OXIDATION AND CHLOBIDATION. 91 



otherwise altered by surface waters, while at 300 feet and below, in the Valley 

 View, the pyrite in the country rock is usually unaltered. 



Where veins do not outcrop, but are covered with a blanket of overlying 

 rock, there is usually comparatively little oxidation. The ore in the Fraction, at 

 a depth of a little over 200 feet, is a sulphide ore; in this case the vein has been 

 protected by a covering of soft volcanic rock (Fraction dacite). Similarly, heavy 

 sulphide ores were found in the Montana Tonopah at a depth of about 460 feet, 

 the veins of this mine apexing under the later andesite, which is decomposed 

 and not readily susceptible of fracturing. The depth of general oxidation of the 

 country rock is only about 90 feet in the Montana Tonopah shaft, between 115 

 and 185 feet in the Wandering Boy, and a little over 200 feet in the Stone 

 Cabin. In the \Vandering Boy the vein is oxidized on the 300-foot level, while 

 the country rock is unoxidized. 



A single fracture line often locally divides the oxidized from the unoxidized 

 ore and rock. This line of demarcation frequently coincides with a fault line, on 

 which account it was suspected that some of the oxidation might be earlier than 

 the faulting; but other considerations render it more probable that, by faulting, 

 rocks of different degrees of porosity and permeability are brought together and 

 thus the result is accomplished. 



SILVER CHLORIDE IN OXIDIZED ZONE OF VEINS. 



In the ores, the effects of oxidation are to change pyrite to limonite, and 

 also to deposit wad (oxide of manganese), which is formed from the manganese 

 carbonate in the primary ores; while horn silver (cerargyrite) becomes plentiful. 

 This abundance of horn silver, being characteristic of the oxidized zone, is evidently 

 due to the effects of chlorine contained in the surface waters. Silver bromides 

 and iodides also sometimes accompanj- the chloride. Free gold has been deposited. 



The large quantities of the haloid metallic compounds in the weathered 

 portions of veins in the desert regions of America have been especially discussed 

 by Prof. R. A. F. Penrose, jr.," who suggests that they are probably due to 

 the arid climate which has prevailed in the present and during the more recent 

 geologic periods, and which has rendered the scanty ground waters saline. It is 

 suggested that these saline waters have accomplished this alteration. 



At Tonopah it is regarded as probable that the primary ore contains some 

 silver chloride, and it is possible that the chloride therein contained may have 

 been concentrated in the zone of weathering, and may also have contributed to 

 the predominance of chlorides in this zone. 



ojour. Geol., vol. 2, p. 314. 



