MINERAL RESOURCES OF AROROY DISTRICT. 417 



increasing value in moderate depth is clearly shown in many cases, but 

 most clearly by samples taken in the 30-meter raise to surface from the 

 upper level of the Nancy. In this case there was a progressive increase 

 downward from values of less than 1 gram to over 6 grams per ton. The 

 increase in air circulation in the veins as compared with the country rock, 

 is evidence i].. >()ose nature of the ore in the larger 



veins may allow mechanical transportation of the fine gold by down- 

 ward flowing surface waters. An adit run on the vein in the lower level 

 of the Colorado developed the fact that the air was much better than 

 in similar cross-cut tunnels of much shorter length in the country rock. 

 In the lower level of the Nancy, a cross-cut was carried from the side of 

 the hill until it cut the vein at a depth of 90 meters below surface. 

 Before the vein was reached, there was great difficulty with the air, but 

 as soon as the banded, alternate broken and massive quartz was cut, 

 there was found to be a sufficient supply. 



It had been considered that the segregations of the different minerals 

 into bands must affect their values, and it was taken for granted that 

 the more massive bands of quartz were practically barren, but careful 

 sampling did not confirm this prediction. As a rule, it may be said that 

 the best values occur in rather irregular pay streaks, which have a ten- 

 dency to foUow the footwall. 



The prospects are not so favorable in the sulphide zone. The hard, 

 blue quartz will, of course, be more expensive to mine and mill than 

 will the broken material handled in the oxidized zone, and whatever has 

 been added to the values by concentration from above, will be lacking at 

 this level. However, practically all the work so far has been done in 

 the deep oxidized zone, and the depth to which the latter has been shown 

 to extend insures the future of the district. 



The comparatively great depth of the zone of oxidation foiind here, 

 considering the low elevation of the district, is consequent upon its recent 

 geologic history. The evidence of the drowned valley of port Barrera, 

 and to a less degree of the Lanang Elver, shows that the land in recent 

 geologic times stood at a level probably over 50 meters above its present 

 elevation. It remained at this level long enough for a broad valley to 

 be eroded and consequently long enough for the oxidizing surface waters 

 to reach a greater depth than would have been possible under conditions 

 existing to-day. Consequently, it will probably be found that while the 

 sulphide zone domes up under the hills, the curve which it follows will 

 be much more gentle and the zone of oxidation deeper than would other- 

 wise be the case. 



In the oxidized zones of different veins, some difference of values is 

 to be noted between the veins carrying noticeable amounts of manganese 

 and those in which iron oxide forms the metallic portion of the gangue. 

 As far as a limited observation extends, all the veins which on their 



