734 DYNAMICAL GEOLOGY. 



nients may be such as to produce a galvanic effect, further promoting 

 decompositions and recompositions. When the solutions differ, after 

 intervals of time, there will be a succession in the changes ; and layers 

 of different species may be formed. 



Thus, a layer of quartz may be succeeded by one of fluorite, or of zinc blende, or of 

 calcite, or of quartz again, etc. In the course of the changes, a layer of cubes of fluo- 

 rite, underlying one of quartz, may be entirely dissolved away, and the cubical cavi- 

 ries filled up by another species, as blende, etc. 



The rock of the walls (especially of the lower wall, where the vein is inclined), when 

 not united iirmh' to the vein, often undergoes deep alteration, and may become pene- 

 trated by ores from the vein itself, carried in by infiltrating solutions. These altera- 

 tions are most extensive in the upper part of veins, where it often happens that the 

 metals are removed by infiltrating waters ; excepting for the most part the iron, which is 

 left in the state of red oxyd, giving its color to the earth}' mass at the top of the vein 

 (called then the iron hat). Hence the occurrence of a line of red earth in the soil may 

 be an indication of a vein of ore beneath. 



Gold-bearing quartz veins generally lose the pyrite, and perhaps other ores which 

 they contain, and thus become cavernous to a considerable depth. To this distance, 

 they are mined with comparative ease; but, beyond, they are extremely hard and much 

 more difficult to work. 



4. Veins of Different Ages. — In the progress of the uplifting and 

 folding of a region undergoing metamorphism, fissures formed at one 

 time and filled would be liable to be broken by cross-fissures at some 

 subsequent time in the epoch (perhaps a following day, week, or year), 

 and these, again, by others. Thus, a succession of veins faulting one 

 another might be formed during one epoch of disturbance ; and they 

 might differ in constitution as the bands in a banded vein differ. 



Again, veins may be intersected and faulted, by fissures formed 

 during subsecment epochs of disturbance. 



It is evident, therefore, that a vein which faults another does not 

 necessarily belong to a later independent epoch. When actually later 

 in epoch, it will usually appear in a different distribution of the new 

 veins over a wide region of country, and in their direction, and their 

 wholly distinct mineral composition. 



5. Filling of Amygdaloklal Cavities. — The cavities in a lava or igneous rock, such as 

 are formed by expanding vapors while the rock is liquid, differ from veins in size, but 

 not essentially in the method by which they are filled with minerals. In amygdaloids, 

 these minerals are usually chlorite, quartz, prehnite, datolite, analcite, or some of the 

 zeolites, or caleite; and in each cavity they often are in successive layers, analogous to 

 the layers of a banded vein. They are introduced by infiltrating waters, which derive 

 the ingredients mainly from the inclosing rock, through the decomposition of some of 

 its minerals. Quartz (glassy quartz, chalcedony, agate, carnelian, etc.) and calcite are 

 the most common of these minerals, just as they are in veins. Most of the species in 

 amygdaloidal cavities are hydrous, — showing that they were formed at a much lower 

 temperature than the materials of a grairy tic vein ; and some of them may perhaps be 

 formed even at the ordinary temperature. 



At Plombieres in France, the cement and brick of walls, of Roman origin, have be- 

 come penetrated in places with zeolites, through the action of the water of a warm 

 mineral spring having a temperature of 140° to 160° F. (Daubree.) 



