268 University of California Publications in Geology [You 13 



As values are apparently related to gravitative stress (all veins 

 having been formed during the same uninterrupted period), so are 

 they related to horizontal stress, vertical veins parallel to a line of 

 compression being typically productive, and vertical veins normal to 

 it being typically unproductive. Furthermore, in a given vertical 

 vein, parallel to a line of compression and rising from a strong shear 

 joint or fault of low dip, but diminishing and finally vanishing in all 

 directions in its plane outward from the flat shear, the values are 

 greatest at that central point, and are arranged fanlike on the plane 

 of the vein, diminishing to zero at the edges. Thus at the center of 

 the fan is native silver in large proportion, associated with dyscrasite, 

 and perhaps breithauptite, also with niccolite or smaltite ; in the semi- 

 circular zone immediately next to it are chiefly niccolite or smaltite 

 or both with more or less gangue ; while in the third zone is only 

 gangue. These, of course, are typical relations, being departed from 

 in different ways and degrees under various conditions. 



Vein structures. — Although branching, reticulate, and offset veins 

 are perhaps the rule, and twin or companion veins are more or less 

 common, nevertheless the typical joint vein is a simple tabular body. 

 Its walls, however, are not often sharp and free, but usually are 

 "frozen" to the vein, and frequently blend imperceptibly into it. 

 Inclusions in the veins, and parallel veinlets in the walls, are the rule. 

 Frequently there is such a gradual transition from inclusions of wall 

 rock in the vein, to thinner and thinner veinlets in the walls, that it 

 is impossible to say where one begins and the other ends. The aspect 

 is altogether that of reticulate and branching fractures whose walls 

 have been replaced by a volume-for-volume reaction. Veins fre- 

 quently are banded, but not continuously nor symmetrically, the band- 

 ing not being due to crustification, but apparently to stages in the 

 growth of the vein by replacement. Crustification has been recognized 

 in a few cases ; but it is quite rare. Inclusions of wall rock are of all 

 sizes and shapes, in most cases retaining the same orientation as the 

 unaffected walls. 



A few instances have been noted where a vertical or steep vein 

 has curved in its lower portion into a flat position, its mineralization 

 changing as it did so according to the rule expressed by the diagram 

 (fig. 3). In more cases, where a vertical vein rests upon or inter- 

 sects a flatly inclined joint, the mineralization of one blends with the 

 different mineralization of the other, as if they had been formed 

 simultaneously. 



