134 
AMERICAN GEOLOGY. 
STRUCTURE OF MINERAL VEINS, TOGETHER WITH THE 
KINDS OF VEINSTONE WHICH CONTAIN THE ORES OR 
METALS. 
§ 88. A metallic vein is originally a fissure of an unknown depth 
and length, hounded by two walls of rocks, whose composition 
differs from the contents of the vein fissures, and from which 
the latter easily cleaves. When the fissure or vein is not verti¬ 
cal, the wall against which the vein rests is called the foot wall, 
and the other the hanging wall. These walls are sometimes 
called the floor and roof of the vein. 
The structure of a vein is simple, and we have no occasion 
to multiply varieties. A dyke is the simplest and least compli¬ 
cated of that class of veins. It is a fissure filled with stony 
matter, with specks of sulphuret of iron disseminated in small 
quantities through its substance. It seems to have been formed 
and filled at one time. A mineral vein consists of stony mat¬ 
ter and the oxide and sulphurets of the metals, and sometimes 
with pure metals; as the native copper and silver of lake Supe¬ 
rior, and the native gold and copper of North Carolina. It is 
necessary to learn the character of the veinstone, before an 
opinion can be formed of the probable value of the vein. If 
the veinstone is solid and compact, the judgment will be unfa¬ 
vorable to its productiveness. If it breaks easily, or if it is 
porous and seamy, the judgment will be favorable for its pro¬ 
ductiveness. In a solid veinstone or gangue containing dissemi¬ 
nated metal in isolated particles, and which do not run together 
as it were, the encouragement for a valuable vein is small, as 
long as it continues in this condition; and a vein of this des¬ 
cription rarely assumes a favorable character. If, however, 
the veinstone is striped vertically with metal, or if the masses 
are elongated and run together, the prospect is favorable; the 
metal is assuming a vein-like character. The metal may 
possess only the disseminated character at the top of the vein, 
and may pass into the veiny character below. The beginning 
