Parr IX. § 1] MINERAL VEINS. 591 
their reception. In certain rocks (limestones, dolomites, &c.) intricate 
channels and large irregular caverns have been dissolved out by the 
solvent action of underground water; in other cases fissures have 
been formed by fracture, or the rocks, exposed to great compression, 
have been puckered up or torn asunder, so that irregular spaces have 
been opened in them. Metallic ores and crystalline minerals intro- 
duced by infiltration, sublimation or otherwise, into the cavities 
formed in any of these ways, may be grouped according to the shape 
of the cavity into veins or lodes, which have filled up vertical or 
highly inclined fissures, and stocks which are indefinite aggregations 
often found occupying the place of subterranean cavities. 
The first two types of ore-deposits do not require special treat- 
ment here. ‘The stratified type has the usual character of sedi- 
mentary formations (Book IV. Part I.); the crystalline type forms 
part of the structure of schistose and massive rocks (Book II. Part II. 
§ vi. 2 and 3) ; the third type, however, from its economic importance 
and its geological interest, merits some more detailed notice. 
§ 1. Mineral Veins or Lodes. 
A mineral vein consists of one or more minerals deposited within 
a fissure of the earth’s crust. Such fissures being usually highly 
inclined or vertical, so also are mineral veins. Cases occur, however, 
- among crystalline massive rocks, and still more frequently among 
limestones, where the introduction of mineral matter has taken place 
along gently inclined or even horizontal planes, such as those of 
stratification, and the veins then look like interstratified beds. 
Mineral veins are composed of masses or layers of simple minerals or 
metallic ores alternating, or more irregularly intermingled with each 
other, distinct from the surrounding rock, and evidently the result 
of separate deposition. They are in no respect to be confounded 
with veins of rock injected in a molten condition from below, or se- 
gregated from a surrounding pasty magma into cracks in its mass. 
Variations in breadth.—Mineral veins vary in breadth from a 
mere paper-like film up to a great wall of rock 150 feet wide or 
more. The simplest kinds are the threads or strings of calcite and 
quartz so frequently to be observed among the more ancient and 
especially more or less altered rocks. These may be seen running 
in parallel lines or branching into an intricate network, sometimes 
uniting into thick branches and again rapidly thinning away. Con- 
siderable variations in breadth may be traced in the same vein. 
These may be accounted for either as due to unequal solution and 
removal of the walls of a fissure, as in the action of permeating 
water upon a calcareous rock ; or to the irregular opening of a rent, 
or to a shift of the walls of a sinuous or irregularly defined fissure. 
In the last-named case the vein may be strikingly unequal in 
breadth, here and there nearly disappearing by the convergence of 
‘the walls and then rapidly swelling out and again diminishing, 
