Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/331

Rh MINERAL vEINs.] structure, partly from the abundant crumpling which most foliated rocks have undergone, whereby the continuity of the individual bands is much disturbed or entirely destroyed. The joints among foliated rocks to which the regular and parallel folia impart a marked ﬁssility resemble those among sedimentary strata. Where, however, the foliation is of a more massive kind, as in the coarser varieties of gnciss, the system of jointing approximates to that of granite or one of the more crystalline igneous rocks. IX. MINERAL Vnrxs. The ﬁssures which so abundantly traverse the crust of the earth have in many instances served as places for the deposit of mineral 111attcr quite distinct from that of the rocks through which they run. As metallic ores frequently occur among the minerals so deposited, and have been ex- tensively worked, a large amount of information has been obtained by mining operations regarding these ﬁssures, or, as they are termed, mineral veins. A general though not invariable relation exists between the nature of the minerals in the ﬁssures and that of the contiguous rocks. When the latter are calcareous, calcite usually forms a conspicuous feature in the veins; among siliceous rocks quartz is abundant. These and the other minerals are for the 1nost part well crystallized or at least largely crystalline in the veins, even when the adjoining rocks are granular or amorphous. They are termed veimstones. Since the joints, faults, and ﬁssures which have been ﬁlled with new mineral substances are commonly highly inclined or vertical, mineral veins generally run as steep wall-like bands across the rocks in which they occur. Their minerals are arranged i11 strips, which on the whole run parallel with the walls of the vein (ﬁg. 58). / / / ul abcidcgaw I-‘lo. -3S.—a, Coating of one mineral, say quartz; b, coating of a. second mineral, say ﬂuor spar; c, coating of ﬁrst mineral, or of a third, say sulphate of baryta; d, rib of ore, as copper or lead; ac, w, walls of the lode. Mineral veins become metalliferous, when among their vein-stones there occur detached crystals or particles, or branching strings a11d threads, or concretionary masses or parallel bands of native metal, or of the sulphides, oxides, or other ores of metals. The association of these various sub- stances within the two walls or cheeks of a vein is often in a double set of parallel bands, those in one—11alf of tlie vein being repeated in the other. The middle of the vein, for example, may consist of galena ﬂanked and partially mixed up with zinc-blende and pyrites. On either side of this central rib there may be a layer of some veinstone, perhaps fluor spar, then a duplicate band of another veinstone, such as barytes or quartz, and so on, to the outer edge of the vein (ﬁg. 58). In other cases, as in auriferous veins of quartz, the vein consists wholly or almost wholly of one vein-stone through which the ore is disseminated in minute grains and strings, so that the vein-stone must be extracted and crushed to obtain the metal by washing. GEOLOGY 317 Mineral veins (lodes, ral-e-veins) vary in thickness from only an inch or less up to many fathoms. Extraordinary variations may be traced even in the course of the same vein, a breadth of several feet or yards rapidly diminish- ing until the two walls nearly or quite meet, to the exclu- sion of the minerals of the vein. Similar diversities may be observed in the horizontal extent of veins, some being traceable for miles, others disappearing in a few yards. They sometimes occupy ﬁssures without any throw, but- most frequently seem to occur along lines of fault. In some cases indeed it can be shown that dislocation has taken place after some portion of the vein had been formed, but before the completion of the process. They usually send out branches, and in some mining districts do this to such an extent that it becomes hardly possible to identify the main vein among its numerous offshoots. The direction of the veins varies in different districts. Two series may often be traced,——a principal series running in one general direction, and a minor set crossing the ﬁrst at right angles or obliquely. Great differences in the rich- ness of a metallic lode may be observed as it is worked vertically and horizontally, some of these depending in a way not easily explained upon the nature of the surround- ing rock. Among the Cornish lodes, for example, some contain copper only where they traverse the Devonian slates, and lose it where they enter the granite, where tin takes its place. In the lead tracts of the north of England the metal diminishes where the veins lie in shale, and augments where they run through limestone. In some rocks, more especially in limestones, large sub- terranean cavities have been ﬁlled with vein-stones and ores. The iron mines of the English lake district, for example, lie in the Carboniferous Limestone, where tunnels and caverns anciently dissolved out of the rock by per- colating or running water have been subsequently ﬁlled up with haematite. In the lead districts of the north of England also, similar cavities have received a plentiful deposit of vein—stones and galena with its accompanying ores. Various theories have been proposed to account for the inﬁlling of mineral veins. Of these the n1ost noteworthy are—(l) the theory of lateral segregation,——which teaches. that the substances in the veins have been derived from the adjacent rocks by a process of solution and redeposit; and (2) the theory of inﬁlling from below,—according to which the minerals and ores were introduced from below dissolved i11 water or steam, or by sublimation, or by igneous fusion and injection. The fact that the nature and amount of the minerals, and especially of the ores, in a vein vary with the nature of the surrounding rocks seems to show that these rocks have had a certain inﬂuence on the precipitation of mineral matter in the ﬁssures passing through them. But that this mineral matter came chieﬂy from below appears almost certain. The phenomena of the ascent of hot water in volcanic districts afford a close analogy to what has occurred in mineral veins. It is known that at the present time various minerals, in- cluding silica, both crystalline and calcedonic, and various metallic sulphides, are being deposited in ﬁssures up which hot water rises. At the same time it is conceivable that to so1ne extent there may be a decomposition of the rocks on either side of a ﬁssure, and that a portion of the mineral matter abstracted may be laid down in another form along the walls of the ﬁssure, or, on the other hand, that the rocks on either side of the ﬁssure may be permeated for some distance by the ascending waters, and that some of the mineral substances carried up in solution may be deposited in the pores and cavities of these rocks as well as in the ﬁssure itself.