88 The Philippine Journal of Science wis 



of the valley, which drains the ore body, during the dry season. 

 The variation is more than 100 meters in some cases. This 

 tends to retard oxidation slightly during the rainy season, when 

 the vein matter is so filled with water as to be protected from 

 atmospheric influences, and also during the dry season, when 

 the chemical activity of water is lacking. These extreme periods 

 exist for only three or four months each year. During the re- 

 maining months there is moderate rainfall, which creates very 

 favorable conditions for oxidation. The net result is probably 

 much greater than in colder countries where surface waters are 

 frozen for six or seven months. 



The Philippines escape the destructive action of great changes 

 in temperature, but since such action only affects the surface rock 

 it has little bearing upon ore bodies in general. This is more 

 than offset by the heavy rainfall, the humidity of the atmos- 

 phere, and, in veins, by the shattering due to earthquakes and 

 earth movements. The temperature of the percolating waters, 

 also, plays an important part in oxidation and hydrations, and 

 the greater average temperature of the surface waters causes 

 greater chemical activity. 



The zone of oxidation extends from the surface to below the 

 extreme lowest limit of ground-water level. Some of the veins 

 are so shattered as to permit easy circulation of surface water, 

 which causes almost complete oxidation throughout. This is 

 also true of several veins in Baguio which, though but little 

 shattered, are formed of granular, porous, or crystalline quartz 

 which permits extensive circulation. Other veins made up of 

 compact and shattered material or those in which channels are 

 found along one or both walls, or through the veins, often con- 

 tain unoxidized material even near the surface. The majority 

 of the veins in Paracale are but little oxidized, due in part to 

 the unshattered condition of the ore body, in part to the low 

 relief of the district and to the consequent nearness to the sur- 

 face of the permanent ground-water level. In Baguio, also, 

 there are a few cases where unshattered quartz or calcite veins 

 have resisted oxidizing agents. 



Leaching has played an important part in the gold veins, but 

 its effect has been one of enrichment rather than impoverish- 

 ment. In practically all cases the richest ore is at or near the 

 outcrop, and values decrease almost uniformly with depth. This 

 is caused by: (1) Rapid erosion of impoverished material, (2) 



