Mining

And others are found in veins,' that is, in cracks, crevices, clefts, caves, pipes, holes in rocks which were made and consolidated ages before the veins were formed.

These cracks have been caused by subterranean disturbances which have split the solid rocks often for many miles in a nearly straight line, sometimes raising one side (or lowering the other)

and sometimes leaving an opening between the two sides of from a few inches up to more than 100 feet in width (see Fig. 29). Such a fracture of the strata, when accompanied by displacement, is

[graphic][merged small][merged small][merged small]

FIG. 29.-Vein in fault.
A, vein; b, limestone
rocks; c, sandstone.

commonly called a fault."

When a fault takes place in hard rock, an open crack is often left; but when the fault is in soft rocks, such as the ordinary coal measures, shales, and binds, no

1 In South Wales and in those numerous countries abroad where Welsh miners are engaged, the word "vein " is applied to stratified seams of coal as well as to mineral veins.

open crack is left, the strata on each side being squeezed together (see Fig. 30). Sometimes a crack may be opened in rocks without raising one side above the other.

Holes are sometimes made in rocks (especially limestone rocks) by the action. of water charged with acids, or by abrasion of gravel-charged streams. These holes are sometimes wide chasms, and sometimes narrow clefts, and sometimes like a pipe; in many cases the large chasms are connected by narrow clefts forming a chain

miles in length.

These cracks, caves,

SECTION
ACROSS

WALL

QUARTZ IRON LEAD ZING COPPER SILVER

FOOT

GOLD

VEIN

chasms, and pipes have been filled up FIG. 31.-Section of gold-vein. with a mineral deposit containing vein-stuff

(ie. valueless minerals) and ore (i.e. valuable mineral) in constantly varying proportions (see Figs. 31 to 33).

Where the veins are formed in cracks caused by violent

FIG. 32.-Silver-lead, Clausthal, Germany: from a photograph by the author.
a, lead ore containing silver, as seen in a section of the vein; b, vein-stuff or
spar (waste). The width of the vein here shown is 3 feet 9 inches.

disruption of hard rocks, they reach down to great depths, and in most cases the bottom of the vein has never yet been proved, and it may be beyond the reach of the miner (the deepest mine has not yet explored a vein for a vertical

height of 5000 feet, though a borehole has reached nearly 6000 feet). There is no reason to suppose that, within such depths as are accessible to human beings, these veins will be either richer or poorer in valuable ore as the mines get deeper.

But as regards the veins which are found in water-made caves, the bottom of these is reached when the bottom of the cave is explored, and cannot go

FIG. 33.-Water-worn cavity and mineral. a, limestone with water-worn cavities; b, mineral vein.

deeper than the stratum which is capable of being formed into caves by aqueous action.

Thus if iron ore (hæmatite) were found in water-made caves of the mountain (Carboniferous) limestone, it would be vain to explore the shales or sandstones underlying the limestone for a continuation of these deposits of ore.

The following list shows some of the minerals which are now mined in veins :

Sometimes minerals are found in volcanic craters and in pits or channels like huge shafts (see Fig. 34) formed by volcanic agency. Such, perhaps, are the Mount Morgan Gold-mine of Queensland, the Kimberley Diamond-mine of South Africa, the sulphur-mines of Sicily.

FIG. 34. Section of volcanic shaft. a, country rock; b, volcanic shaft or

The following table is interesting, as showing the tonnage and value of the minerals got annually in Great Britain. It has been comvein, containing piled from the Government statistics:

mineral.