Agricola, Georgius, De re metallica, 1912/1950
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              <s>Lastly, the seams, which are the very finest stringers (
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              fibrae
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              ), divide
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              the rock, and occur sometimes frequently, sometimes rarely. </s>
              <s>From
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              whatever direction the vein comes, its seams always turn their heads
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              toward the light in the same direction. </s>
              <s>But, while the seams usually run
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              from one point of the compass to another immediately opposite it, as
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              for instance, from east to west, if hard stringers divert them, it may
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              happen that these very seams, which before were running from east to
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              west, then contrariwise proceed from west to east, and the direction of
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              the rocks is thus inverted. </s>
              <s>In such a case, the direction of the veins is
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              judged, not by the direction of the seams which occur rarely, but by those
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              which constantly recur.</s>
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              <s>A—SEAMS WHICH PROCEED FROM THE EAST. B—THE INVERSE.</s>
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              <s>Both veins or stringers may be solid or drusy, or barren of minerals,
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              or pervious to water. </s>
              <s>Solid veins contain no water and very little air. </s>
              <s>The
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              drusy veins rarely contain water; they often contain air. </s>
              <s>Those which
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              are barren of minerals often carry water. </s>
              <s>Solid veins and stringers con­
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              sist sometimes of hard materials, sometimes of soft, and sometimes of a
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              kind of medium between the two.</s>
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