Page:A Treatise on Geology, volume 2.djvu/190

 176 silver ore, with sometimes a little orpiment; and rarely a little copper nickel, glance cobalt, native silver, lead glance, iron pyrites, and sparry ironstone. The veinstones are heavy spar, green fluor, calcareous spar, and a little brown spar. Occurs in the intersections or in the middle of veins.

(The distinction of age between this and the last system is obscure.)

The seventh deposit consists of red ironstone, containing also a little iron glance, quartz, and heavy spar. Occurs in the upper parts of veins.

The eighth deposit contains copper pyrites, mountain green, malachite, red and brown iron ochre; with veinstones of quartz and fluor. It is of small importance.

In the valuable lead mines of Aldstone Moor, cases of intersection so complicated as those of Cornwall and other tracts of primary strata seldom or never occur. The main facts are the general east and west direction (by compass) of the lead veins, and the intersection of these by cross courses which range, like these in Cornwall, mostly west of north and south of east. Their "throw" is sometimes very great. The underlie of the veins is seldom considerable; and being mostly in the same direction in each mining field, intersections of the veins are not commonly met with. The cross courses are, as in Cornwall, commonly wider than the veins, and seldom produce any thing valuable. The veinstones are quartz, fluor, carbonate of lime, sulphate of barytes, &c.

That veins are enriched near the places where they are intersected by cross courses, is an opinion common in Cornwall, and for which good evidence appears: sometimes this happens only on one side of the cross course, as at Huel Creber mine, near Tavistock. Reciprocally, the cross courses are productive near the places where they cut the veins. When veins cross one another, it is supposed that the intersections are seldom enriched if the veins differ much in underlie.

Slides often contain ore, in the part between the