236 



STRUCTURAL GEOLOGY. 



be babe b 



-d- 



- Ar- 



(Fig. 141). Sometimes the bands are beautifully 

 regular and distinct, like agate (Figs. 130, 140) ; some- 

 times on a larger scale, and irregular. Very often we find 

 several corresponding layers of 

 agate on the two sides, and the 

 center filled with combs of quartz- 

 crystals with interlocking teeth 

 (Fig. 140, I). 



Irregularities. Small veins, 

 the fillings of small cracks, are 

 extremely irregular, running in 

 all directions, and intersecting 

 each other in every conceivable way. Great fissure-veins 

 are far more regular. But even these are more or less 

 irregular, partly from the irregularity of the original fis- 

 sure and partly from subsequent movements. Perhaps 

 the most important of these is displacements by fissures 

 or other veins, as explained below. 



Age of Veins. Often in the same locality we find 

 two or more systems of veins, formed at different times, 

 crossing each other. In such cases, as in dikes, the fissure 

 or vein which cuts through the other (Fig. 142, a) is of 



FIG. 141. a, galena; 66, bary- 

 ta ; cc, fluor-spar ; d, wall. 



FIG. 142. 



course the younger. The absolute age, i. e., the geological 

 period in which the fissure was made, can be known only 

 by the age of the strata through which it breaks. 



Recovery of Lost Veins. Suppose 1} (Fig. 142) is a 

 valuable vein, and we work down until we strike a. The 



