72

BOOK III.

Lastly, the seams, which are the very finest stringers (fibrae), dividethe rock, and occur sometimes frequently, sometimes rarely. Fromwhatever direction the vein comes, its seams always turn their headstoward the light in the same direction. But, while the seams usually runfrom one point of the compass to another immediately opposite it, asfor instance, from east to west, if hard stringers divert them, it mayhappen that these very seams, which before were running from east towest, then contrariwise proceed from west to east, and the direction ofthe rocks is thus inverted. In such a case, the direction of the veins isjudged, not by the direction of the seams which occur rarely, but by thosewhich constantly recur.

A—Seams which proceed from the east. B—The inverse

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Both veins or stringers may be solid or drusy, or barren of minerals,or pervious to water. Solid veins contain no water and very little air. Thedrusy veins rarely contain water ; they often contain air. Those whichare barren of minerals often carry water. Solid veins and stringers con-sist sometimes of hard materials, sometimes of soft, and sometimes of akind of medium between the two.