IX, A, 3 Dalburg and Pratt: Iron Ores of Bulacan 233 



were encountered everywhere without any adhering or inclosed pieces of 

 diorite. This circumstance indicates strongly that the once existing rock 

 with which the present ore blocks were associated was comparatively easily 

 destroyed so that the ore, freed through weathering, is now nowhere to be 

 found in continuity with them. In this connection the occurrence of a 

 dark-colored limestone of which several pieces were found at a place on 

 the same slope is interesting. It is possible that the ore masses were 

 enveloped in this easily destroyed limestone. It appears to me very plausible 

 that the magnetite blocks at Bato-balani were formed from contact phe- 

 nomena between diorite and the limestone which is still found in traces 

 over the former surface of the igneous rock. * * * One could suppose 

 that the ore formed in the limestone under the influence of the solutions 

 and gases coming from the cooling diorite magma. I did not observe other 

 contact minerals, such as garnet, at this place, but in complete accordance 

 with this theory is the occurrence of nests of yellowish white, needlelike 

 quartz which are found sparingly in the magnetite. In places the ore 

 particles build a sort of frame or skeleton, the spaces of which are filled 

 with quartz. * * * xhe igneous rock that occurs with the magnetic 

 iron ore is an augite-bearing hornblende-diorite. It has a speckled appear- 

 ance, resulting from the arrangement of white to gray plagioclase and the 

 scattered, rounded, or elongated grains of greenish black hornblende which 

 dot the abundant groundmass. 



It is concluded that the Bulacan iron ores are similarly the 

 result of contact phenomena caused by intrusions. The intrusive 

 rocks are identiiied with some hesitation as imperfectly defined 

 dikes, the occurrence of which has been noted. Weld," in his 

 study of very similar deposits near Hongkong, found that the 

 Hongkong granite was the rock whose intrusion into older sedi- 

 ments had caused the deposition of magnetite-hematite ores. 

 It is not impossible that the Philippine granites, of which the 

 granite in Bulacan is representative, are to be correlated with 

 the Hongkong granite, but the directly resulting suggestion that 

 the granite in Bulacan is the intrusive rock involved in the 

 genesis of the ore deposits there is refuted by the clearly estab- 

 lished priority in age of the granite over the sedimentaries which 

 are affected by the intrusion. 



For contact deposits, however, the Bulacan ores occur in notably 

 large part as a replacement of the intruded rocks, limestone, 

 and other sedimentaries ; they are not confined to a narrow con- 

 tact zone. In minor occurrences the ore has also filled fissures 

 which are not on the immediate contact. Some of the more 

 common contact-metamorphic minerals, such as garnet and wol- 

 lastonite, were not identified in the Bulacan ores in spite of the 

 fact that they were especially sought. The Camaching limestone 



" Lot. cit. 



127944 3 



