time, and transported by the wind to great distances. Volcanoes vomit detached substances or fine particles of dust, which with the aid of water form volcanic tufas of various kinds. (b) Chemical precipitates from aqueous solutions.— By chemical agency many kinds of deposit are formed. For instance, calc-tuff, siliceous tuff, bog iron-ore, incrustations of salt, and many mineral formations in clefts and cavities of rocks. The crystalline particles of ice which fall from the air in the form of snow may be considered as a chemical precipitate. Snow, as we have seen, forms the névé and glaciers of high mountain regions. (c) Zoogenic deposits are products of animal agency. Their massive accumulation is partly a mechanical process. Thus we have rocks formed entirely of siliceous infusoria, also the chalks, banks of shells, coral-reefs, guano and coprolite beds, &c. From the condensation of these rocks, hornstone, lydian-stone, limestone, &c., may have resulted. (d) Phytogenic deposits are such as consist chiefly of vegetable substances; these have either grown in situ, or have been washed together. From these deposits, by process of consolidation and subsequent conversion, the different coal formations have resulted. The above-mentioned processes of rock-formation are those which admit of direct observation. There are others at whose nature we only arrive by reasoning from the results. Such are: 3. METAMORPHOSIS, OR TRANSMUTATION OF PREVIOUSLY EXISTING ROCKS. This is a process constantly at work-it has even begun to affect most of the distinctly sedimentary rocks. Few of these but have undergone some change. Thus the changes from argillaceous mud to shale and then to clay-slate, from sand to sandstone, from loose stones to conglomerate, from calcareous silt to limestone, from peat-moss to browncoal, or ordinary black coal, &c., are, properly speaking, all cases of metamorphosis, although the rocks we have just named are not usually termed metamorphic. That term is reserved for the further stages of transmutation, where the change is so complete that the first state of the rock can no longer be easily or with certainty recognised by mere observa tion. The genuine metamorphic rocks are mica-schist, gneiss, and the other crystalline schists, whose identity with their originals can only be proved by deduction from a variety of collateral circumstances. The foregoing are the only processes of rock-formation known to us by observation, or which can be ascertained by deduction from known facts. These processes are, however, undoubted and indisputable, and our chief difficulty consists in determining in each instance to which mode of formation a rock owes its origin. Here many difficulties and justifiable doubts present themselves. Let us therefore attempt the application of these experiences and their consequences to the several groups of rocks which we have described in the preceding pages. No unprejudiced observer of geological phenomena can doubt that those which we have classed and named as igneous rocks were once in a fluid or viscous state, and that whilst in that state they broke through pre-existing rocks, overflowed them, and afterwards consolidated. Ample proofs of these operations of nature are found in the relation of the bedding of the igneous to those of their surrounding rocks, the disturbances which they have frequently (but not invariably) caused in the rocks broken through, the fragments of the latter which they enclose, and the veins or branches which they have thrust into those adjoining. These general conditions established, there still remain many special phenomena of formation to be explained and accounted for, which we propose briefly to consider in this place. The great mutual resemblance of all igneous rocks both chemically and mineralogically bespeaks a like process of formation for all, i.e. they were all forced upwards from the interior towards the surface of the earth in a |