Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/460

Rh 442 MINING [PROSPECTING. Faults. deposits of the Ulverston district (fig. 5 1 ) and the brown haematite deposits (churns) of the Forest of Dean, which FIG. 5. Vertical Section, Roanhead Mine. A, Mountain Limestone ; B, red haematite ; C, sand and clay; D, gravel. Scale ao * 00. occupy irregular cavities in the Mountain Limestone. These may have been formed by the percolation of water bring ing down iron in solution from overlying Triassic rocks. Other examples of masses are the calamine deposits of Altenberg 2 (fig. 6), Sardinia, and Lombardy, the iron ore deposits in Missouri, such as Iron Mountain and Pilot Knob, the huge &quot; necks &quot; or &quot; pipes &quot; of dia- mantiferous rock in South m upright FJQ 6 _ berg. B, slate ; d, dolomite c &amp;gt; calamine ; L, clay. pipes rock in Africa, and the granite decomposed in situ worked for china clay in Cornwall. Under this head also are included by most authors the so-called &quot;stockworks&quot; or &quot;reticulated masses,&quot; names applied to masses of sedimentary or igneous rock which are penetrated by so many little mineral veins as to make the whole worth excavating. It must be understood that we cannot expect nature to make distinct lines of demarcation between the different kinds of deposits. Though we may be able to see clearly that a seam of coal is contemporaneous with the enclosing rocks, and that a vein intersecting beds of shale and sandstone was formed subsequently, cases frequently occur where the origin of the mineral is uncertain. For example, we have the lead-bearing sandstone ofMechernichand the silver-bearing sandstone of Utah. The grains of sand are of sedimentary origin ; but opinions differ as to whether the lead and silver respectively were deposited with the sand or were introduced subsequently by solutions percolating through the beds. In the case of the well-known bed of Cleveland ironstone, Dr Sorby considers that the iron was &quot;derived partly from mechanical deposition and partly from subsequent chemical replacement of the originally deposited carbonate of lime.&quot; 3 Furthermore, a bed may be so folded and contracted as to lose its original sheet-like form in places and assume the shape of an irregular mass. This may happen even with a coal seam. 4 All kinds of deposits are subject not only to irregularities of origin dependent upon their mode of formation but also to dislo cations or shiftings known as faults, heaves, or throws. We will take the case of a bed (fig. 7). AB is a seam which ends off suddenly at B, whilst the continuation is found at a lower level at CD. The bed was evidently once continuous ; but a fracture took place along the line XY followed by a displacement. As a rule .. Fig. 7. Fig. 8. the portion of rock on the hanging-wall side of a fault appears to have slid downwards, but occasionally this is not so, and we have a reversed fault (fig. 8). It is very evident, in some cases, that the motion took place, not along the line of greatest dip, but in a dia- 1 Fr. Moritz Wolff, &quot; Beschreibung der Rotheisenerzlagerstatten von West Cumberland mid North Lancashire,&quot; Stahl und Eisen, 2 Jahrgang, No. 12. plate vi. 2 M. Braun, Zeitschr. d. d. geol. Gesellsch., 1857, vol. ix. ; and A. von Groddeck, Die Lehre von den Lagerstdtten der Erze, Leipsic, 1879, p. 242. 3 Quart. Jour. Geol. Soc., vol. xxxv. (1879), p. 85, Anniversary Address of the President. 4 J. Gallon, Lectures on Mining, vol. i. p. 63, and Atlas, plate viii. fig. 44. gonal direction, causing a displacement sideways as well as down wards. Nevertheless, where beds or veins are not horizontal, a mere shift along the line of dip is sufficient to cause an apparent heave sideways. This will be understood from fig. 9. Let AB and CD represent two portions of a lode dislo cated by the fault EF. The point B corresponded originally with B, and the dislocation was caused by a simple sliding of B along the line of dip BB. An instance of the complication caused by a succession of faults is shown in fig. 10. 5 2. Prospecting, or Search for Mineral. The object of the pro spector is to discover valuable deposits of mineral. This search is beset with many difficulties : the outcrops of FlG. 10. Vertical Section, Penhalls Mine, Cornwall. G-, G, G, small veins called gossans in the St Agnes district. mineral deposits are frequently hidden by soil; the nature of the deposit itself is generally entirely changed near the surface; and, in addition to this, the explorer may have to pursue his work in trackless forests far away from any settlements. The prospector seeks for natural sections of the rocks, such as occur in cliffs or in river valleys and their tributary gullies and gorges ; he examines the materials constituting the river-beds, often digging up and washing portions in a pan, in order to ascertain whether they contain traces of the heavy ores or metals. If, while prospecting in a valley, he discovers stones that have the appearance of having once belonged to veins, he endeavours to trace them to their source, and is perhaps rewarded by finding similar frag ments, but less water-worn, as he goes up the stream ; further on he may come upon large blocks of veinstuff lying about, and finally find the vein itself laid bare in a gorge, or at the bottom of a brook, or possibly projecting above the soil in the form of huge crags of quartz. Thus at the Great Western quicksilver mine in California the outcrop of the vein appears as a dike over 100 feet wide, and having precipitous sides in places 75 feet high. 6 Loose pieces of veinstuff found lying about are known in Cornwall as shoad-stones, and sheading is the term given to the process of tracking them to the parent lode. The upper portion of a deposit is frequently much altered by atmospheric agencies, and bears little resemblance to the undecom- posed bed or vein which will eventually be met with at a greater or lesser depth. The principal difference consists in the change of sulphides into oxides or oxidized compounds. Thus iron pyrites, which is such a common constituent of mineral veins, is converted into hydrated oxide of iron, and a vein originally consisting largely of iron pyrites and quartz now becomes a cindery mixture of quartz and ochre, known in Cornwall as gossan. This gossan, or iron hat, may often furnish important indications concerning the nature of the lode itself, because such minerals as pyromorphite or cerus- site point to the existence of galena, whilst melaconite, cuprite, malachite, and azurite are the forerunners of chalcopyrite or copper glance. The gossan itself may contain a sufficient quantity of valu able ores to be worth working. The seams containing native sulphur in Sicily often show no trace of that element immediately at the surface, as the sulphur-bear ing limestone weathers into a soft white granular or pulverulent 5 J. W. Pike, &quot; On some remarkable heaves or throws in Penhalls Mine,&quot; Quart. Jour. Geol. Soc., vol. xxii. p. 537. 6 Luther Wagoner, The Geology of the Quicksilver Mines of Cali fornia,&quot; Engineering and Mining Journal, vol. xxxiv. p. 334.