Agricola, Georgius, De re metallica, 1912/1950

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              for a distance of several fathoms, then from the upper part of tunnels or even
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              drifts that have already been driven, other drifts are driven again
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              and again until that part of the vein is reached which does not yield metal.
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              </s>
              <s>The timbering of these openings is done as follows: stulls are set at
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              intervals into hitches in the hanging and footwall, and upon them
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              smooth poles are laid continuously; and that they may be able to
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              bear the weight, the stulls are generally a foot and a half thick. </s>
              <s>After the
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              ore has been taken out and the mining of the vein is being done elsewhere,
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              the rock then broken, especially if it cannot be taken away without great
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              difficulty, is thrown into these openings among the timber, and the carriers
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              of the ore are saved toil, and the owners save half the expense. </s>
              <s>This then,
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              generally speaking, is the method by which everything relating to the
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              timbering of shafts, tunnels, and drifts is carried out.</s>
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              <s>All that I have hitherto written is in part peculiar to
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              venae profundae,
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              and in part common to all kinds of veins; of what follows, part is specially
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              applicable to
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              venae dilatatae,
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              part to
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              venae cumulatae.
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              </s>
              <s> But first I will
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              describe how
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              venae dilatatae
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              should be mined. </s>
              <s>Where torrents, rivers, or
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              streams have by inundations washed away part of the slope of a mountain or
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              a hill, and have disclosed a
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              vena dilatata,
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              a tunnel should be driven first straight
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              and narrow, and then wider, for nearly all the vein should be hewn away; and
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              when this tunnel has been driven further, a shaft which supplies air should be
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              sunk in the mountain or hill, and through it from time to time the ore, earth,
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              and rock can be drawn up at less expense than if they be drawn out through the
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              very great length of the tunnel; and even in those places to which the tunnel
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              does not yet reach, miners dig shafts in order to open a
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              vena dilatata
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              which
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              they conjecture must lie beneath the soil. </s>
              <s>In this way, when the upper
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              layers are removed, they dig through rock sometimes of one kind and colour,
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              sometimes of one kind but different colours, sometimes of different kinds but
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              of one colour, and, lastly, of different kinds and different colours. </s>
              <s>The thickness
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              of rock, both of each single stratum and of all combined, is uncertain, for
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              the whole of the strata are in some places twenty fathoms deep, in others
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              more than fifty; individual strata are in some places half a foot thick; in others,
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              one, two, or more feet; in others, one, two, three, or more fathoms. </s>
              <s>For
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              example, in those districts which lie at the foot of the Harz mountains,
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              there are many different coloured strata, covering a copper
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              vena dilatata.
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              When the soil has been stripped, first of all is disclosed a stratum which
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              is red, but of a dull shade and of a thickness of twenty, thirty, or five and
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              thirty fathoms. </s>
              <s>Then there is another stratum, also red, but of a light
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              shade, which has usually a thickness of about two fathoms. </s>
              <s>Beneath this is a
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              stratum of ash-coloured clay nearly a fathom thick, which, although it is
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              not metalliferous, is reckoned a vein. </s>
              <s>Then follows a third stratum,
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              which is ashy, and about three fathoms thick. </s>
              <s>Beneath this lies a vein
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              of ashes to the thickness of five fathoms, and these ashes are mixed with
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              rock of the same colour. </s>
              <s>Joined to the last, and underneath, comes a
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              stratum, the fourth in number, dark in colour and a foot thick. </s>
              <s>Under this
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              comes the fifth stratum, of a pale or yellowish colour, two feet thick; under-</s>
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