WORKINGS PARTLY IN EARLIER ANDESITE. 189 



MACNAMARA WORKINGS. 

 LATER AXDE8ITK AT SURFACE. 



The MacNamara shaft is situated a short distance northwest of the West 

 End, and probably in the same fault block. The geology partakes of the same 

 perplexing character as that described in the West End (see p. 184). The shaft 

 was first sunk to a depth of 200 feet, from which point drifts were run 50 

 feet to the north and about 300 feet to the south. The rock in which the shaft 

 started and which outcrops in the vicinity is undoubted later andesite, such as 

 covers the whole surface of this fault block. 



CHARACTER OF ANDESITE ON 200-FOOT LEVEL. 



The rock encountered on the 200-foot level differs in character very slightly 

 from that at the surface, except that the latter has the purplish color due to 

 partial oxidation, while the former has a green color characteristic of andesite, 

 containing a large proportion of chlorite as a result of subterranean alteration 

 processes. Also the andesite at the surface is decidedly fresher than that on the 

 200-foot level, where it is always highly altered. 



CORRELATION OF MACNAMARA AXD WEST END AXDES1TES. 



There would, however, be hardly sufficient reason for dividing the upper 

 and the lower andesite were it not that study and comparison make it seem clear 

 that the rock on the 200-foot level is practical!} 1 identical in characteristics with 

 that on the 220-foot level of the West End, which the writer, for reasons 

 previously given, is obliged to believe to be a phase of the earlier andesite rather 

 than of the later andesite. 



The MacNamara rock can be matched almost exactly with specimens of the 

 West End rock. When studied under the microscope it is found to be altered 

 largely to chlorite and calcite, with pyrite, quartz, siderite. and sericite. If it is 

 the earlier andesite, therefore, it belongs to that phase which has altered to calcite 

 and chlorite rather than to that which has altered to quartz and muscovite. such 

 as the phase found on the 700-foot level of the Siebert shaft and below, which is 

 believed by the writer to have been formed usually at some distance from the 

 mineral-bearing veins rather than in their immediate proximity. 



This rock contains calcite blotches and veinlets, and occasional stringers of 

 mixed quartz and calcite, one of which, it is claimed, afforded assays showing a 

 value of $2. 



