vin, a, 2 Edding field: Ores of the Philippines 89 



leaching out of valueless minerals, (3) mechanical concentra- 

 tion of valuable minerals, and (4) chemical concentration of 

 valuable minerals. 



Owing to the steep slopes characteristic of most of the dis- 

 tricts, the erosion of the outcrops of the veins is very rapid. 

 During the rainy season, with its high ground-water level, only 

 a small part of the vein is subject to rapid leaching. At the 

 water level the leached values tend to be redeposited, forming 

 a zone near the surface. A large part of the partially leached 

 material above this level is washed away, and the gold content 

 may eventually form placers. With the descending of the 

 ground-water level, leaching action takes place extensively, and 

 calcite, copper, iron, manganese, and other minerals are dis- 

 solved, and either flow away in solution or are redeposited lower 

 down in the waterways. 



The solubility of calcite is very great on account of the large 

 amount of carbonic acid in the surface waters. This has been 

 found to vary from 0.22 to 0.45 per cent by volume. Roth 8 

 states that 1 liter of pure water, either cold or boiling, will 

 dissolve about 18 milligrams of calcium carbonate. Water 

 saturated with carbonic acid will dissolve from 700 to 880 

 milligrams at 10°C., 9 or about fifty times as much as pure 

 water. This has proved an important factor in the leaching 

 of the numerous calcite-bearing veins in the Philippines. 



In the upper workings of the Colorado mine the ore is honey- 

 comb quartz stained with iron and manganese, but no calcite has 

 been found. However, the quartz in the vein shows perfect 

 mold forms for calcite crystals of characteristic shape, and 

 proves that at one time the vein contained a large percentage 

 of calcite which was afterwards leached from most of the vein 

 in the zone above average ground-water level. 



In the Eastern mine, Aroroy, honeycomb quartz is also found, 

 but in this mine calcite can be found very near the surface. 

 The leaching has taken place only along certain channels and to 

 a much more limited extent than in the Colorado mine. 



Calcite leaching is the only instance where marked impoverish- 

 ment of any one constitutent occurs, except in copper-bearing 



8 Allgemeine und chemische Geologic Wilhelm Hertz, Berlin (1879), 

 1, 48. 



8 Geike, Textbook on Geology. 4th ed. New York (1903), 471. 



