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be attributed to the fact that at a depth the fissures occupied by the 

 dikes or veins were confined to the original line of breakage on account 

 of the superincumbent pressure, while nearer the surface this pressure 

 was relieved and numerous transverse breaks of more or less superficial 

 character were encountered and the fissures were more easily deflected 

 and divided. " 



It is of special importance with these deposits to distinguish 

 between shrinkage cracks and fissures, both along dikes and elsewhere. 

 The reason for this is that fissures are more persistent and likely to 

 extend to greater depths than are shrinkage cracks. 



The ore of the veins, as elsewhere, occurs in chutes. That is, cer- 

 tain parts of the fissures are barren while others contain ore of value. 

 These chutes are of greater vertical than horizontal dimensions. They 

 differ from the chutes generally recognized in other mining regions 

 in being isolated bodies of ore along the fissure planes, rather than 

 rich portions of otherwise clearly defined veins. They vary in breadth 

 from one to several hundred feet, and from a few inches to several 

 feet in width. Their distribution is not influenced by a difference in 

 country rock, as that is uniform. They are probably due more to the 

 location of cross fractures and fissures, and to localization of the ore- 

 bearing solution. They are also affected by the distribution of the 

 dikes. 



Chapter IV treats of the source and mode of deposition of the ores. 



The ore. Dr. Penrose concludes, was probably derived from both 

 deep and shallow sources, from the whole area tributary to the fissures. 

 An investigation of the actual gold contained in the country rocks 

 was not made, but the metal doubtless exists in both the old and new 

 formations, though he is of the opinion that the later eruptives were 

 probably the most prominent source, as in them the conditions for 

 concentration were best realized, as they were in the neighborhood of 

 volcanic vents where the rocks were penetrated by hot waters impreg- 

 nated with the mineral solutions. The solution and deposition of 

 gold along with quartz from alkaline solution is readily understood, 

 but the formation of the telluride of gold requires further investiga- 

 tion before it can be explained. 



As regards the associated minerals, the fljj^orite was probably 

 evolved from the phonolite magmas, in a volatile or soluble form, and 

 acted on solutions carrying lime ; if it was in the form of hydrofluorsilic 

 acid, the association of the quartz with the fluorite is readily explained. 



