84 The Philippine Journal of Science 1913 



Assuring- takes place. It is evident that the active volcano 

 would have little or no part in ore formation, since the magma 

 below, in its extremely heated condition, would give off nothing 

 but gas or molten rock, which would escape from the volcanic 

 vent. Its chief agency is in the formation of fissures which 

 are filled later by ascending mineral-bearing solutions, probably 

 derived from, or influenced by, the same magma at a t cooler stage. 



MINERAL SPRINGS 



Hot springs are abundant throughout the Islands. In some 

 localities they have been found depositing arsenic, iron, lime, 

 salt, or sulphur. In the Benguet mineral district, large masses 

 of silica containing small values of gold have been found, which 

 were undoubtedly deposited by springs. Silicification of certain 

 rocks has taken place to a marked extent in this region, and has 

 led to the name "Benguet formation" for this class of rock. It 

 would appear that vein formation is still going on and that gold 

 has recently been, or is at present being, deposited in the fissures 

 up which the gold-bearing solutions are coming. 



GOLD ORE VEINS 



Gold-bearing veins are found filling fissures in andesite, an- 

 desite agglomerate, diorite, and granite gneiss, and also along the 

 contacts between two classes of igneous and between igneous 

 and sedimentary rocks. By far the greatest number of veins 

 are found in the andesite. These fissures seem to have been 

 formed without appreciable movement or vertical displacement, 

 as shown by the absence of slickensides on the walls of the fis- 

 sures and the comparative absence of faults in a region which 

 must have undergone more than one period of Assuring. 



The faulting of a quartz vein is seen in the "Tejon dike" on 

 the Ascension group, Baguio, Benguet. This dike, or vein, and 

 another or possibly the same vein in Emerald Creek have been 

 faulted twice, as shown in fig. 1. One section of the vein has 

 been entirely displaced, and the direction of strike of the western 

 portion changed by about 30°. This faulting may have been due 

 to a local shifting of the rock comparatively near the surface. 

 The only other example, so far recorded, is on the Eastern 

 property, Aroroy, where only one of a series of six or eight paral- 

 lel veins is faulted and has a lateral displacement of about 3 

 meters. The fault line was perpendicular to the vein. It showed 

 well-marked slickensides, and contained drag material from the 



