Gold-bearing veins may be described as comparatively thin sheets traversing what are called country rocks, which were formed earlier than the veins themselves; and occupy crevices formed by fracture of the inclosing rocks, or have been formed along the lines of junction of such rocks by changes in those adjacent. It is this origin at a later date than that of the rock formation which constitutes the essential difference between vein a bed.
The above description includes all veins of porphyry or other intruded rocks, such as granite or basalt, as well as those containing the precious minerals. Veins filled with porphyry or similar rocks or other kind are usually dikes. The term gold vein is more properly restricted to those which are more or less filled with free gold and/or gold bearing minerals, whether in workable quantities or otherwise. Building stone should be excluded from this consideration, as many valuable varieties (besides those found in beds) are the product of dikes. In the United States the term vein, lode or ledge are used indiscriminately in different localities, but all have the same meaning, while in Australia and South Africa the term reef is often applied to mineral veins as well as to bedded deposits like gold bearing conglomerates. Basically the term lode is applicable to any zone or belt or mineralization rock, lying within boundaries clearly separating it from the neighboring rocks.
Like beds, veins have dip, strike and walls. As a usual thing the dip of true veins is apt to be steeper than that of the majority of beds, though such distinction is not absolutely necessary. The dip is measured from a horizontal line as in the case of beds and expressed in degrees. If the vein is vertical the dip is 90o. in some cases the dip may be so flat that when the vein is exposed by the wearing away of hanging wall it may appear almost like a bed. Such cases are often called blanket-veins, but a very slight examination will show in most cases their true slip.
Owing to their different origin, the strike of true veins is more likely to have a uniform direction than in beds, as the inclosing rocks have not usually been subjected to so much movement since the formation of the fissure as has been suffered by the strata containing the beds; or if movement has taken place, the weakness of the fissure has directed the motion into that plane and simply caused a reopening of the fissure, and not infrequently a refilling of it with a different class of gold deposit. taking into consideration the great amount of motion of the walls of the lode as shown by their shattered condition, that on a reopening of the fissure the contents of the lode were concentrated with gold values.
The outcrop of a gold vein is called cropping and is the portion of the vein exposed on the surface. It does not follow that the visible outcrop corresponds with the true strike of the lode. This can occur when the vein is vertical, or outcrops in a level plain, when it would shoe a comparatively straight line and the true course, whatever the dip might be. It is not often that such a case occurs. On the contrary, it is possible to find the bulk of gold bearing mineral veins in rough and broken mountain regions, and in these cases the tracing of the outcrop becomes a more difficult matter, especially when the vein crosses ravines or valleys filled with gravel or debris which hide its presence. All veins with a pronounced dip have a crooked or serpentine outcrop and the flatter the dip the more sinuous this outcrop will appear.