1914] Whitman: Notes on the Copper Ores at Ely, Nevada 317 



also have the same sort of outcrops in places here and there on the 

 sides, even to the bottoms. 



In a few places the undersides of these outcrops have been exposed 

 in prospect tunnels (pi. 30, fig. 1) ; and in such eases are seen to 

 extend irregularly down as fingers into the underlying limestones as 

 complete replacements of it, as if they had resulted through the agency 

 of slowly descending surface waters. Deep drilling operations have 

 shown that masses of jasperoid occur at various levels within the 

 limestone; and that fact may account for their exposure on hillsides 

 as well as hilltops, and it is also consistent with the idea that they 

 originated from emanations given off by the cooling ore-porphyry. 

 However, within the limestone in certain places, concretions of meta- 

 somatic silica have formed, from one to ten or twelve inches in 

 diameter. These resemble the jasperoid and evidently resulted from 

 the weathering of the limestone. 



In places the ore-porphyry weathers to a soft brown earth which 

 is easily eroded away, while in others it becomes silicious and resembles 

 the jasperoid, demonstrating that this igneous rock as well as the 

 neighboring limestone is capable of yielding a compact silicious rock 

 like the jasperoid, as a result of weathering processes alone. 



Thus it is argued that, since the blout caps over the ore-porphyry 

 areas cannot be parts of any original shell of silica, because they are 

 conformable to the present topography, and since similar silicious 

 rock evidently results from the weathering both of the ore-porphyry 

 and the limestone, and since an intimate examination of the under- 

 sides of the outcrops of jasperoid reveals an irregular contact result- 

 ing from replacement of the limestone, such as might have resulted 

 from the action of descending waters, therefore the weight of evidence 

 is in favor of the origin of the jasperoid through the action of descend- 

 ing meteoric waters. 



PYROCLASTIC DYKES 

 In connection with the geological history of the region, and throw- 

 ing light on the age of the ores, there is still another phenomenon 

 which is also of interest in other connections. This is the occurrence 

 of pyroclastic dykes in the ore-porphyry. These were studied at two 

 points about a mile apart, namely in Liberty Pit of the Nevada Con- 

 solidated Copper Company, and in the Morris workings of the Giroux 

 Mining Company. The dykes are extremely irregular, not being eon- 

 fined by walls which in any sense could be regarded as those of a 



