VEINS OF THE EARLIER ANDESITE. 85 



Sometimes, also, either the hanging-wall or the foot-wall streak may be wanting. 

 Next, -streaks of quartz parallel with the walls may be found, or the quartz may 

 form a network in the andesite. Thus the process may be traced to the stage 

 where the whole of the andesite is replaced by quartz, forming a solid vein 

 several feet in width. As a rule, however, more or less decomposed andesite 

 forms part of the vein. 



PORTIONS OF VEINS DUE TO CAVITY FILLING. 



As exceptions there are found streaks of quartz, usually small, within the 

 vein, which show crustification and comb structure and thus bear evidence of 

 having been formed in cavities. These cavities, however, were often of irregular 

 shape and were not fissures, properly speaking, but spaces of dissolution, and 

 were the effect of the mineralizing waters themselves. 



The largest example of a crustified vein is found in certain parts of the 

 Montana Tonopah workings, where the cavities were sometimes 2 or 3 feet in 

 diameter and gave rise to well-banded ores (PL XIII). 



CROSS WALLS AND ORE SHOOTS. 



The fractures transverse to the main system had a not inconsiderable effect 

 in determining the course of the ore solutions. Along important transverse 

 fractures it has been found that the vein frequently widens or narrows abrupth*, 

 the cross fractures playing the same part as the lateral wall fractures, even if not 

 to such an extent, and so earning the name of cross walls which has been given 

 them. To these cross walls, more or less pronounced, the division of the water 

 circulation along the main zone into columns of unequal importance was due, and 

 hence the mineralization accomplished by these waters was correspondingly 

 localized. It is probable that the recognized ore shoots or bonanzas had their 

 origin in this way. 



NATURE OF MINERALIZING AGENTS. 



That the mineralizing agent was water is evident from the character of the 

 vein and from the nature of the alteration of the wall rock. That its action was 

 probably connected with the earlier andesite eruption is shown by the fact that 

 it followed this and, at least so far as mineralizing activity was concerned, was 

 of limited duration, for its effects have not been determined in the succeeding 

 later andesite. It appears probable, therefore, that the mineralizing agents were 

 volcanic waters, such as are usually among the after effects of volcanic outbursts, 

 and that they were hot and ascending. A consideration of their effects, as dis- 

 played both in the veins and in the country rock, will throw further light on 

 their nature. 



