IX, A, 3 Dalburg and Pratt: Iron Ores of Bulacan 217 



is composed of a dense aphanitic grayish white rock, thin 

 sections of which were examined by Rowley. He classifies the 

 rock as an acid intrusive, a quartz porphyry with phenocrysts of 

 quartz and less abundant feldspar. The quartz is much cracked 

 and corroded; the feldspars are altered and clouded, and are 

 also cracked and ragged in outline; both orthoclase and plagio- 

 clase were identified, but the orthoclase variety is predominant. 

 The groundmass is microcrystalline, and is composed of consertal 

 anhedrons of quartz and feldspar with traces of chlorite. 



While this most clearly defined dike consists of a quartz- 

 bearing acid rock, it is believed that the majority of the dike 

 rocks are basic in character. They are dark in color, and are 

 felsitic, finely porphyritic, or holocrystalline in texture. A 

 sample of a dike of black felsite within the granite near Banco 

 west of the Hison iron-ore deposit was examined by Smith; 

 it was found to have an ophitic texture and to consist principally 

 of plagioclase feldspar and hornblende with secondary epidote. 

 Another rock occurring as a dike at the edge of the granite 

 near the Montamorong iron-ore deposit he found to be quite 

 similar in texture and composition. A second dike-rock from 

 Montamorong he classified as a diabase — a holocrystalline rock 

 whose texture is ophitic and whose essential minerals are plagio- 

 clase feldspar and pyroxene. An apparently intrusive rock at 

 Santol was likewise identified as an ophitic diabase containing 

 feldspar, green hornblende, and considerable magnetite. An 

 intrusion in the base of the sedimentaries at the Tumotulo iron- 

 ore deposit he classed as porphyritic andesite, with plagioclase 

 feldspar, hypersthene, augite, and magnetite, while a sample 

 taken from a small intrusive area at the western margin of 

 the granite near Maasim River he determined as diorite — holo- 

 crystalline with plagioclase feldspar, amphibole, minor quartz, 

 and magnetite. 



Deep-seated rocks. — The only rock which is clearly of the 

 deep-seated type in this area is the granite which occurs in the 

 one rather extended exposure near the eastern edge of the 

 Cordillera. An area of quartz-diorite, which may be either 

 intrusive or deep seated, occupies the upper valley of Bayabas 

 River to the east of the area included on the map, and still 

 farther east at the headwaters of Angat River very coarsely 

 crystalline diorite was observed which is probably of deep-seated 

 origin. 



The granite is a somewhat decomposed, holocrystalline rock 

 made up very largely of quartz and feldspar. It yields a quartz 

 sand upon decomposition, and level exposures are invariably 



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