As with other mineral substances, we find that ores of the useful metals have been sometimes thrown down in a fissure at one time and not at another, the deposit of one ore sometimes repeated, at others not. Thus there may have been a coating of a zinc ore at one time, of copper ore at another, and a covering of tin ore upon these, sometimes separated by other mineral substances, at others in deposits one above the other. Again, we find, in the successive dislocations which are sometimes seen to have effected the lines of fissures, that, while the lines of least resistance to the applied force have been chiefly through the contents of the original fissure, occasionally a new fissure has been made through portions of the adjoining rock; so that the minerals which may have been subsequently deposited in the new crack or fissure will be partly in the old line, and partly amid the newly-broken and adjacent rocks.

It would be out of place to attempt a general notice of those veins which, because they contain the ores of the useful metals, are commonly termed mineral: it will be sufficient to observe that from decomposition the upper or exposed parts of many do not show the ores in the manner they occur beneath. Thus, above veins wherein the ore from which the largest amount of copper is produced, namely, the compound of copper, iron, and sulphur known as copper pyrites, a mass of ferruginous matter is often found, known by many of our miners as gossan, and by the French miners as chapeau de fer. This is the result of a decomposition arising from exposure to atmospheric influences of various kinds, and occasionally from other influences. It is probable that the sulphur by a union with the needful oxygen became sulphuric acid, and that, this formed, the copper was attacked and removed, to be dealt with like any other solution of sulphate of copper. And beneath this gossan, or the back of the lode, as it is often termed, we observe appearances strongly reminding us of the common electrotype process for procuring copper from a similar solution. The pure metal is gathered together in chinks and cavities between the main mass of gossan and the body

of the undecomposed copper pyrites, mingling, perhaps, occasionally with the lower part of the former. Sometimes this native copper, as it is called, may retain its metallic character, but at others it becomes converted into an oxide, and this again into a carbonate by the percolation of waters containing common air and carbonic acid. The iron seems in a great measure to have been left behind, and this forms the rusty substance above mentioned. It will be readily understood that, the needful conditions obtaining, other parts of a mineral vein than the mere upper portion may become decomposed in the same manner. In fact, the changes which have been effected in the fissures containing mineral veins, the mode of throwing down a mineral substance, its subsequent removal, its reappearance or apparent transport elsewhere, the pseudomorphous filling up of crystalline cavities, the substitution of one substance for another, the evident alterations produced by new fissures, particularly when these have traversed the original fissures at right angles, the differences of contents of fissures when they take different directions traversing the same country and association of rocks, are objects of high interest; and though no doubt best studied in mining countries, where opportunities are so numerous, and veins are so extensively laid open, a voyager, with some little time on a favourable portion of coast, may often nevertheless acquire much information on these heads. To do so, and procure illustrative specimens and a highly valuable collection, interesting in many respects, it is not necessary that the vein should be one containing the ores of the useful metals-the contents of those fissures and dislocations, termed common faults, are often in a scientific point of view equally important.

The cavities of many igneous rocks, and indeed holes and cavities in all, afford good places wherein to search for minerals. In some parts of Iceland (and also in the north of Ireland) it has been observed that crystallized minerals occur most abundantly in the cavities of the rocks which

have suffered decomposition, the minerals having probably crystallized out of a solution of some of the constituents of the rock. In Iceland they are found sparingly in the cavities near the summit of the mountain Bulandstind, consisting of igneous rock, but very abundantly in those at its base (Sartorius v. Waltershausen, Skizze von Island, page 91). The traveller is recommended to observe the manner in which minerals are distributed at different heights in similar situations.

Minerals of the zeolite family are very common in the vesicular cavities of some igneous rocks; and at one time, before their mode of occurrence was properly understood, the quantity of water found in many of them was thought to militate against the igneous origin of the containing rock. They form an interesting class of minerals, and, opportunities offering, should always be collected. They come under the head of hydrated aluminous silicates, with potash, soda, lime, and their isomorphous substances. The great proportion of them contain from 8 to 18 per cent. of water in combination. In the same kind of vesicles, siliceous deposits in the form of agates are not uncommon. In these and in cavities of various rocks, even those of aqueous origin, such, for example, as the dolomitic rocks of the new red sandstone series, in Somerset and Gloucestershire, the agate linings of the cavities have continued only for certain distances, after which the elements of other minerals have entered the hollows, and various crystallized substances have been the result. Cavities, therefore, in all rocks may be searched. With respect to the successive siliceous coatings forming agates, while some kinds of coatings show an adjustment to the walls of the cavity, others have accumulated in flat layers, generally considered to have been formed horizontally. Sometimes part of a cavity has been filled in one way, and the remaining portion in the other. Occasionally, from cavities left after a part of the hollow has been filled horizontally, stalactites of the matter of the agate have descended from above, as in

the annexed figure. It is desirable always to ascertain how far such flat layers correspond with the present horizon; and, if the vesicles or hollows are almondshaped (elongated more in one direction than another), how far these are constant in the same direction, thus pointing out that in which the molten viscous rock moved.

Many nodules in rocks, those which have clearly not been formed as gravel or boulders by attrition, afford examples of the aggregation of similar matter from a mass, such as one of clay, in which that matter has once been more generally diffused. In this way we have siliceous nodules, calcareous nodules, and those valuable nodules, the clay ironstones. These last are fundamentally carbonates of iron, with a variable addition of the matter of the mud or silt amid which the carbonate of iron has once been more generally diffused. In many such nodules there has been a shrinking from the centre to

the sides, causing cracks, that have been variously filled with mineral matter, as in the subjoined figure. Occasionally in the cracks so formed, and not quite filled up, various mineral

substances are obtained well crystallized. It may be here observed, as regards the metallic titanium frequently discovered in iron furnaces when blown out, that we have found the oxide of titanium crystallized in the cavities of clay ironstones. Taken as a whole, the observer will do well to look into any cavities or cracks he may discover in rocks, even in the hollows among organic remains, for various mineral substances. Many a crystallized body will thus be frequently found, and the replacement of one substance by another be well seen.

Not only in cracks or hollows, but in the body of the rocks themselves, minerals may be observed well crystal

lized. This is well seen in the class of igneous rocks known as porphyries-that is, where a general paste or base, confusedly crystalline, compact or earthy, may happen to contain isolated and well-formed minerals of different kinds. From experiments in the laboratory, and the results of metallurgical and chemical operations carried on upon the large scale, we know that this isolation of crystals may readily be obtained. In the igneous dykes, as they are termed that is, where igneous matter in fusion has been forced up, filling cracks formed in the rocks which they traverse-we sometimes see good illustrations of the mode in which isolated mineral crystals may be produced. Let us take as an example some of the granitic dykes known as elvans by the miners of Cornwall, and let the annexed figure represent a section of one of them, a a being some

schistose rock broken through or fractured (it may be any rock previously consolidated; granite is thus frequently fractured, and the fissure filled by an elvan). We find that, while the central portion d may be a granite, the parts c c are porphyritic, and b b some compact rock. Upon investigation, we see that all parts are chemically the same, and that these various characters are due to differences in cooling. The central portion retained its heat longest, while the portions adjoining the bounding and fractured rocks were more speedily cooled. In such porphyries various minerals are found, those of the felspar family being very common. Such results from differences of cooling can be imitated artificially with substances under our control. In this way crystals of silicate of lime may be beautifully obtained, isolated in transparent glass.

Whole mountain masses are occasionally composed of