264 The Philippine Journal of Science isu 



magnetite and hematite. W. H. Emmons ^ gives for magnetite 

 and hematite the following associations : 



1. Igneous rocks. 



2. Pegmatite veins. 



3. Contact metamorphic deposits. 



4. Deposits of deep vedn zone. 



5. Secondary minerals in zones of oxide and sulphide enrichment. 



6. Products of dynamo regional metamorphosis. 



It was found in examining the rock sections that by far the 

 commonest rock carrying appreciable amounts of iron ore is a 

 holocrystalline form, apparently a diorite. The amount of iron 

 found in any other form of rock is so small as to be almost negli- 

 gible. This association of iron ore with the holocrystalline rock 

 leads to the conclusion that this rock has had some influence or 

 agency in the formation of the deposits. In fact, it appears that 

 the ore bodies resulted from contact metamorphic conditions 

 following the intrusion of some magma contemporaneous with 

 its cooling. The gases or solutions carrying the iron were de- 

 rived from this magma upon cooling, and the iron was depos- 

 ited along the contact between this magma and some other rocks, 

 particularly a diorite, and also, as was seen in the field, along 

 fissures in the older rocks. 



The most striking feature accompanying the formation of the 

 ore deposits is the exceedingly large amount of quartz released. 

 Quartz is found in practically all specimens of ore and all speci- 

 mens containing iron ore, and besides has replaced mineral con- 

 stituents in a large portion of the rocks, making up the capping 

 of the district. The solutions, then, contained both quartz and 

 iron. Most of the iron was deposited first at the lowest possible 

 levels; the quartz filled the interstices, but being in such large 

 quantity continued upward attacking the overlying rocks and 

 largely replacing the feldspars, giving a capping over the district 

 of silicified rock similar to that in Baguio, where it is called the 

 Baguio formation. 



Many samples show magnetite with large amounts of pyroxene 

 and amphibole. This class of ore is so abundant that it would 

 appear that the pyroxene or amphibole had suffered a migration 

 of the same character as the magnetite and quartz ; that is, the 

 solutions released from the cooling magma also contained con- 

 stituents for forming these silicates. While they may have been 

 formed by alterations of the wall rock during the deposition of 

 the ore, the large amount formed seems to point to some other 



^Econom. Geol. (1908), 3, 620. 



