IX, A, 3 Dalburg and Pratt: Iron Ores of Bulacan 231 



elastics and crystalline limestone. Bowlders of iron ore, in 

 which magnetite occurred in limestone as a replacement or a vein 

 just as it does at Camaching, were noted by one of us in Lenatin 

 River below the Santa Inez deposit. 

 Adams " says : 



* * * Bowlders of iron ore, some of which are from 2 to 3 meters in 

 diameter are encountered about one hour's walk [from Santa Inez] up the 

 river [Lenatin] in the bed of the stream. The mountain to the west of 

 the river was evidently the source of these masses. The lower slope of the 

 mountain was ascended along the bed of a stream which empties into the 

 river just above the bowlders. The country rock exposed by erosion is an 

 andesite containing numerous small specks of pyrite, and in some places 

 bunches of pyrite were found in sheer zones. The larger masses of pyrite 

 were partially altered to hematite. In places there is a small amount of 

 chalcopyrite present and the alteration has given rise to a coating of blue 

 and green copper carbonates. The copper ores have been prospected lately 

 but have not been found in encouraging quantities. On the wall of the 

 ravine, a face of rock was seen which showed a considerable amount of 

 iron ore, coating and replacing the country rock. This has somewhat the 

 appearance of a dyke running up the mountain, although there is no proof 

 that it is, since the dense vegetation obscures the formation excepting in 

 the walls and bed of the ravine. Near the top of the hill there is an outcrop 

 of iron ore. The summit of the hill is capped by a heavy bed of limestone, 

 such as is frequently met with in the eastern cordillera. In descending, 

 exposures of a metamorphosed fine-grained clastic rock were seen in the 

 bed of the ravine to the south of the one which was followed in ascending. 

 This rock contains specks of pyrite, but no bowlders of hematite were 

 seen. A simple and sufficient explanation of the origin of the iron ore is 

 that it has been derived from the pyrite which is found disseminated in the 

 country rock and occurring as masses in the sheer zones. It is probable 

 that the mineralization is a result of contact phenomena resulting from the 

 intrusion of the andesite in the sedimentary formation. 



According to this idea the iron oxides which are hematite and 

 magnetite (Adams noted the occurrence of hematite only) 

 formed through the oxidation of pyrite, which in turn is the 

 product of contact metamorphism. 



Smith " has proposed a similar origin for the Bulacan ores : 



I wish here to record the result of my own very limited observations in 

 addition to the remarks by Mr. McCaskey regarding the occurrence of the 

 ore. The ore is found in the massive crystalline rocks of this region, which 

 are in the main dioritic and andesitic. The iron ores, hematite, magnetite, 

 etc., are alterations of these crystalline rocks in place. They are not sedi- 

 mentary deposits, and therefore any regular strikes and dips, such as occur 

 in sedimentaries, would not be found. The iron deposits, as far as I was 

 able to make out, have absolutely no connection with the later sedimentaries. 

 The diorite is very rich in ferromagnesian minerals, with an unusual amount 



" This Journal, Sec. A (1910), 5, 106. 

 "Mm. Resources P. I. for 1909 (1910), 32. 



