278 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



is doubtful, however, if, when the bonanzas in the Tonopah veins shall have been 

 worked out, the shoot-like form will always be discernible; in the case of the 

 richer eastward-pitching shoots of the Mizpah vein, for example, the spaces 

 between the shoots should probabty be considered together with them, in the 

 larger sense, as parts of one great bonanza, whose eastward pitch and shoot-like 

 form would be less emphasized or not at all. 



In the case of Pachuca, the bonanzas are irregular or roughly elliptical and 

 are not shoot like; yet the fact observed by Ordonez, that the bonanzas on the 

 different veins group themselves into a definite zone running transversely across 

 the strike, is hardly to be accounted for except by the explanation" arrived at 

 in the case of Tonopah, that the bonanzas are due to the influence of an intersecting 

 fracture system. At the Comstock the bonanzas are similar to those in Pachuca, 

 although no local evidence has been found explaining their origin. 



The above explanation is readily acceptable for bonanzas that are elongated 

 into definite shoots, and are actually known to be associated with and dependent 

 upon cross fracturing, as in Tonopah; but it is hot so easily acceptable, perhaps, in 

 the case of wholly irregular bodies, such as those of the Comstock. Yet at Tonopah 

 the bonanzas are irregularly cut off, and do not continue indefinitely downward 

 on the pitch; and to this limitation the explanation of the controlling effect of 

 cross fractures must unavoidably be extended. Indeed, an inspection of the 

 platting of fig. 24, showing the principal observed faults and fractures in the 

 Mizpah mine, and a reflection that this is diagrammatic, while the real fractures 

 and their intersections will be much more varied and localized, shows that the 

 intersections of such mazes (such intersections constituting the tortuous channels 

 of most active circulation) with the main vein fractures will often be quite irreg- 

 ular will only approach a shoot-like form when dominated by some stronger set 

 of cross fracturing, and will cease to produce ore bodies or bonanzas of definite 

 form when there is no controlling fracturing, and now one fracture, now another, 

 invites and controls the circulation. 



EXISTENCE OF A MAJOR PACIFIC TERTIARY PETROMETALLO- 



GRAPH1C ZONE. 



Some further notes may be added to the above references (see p. 275) to the 

 extension of the belt of late Tertiary -Pleistocene andesites. 



In the region of Krakatua (situated between Sumatra and Java) the belt of 

 recent and active volcanism turns eastward and passes through the East India 

 Islands and adjoining island groups, paralleling the Australian coast, then curving 



a Mr. 8. F. Emmons informs me, on reading the manuscript of this report, that the above explanation Imd been 

 adopted at l'iichm-H when he was there in 1901. 



