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and promotes the creation of open spaces. The dislocating force, however, continually crowds the projecting surfaces together, and thus a space already partly filled with mineral deposit may be closed, or an open space may be filled with the detritus of friction. But the space finally left open facilitates communication with the deep region, from which it is filled. According to this conception, the vein-sheet must not be regarded (as is too often done) as a uniform plate of ore. On the contrary, it consists of several portions of very unequal value. The most valuable, doubtless, is the cavity-filling which forms the bonanza proper. In another portion the mineral solutions have been forced to penetrate the country-rock, and impregnate it with ore. A third portion remained altogether impenetrable to the solutions, and represents barren ground. These three kinds of ground may evidently show, at least in the same district, a certain regularity of relation ; and, of course, it is most important to determine for a given district some law of distribution of tlae rich ore-bodies. In certain instances some knowledge of this.distribution has been, in fact, successfully acquired for a given vein, before it had been exhausted by mining. In many other cases we cannot establish the law, even afterwards, because the most necessary records were not made during the exploitation. On the whole, we must confess that our knowledge of the laws of bonanzas is nothing to be proud of. In this respect the work of Professor Moissenet may be consulted. Obviously, in all such investigations, the question of the origin of the fissure must be separated from that of its filling. The former can be answered only upon the broad basis of a knowledge of the stratigraphic relations of the whole vicinity, and with reference chiefly to the physical properties of the rocks, while in the latter their chemical properties come to the front. As a rule, however, the country-rock of an ore-vein is more or less altered, not only by decomposition, but also by subsequent solidification, thus rendering much more difficult the comparison with conditions existing far from the vein. This alteration of the country-rock is universally ascribed to the mineral solutions which deposited the ore; and it is not improbable that a close study of it might enable us to draw conclusions as to the nature of these solutions. Unfortunately, petrography is still confined mainly to fresh, typical rocks, and the study of the decomposed countryrock of ore-veins has not been cultivated so much as could be wished. All veins which exhibit friction-phenomena, such as crushed country-rock, slickensides, and striations, are structurally fault-fissures. Such a vein may be conceived, therefore, as the boun-dary-surface of a mass which has undergone movement. The vein-phenomena of the Hartz especially support this conception. Some vein-fissures are confined to a given rock, and do not extend into the adjacent rock. These cannot be ascribed to structural dislocation, but must rather be considered as caused by changes of volume in the immediate formation. They are often called fissures of contraction. The most striking example which has come to my knowledge is shown in Fig. 36, which is from the gold-district of Beresov, in the Ural Mountains. Palteozoic slates are there traversed by a number of granite veins, 66ft. to 131 ft. thick, and striking chiefly north and south; and each of these granite veins is again traversed by east and west gold-quartz veins, which at the borders of the granite either become barren or cease altogether. Near the Beresov is the Pysminsk district, in which the granite veins are replaced by diorite and serpentine; but, strange to say, the gold-quartz veins occupy in these rocks the same position as in the peculiar Beresov granite, locally called beresite. Judging from Beresov alone, one might suspect the veins to have been filled from the granite; but the occurrence in Pysminsk suggests caution. Finally, the veins of the well-known very deep mines of Przibram might be ascribed to the contraction of the eruptive dykes in which they occur (although they depart here and there into the stratified rocks) ; but we cannot dream of deriving their metallic filling from the dykes. The Commission already mentioned, established to test the applicability of the lateral-secretion theory to Przibram conditions, found the material of the dykes to be the same in depth as in the upper zones. The largest amount of metallic contents attributed to the diorite dykes would account for a portion only of the thickness of ore in the veins. The greater part must certainly be regarded as of deep origin; and it is more convenient to treat the entire metallic contents of the veins as derived from greater depths. Granting, then, that the vein-spaces at Beresov were formed by the contraction of the granite dykes, the vein-filling must be ascribed, like that of other deposits, to metallic solutions ascending from the deep region. With regard to structure, the fillings of ore-veins very often exhibit distinct crustification, and sometimes even a symmetric succession of crusts from both walls to the central druse. But this phenomenon often retires into the background ; crustification becomes indistinct or disappears, as is frequently the case in gold-quartz and other metamorphosed veins, in which its last traces appear in the crystal-tips of the central druse and the occasional indication of fibres perpendicular to the walls. Sometimes one part of a vein shows distinctly a crustification which in other parts is discerned with difficulty, or is even wholly absent. Fig. 53 represents a specimen from the Drei Prinzen Spat vein in the eighth level of the Churprinz Friedrich August Mine at Freiberg. It is interesting also by reason of the two dislocations which it exhibits. The oldest vein (a) of quartz, with irregularly disseminated galena and zinc-blende, is traversed and faulted by a second, very clearly crustified, vein, the filling of which consists of hundreds of very thin alternate crusts of (b) fluorite and quartz and (c) barite, symmetrically arranged on both sides, with a central druse (d) containing a gray earthy mass. A quartz seam (ef) then faults both veins. The manager of the mine assured me that the specimen occurred in the vertical position in which it was sketched. (In order to be certain at all times on this important point, it is advisable, before removing a specimen from its natural position, to mark it in colour with a vertical arrow, head downward.) Very often the crustification of a vein-formed ore-deposit is only to be traced in the appearance of the whole, since each of many irregular veinlets may represent separate mineral crusts. Accurate