vm, a, 2 Edding field: Ores of the Philippines 83 



VOLCANOES 



At one time or another volcanoes have been exceedingly- 

 active in all parts of the Philippines as shown by the enormous 

 deposits of volcanic tuff which in places are found to a depth 

 of over 200 meters. The influence the volcano has had upon 

 ore deposits is very uncertain. In examinations of mining 

 districts throughout the world, we are confronted by numerous 

 instances where the mineral district is in a section once the seat 

 of volcanic action. This is true of Cripple Creek, Colorado, and 

 numerous other prominent localities in Nevada, Utah, Montana, 

 Arizona, and New Mexico. T. A. Rickard 5 has discussed the 

 Cripple Creek Volcano exhaustively, and in a closing paragraph 

 emphasizes the influence it had upon ore deposits in the district, 

 principally by producing conditions in the rock favorable to ore 

 deposition. Francis Church Lincoln 6 has stated that the min- 

 eral constituents of volcanic emanations have been shown to 

 include all the economic metals, with the exception of gold and 

 silver, and that silver was found in galena in ejected marble 

 blocks of Monte Somma. It has also been noted that among 

 the products in the fumaroles ejected are such active agents as 

 hydrochloric acid, sulphuric acid, sulphurous anhydride, hy- 

 drogen disulphide, carbon dioxide, sulphur, hydrofluoric acid, and 

 hydrobromic acid. 



If it be considered that the fracturing of the rocks and the 

 creation of the fissures have resulted directly from the cooling of 

 the molten rock at, or near, the surface of the earth, or from 

 the eruptive force of the volcano and the subsequent subsidence 

 of the nearby areas, it is evident that the most active agent lying 

 beneath the fracture zone would be the molten or heated magma 

 which produced the volcanic flows. The fissures would probably 

 be the easiest means for the escape of the resulting gases or 

 for the ascension of the solution which might subsequently be 

 derived from the cooling mass. The neck of the volcano might in 

 some cases also furnish an easy passage. Such a condition may 

 have existed in the case cited by Kemp 7 of the Bassick mine, 

 Custer County, Colorado, where the ore occurs in an old vol- 

 canic neck as a coating on the bowlders. That so few cases of 

 this kind exist is due probably to the fact that the neck of the 

 volcano solidifies in a confined space in which practically no 



' Trans. Am. Inst. Min. Eng. (1900), 30, 367. 

 'Econom. Geol. (1907), 2, 265. 



* Ore Deposits of the U. S. and Canada. 4th ed. New York and London 

 (1901), 281. 



