232 The Philippine Journal of Science 1914 



of iron pyrites and chalcopyrite which have gradually yielded their iron to 

 the percolating ground water traveling along fractures. The present de- 

 posits in my opinion represent merely a segregation of iron oxide resulting 

 from decomposition of the above-mentioned minerals. 



And again, later, Smith and Fanning " state : 



From our examination of the Bulacan deposits we can say that they are 

 very irregular and occur with igneous rocks of the Eastern Cordillera as 

 local enrichments from the alteration of chalcopyrite and other iron bearing 

 minerals of that formation. The rich pockets which we examined near 

 Angat, and from which the natives mine their ore, are located along frac- 

 tures in the formation. 



The observations recorded in this paper do not support a 

 theory of origin through superficial alteration of pyrite or other 

 iron-bearing minerals and rocks. The direct product of such 

 alteration would apparently be limonite and possibly hematite 

 rather than magnetite and hematite. Moreover, microscopic 

 study of thin sections of Bulacan ores by both Eddingfield and 

 Rowley indicates that magnetite and hematite occur as original 

 minerals in quartz and that the associated pyrite is partly of 

 contemporaneous deposition with the magnetite and partly of 

 later deposition ; in either case, the pyrite is generally unaltered. 



F. Rinne has suggested an origin for the magnetic iron ore at 

 Bato-balani, near Mambulao, Camarines, which could be applied 

 plausibly to the Bulacan ores. The Bato-balani deposit has been 

 studied by one of us; it contains hematite and pyrite as well 

 as the more abundant magnetite, and was found to show a close 

 similarity to the Bulacan ores, as will appear from the following 

 translation from Rinne:" 



* * * The mountain (a hill near Bato-balani) on its summit and its 

 slopes over a length of 400 meters and breadth of 200 meters fairly bristled 

 with countless large and small bowlders of magnetic iron ore. In the bed of 

 a small stream on the side of the mountain the ore stood out in bowlders and 

 rounded blocks; in the surrounding hemp clearing, dark, bare, frequently 

 jagged and porous blocks protruded from the ground everywhere, and in 

 the jungle one could distinguish the same blocks in great number in the 

 ground. Here and there in the mottled laterite (in which the bowlders 

 are embedded) the structure of the original rock could be distinguished. 

 * * * Apparently the weathered rocks are to be traced to a dioritic 

 "Eruptivgestein," of which several fairly fresh pieces were found between 

 the ore rocks in the hemp field. * * * j^; might be thought that the 

 magnetite masses here are a segi-egation from an igneous rock, probably 

 from the diorite found between the ore masses. It is surprising, however, 

 in explaining the magnetite as a magmatic segregation that nowhere was 

 the contact between the diorite and the ore to be seen. The ore masses 



"Zfotd. for 1910 (1911), 59. 



^^Zeitschr. f. prak. Geol. (1902), 10, 117. 



