added. Repeated movements may break up the rock (country or vein matter, or both), making a rubble-filled fissure which upon the deposition of vein matter in the interstices is called a brecciated vein. A fissure may have numerous offshoots which the miner

calls "spurs" or "angles," or may consist not of a single main fissure but of many small irregular fractures filled with mineral, which is called a reticulated vein. A system or group of veins, approximately parallel in direction, but which die out at various

MINING, MINERAL AND GEOLOGICAL LAW
OLOGICAL

147

lengths and are replaced by others on either side and are connected with each other by cross veinlets or stringers (usually diagonal), is called a linked vein. This name was first used by Becker in his "Geology of the Quicksilver Deposits of the Pacific Coast" (Monograph 13, United States Geological Survey, p. 410). The diagram which he gives is reproduced in Fig. 33, p. 149, which gives the idea better than verbal descriptions.

The movements of the rock walls against each other during the disturbances accompanying the creation of the fissures often produce smoothed, striated surfaces in the vein termed "slicken

sides." Veins are not always or even usually perpendicular, although they usually approximate more or less closely to the perpendicular. The angle between the vein and the horizontal is the "dip." The angle between the vein and the perpendicular is the "hade," or sometimes the "underlie." The wall on the lower side of a dipping vein is the "foot-wall" and on the upper side the "hanging-wall." Open cavities in a vein are called vugs" ("vughs") or "druses." These are often lined with fine crystals. The line along which a lode or vein comes to the surface is called the "outcrop" ("outgoing" or "back"). The outcrop sometimes forms a ridge or reef if harder than the sur

rounding rock, and a trench if more subject to decomposition. The outcrop of veins which contain pyrites usually consists of a mass of brown and rusty matter stained with, or perhaps chiefly composed of, iron oxides formed by the weathering of such iron minerals. This is termed "gossan" or sometimes the "iron hat" or "iron cap."

The ore is called

ring ore or cockade ore from the "Ring and Silverschnur mine"
in the Harz.

(See p. 146.)

From Beck; Nature of Ore Deposits.

If the mineral filling of a fissure vein is deposited in successive layers of different minerals, or different mixtures of minerals, as is frequently the case, it is often called a banded vein or ribbon vein, and if the layers are duplicated on both walls it is said to be a symmetrical banded vein (Figs. 35, p. 150, and 36, p. 151).

If the minerals in a fissure vein or lode are markedly crystalline with the long axes projecting toward the center, a section across the

vein will have a rough resemblance to a comb, and miners sometimes call them "comby" lodes (Figs. 26, p. 144, and 35, p. 150).

A segregated vein is one in which it is believed that the filling

FIG. 34.

Replacement of a layer of shale by pyrite (black) and spongy quartz, showing characteristic small vugs with outer shells of pyrite and inner linings of quartz crystals. (See p. 147.)

has been derived from the adjacent country rock by the carrying in of mineral matter dissolved therefrom by water percolating through it into the fissure. This name is also applied to lenticular

bodies, which are limited on the strike and the dip, of quartz or other vein material which are sometimes found in igneous and metamorphic rocks (Figs. 37, p. 152, and 38, p. 153).

In Australia the word "reef" is the common name for an ordinary vein formation. Strictly speaking, the term should only be applied to a vein which projects above the surface, forming an elevated outcrop, but the Australian use seems to include all kinds of veins, underground as well as those projecting above the surface. Here also the name "saddle reef" is used, being applied to a particular form of vein which occupies curved, lenticular spaces at the highest points in anticline and the lowest points in synclines (Fig. 39, p. 153).o

FIG. 35.

Inches

Cross section of banded vein near London shaft, Mineral Point, Colo.; a, county rock; b, quartz and chalcopyrite; c, tetrahedrite; dd', quartz; 1, galena. After Ransome. (See p. 148.)

The mineral matter of the vein may be closely adherent to the walls, in which case miners graphically describe the ore as "frozen" to the country rock, or there may be a layer of clay or decomposed mineral matter between the vein contents and the wall that the miners call "gouge," "flucan," or "selvage" (Fig. 40, p. 154).

.The "ore," or valuable mineral in the vein, is seldom deposited with any approach to uniformity throughout the vein matter, but usually occurs in localized strips and irregularly connected longitudinal bodies which are believed to represent the channels through which the larger part of the vein-filling solution flowed. These are usually extremely variable as to direction, size, shape, etc., and have numerous names applied to them by the miners, such as, "shoot" (also written "chute"), "pay streak," "ore chimney,"

9 James D. Geikie, "Structural and Field Geology," p. 252.