Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/136

126 few fathoms below the surface, it assumes more determinate marks of being a fissure.

The contents or load of the generality of veins, if at all attached to their sides, are for the most part easily separated. A dark ochreous crust occasionally covers one or both sides of the vein, technically called the walls of the load; and when these, or at least one of them, is regular and determinable, they are more encouraging to the miner than when rough and uneven. A thin coating of flucan is found on one, or occasionally on the other wall, or sometimes on both walls of a metalliferous vein, as described by pl. 6. fig. 4. but it is said that this coating or vein of flucan is most commonly found on that which, in regard to the underlie of the vein, may be called the upper wall.

It has been already noticed, in speaking of the denominations of metalliferous veins, that there is a great diversity in their loads or contents, and that the same vein exhibits so little uniformity in that respect, at different depths, as to assume various characters. Near the surface the load consists for the most part of a sort of ochreous rubble, probably the debris of the neighbouring country; beneath which, though rarely nearer grass than 20 or 30 fathoms, are found some metalliferous indications. These, if gossan be the prevailing substance, consist of the ores of tin or copper. But iron pyrites, blende, fluate of lime, quartz, and sometimes chlorite, or flucan, frequently prevail for many fathoms, occasionally mixed with portions of the country through which the vein passes, though frequently with but slight traces either of tin or copper. A vein is sometimes found to consist of little else than a bed of hard and