Lastly, the seams, which are the very finest stringers {fibrae), divide
the rock, and occur sometimes frequently, sometimes rarely. From
whatever direction the vein comes, its seams always turn their heads
toward the light in the same direction. But, while the seams usually run
from one point of the compass to another immediately opposite it, as
for instance, from east to west, if hard stringers divert them, it may
happen that these very seams, which before were running from east to
west, then contrariwise proceed from west to east, and the direction of
the rocks is thus inverted. In such a case, the direction of the veins is
judged, not by the direction of the seams which occur rarely, but by those
which constantly recur.
