130 GEOGNOSY AND GEOLOGY. 
for a short distance. There appears to be no regular law to which the 
course of veins is subjected : they seem to pursue their own course, without 
being affected in the least by the hindrances which stratification would seem 
to present. Most mining operations have reference to the following up of 
veins, as may readily be seen in almost any mine other than one of coal: 
here the substance sought for always occur in beds, layers, or strata. 
The shape of a vein can only be ascertained by working it. Its dimen- 
sions in a horizontal direction may thus be determined; but the matter is 
more difficult with reference to its vertical descent. Little is known of 
the character of veins at great depths, and this ignorance prevents much 
knowledge of their true character. The upper portions of veins, however, 
can be readily investigated; they are either exposed to view at the surface, 
when the incumbent detritus has been removed, or else they wedge out 
before coming through the containing rock. The horizontal course of a 
vein, with reference to the meridian, is known as the direction, and the 
angle of descent, formed with a vertical plane, is called the hading of the 
vein. In most cases the vein comes to the surface, where it may project 
like a wall. In reference to the distribution of the vein, three principal 
parts are distinguishable, the central and two wings. In a very few cases 
the vein is everywhere uniform; it is more generally ramified, and runs out 
into threads. The wings may vanish in a similar manner, although they 
are sometimes found to be cut off by faults. This cutting off may be 
effected by dykes of igneous matter or by other veins. In such instances 
the vein may, in most cases, be recovered on the other side of the displacing 
body, although not always in the line of continuity. 
A vein frequently swells out in parts of its course, so as to occupy a 
considerable space. The masses filling up these spaces are termed /odes. 
The matter fillimg up a vein is called the matrix or gangue, and may be 
composed of very different substances. These consist of mineral bodies, of 
mixtures of mineral species which do not occur as rocks, of true rocks or of 
their mixtures, either loosely aggregated or cemented by some other 
substance, thus forming a true breccia. A vein may be filled with a metallic 
ore, mixed with some non-metalliferous substance, the latter being termed 
the gangue. While veins containing non-metalliferous matter exclusively 
are of rare occurrence, it is still more seldom that they are found occupied 
by native metals. This latter case occurs more frequently in the copper 
mines of Lake Superior than anywhere else. Gangue, or matrix and ore, 
are most frequently found together. 
The relative ages of veins, as of abnormal rock masses, must be 
determined by their mutual penetration. Investigations of this kind can 
only be carried on in extensive mines, and even there the results are by no 
means satisfactory. . 
The filling up of the vein is either entire or partial. The former case is 
peculiar to abnormal masses, such as granite, syenite, diabase, trap, and 
porphyry. The occurrence of druses is intimately connected with the 
partial filling of veins. These are hollow spaces of ellipsoidal form, their | 
major plane of intersection parallel to the plane of the vein (pl. 39, fig. 81). 
560 
