218 The Philippine Journal of Science 1914 



covered with a deep mantle of coarse, sharp quartz. Bowlders 

 and stream-floor exposures of the granite exhibit pitted surfaces, 

 caused by the unequal resistance of the constituent minerals 

 to weathering processes. Rowley examined petrographic sec- 

 tions of a sample from Calingnag Creek west of the Hison 

 iron-ore deposit, and submitted the following notes: 



The specimen is a light-colored, medium to coarse-grained holocrystalline 

 rock which would be classed megascopically as a granite. In thin section 

 it is seen to be composed principally of quartz and feldspar with subordinate 

 epidote, green hornblende, titanite, and iron oxide. The fabric is consertal, 

 unequigranular. The feldspars predominate slightly in abundance over the 

 quartz which, although it exhibits some true crystal faces, occurs usually 

 in irregular, apparently rounded anhedrons, or is graphically intergrown 

 with the feldspar. The quartz contains many fluid inclusions as well as 

 gas bubbles and dust particles. The feldspar is subhedral to anhedraJ, 

 prismoid to equant; zonal structure is common, and Carlsbad and albite 

 twinnings are to be observed. In places combinations of albite and peri- 

 cline twinning produce the "grating" appearance characteristic of micro- 

 cline. Some of the feldspars are considerably clouded, some show twin 

 lamellations due to strain, and some exhibit a striated appearance sug- 

 gesting crytoperthite. The cloudy feldspar with or without Carlsbad twin- 

 ning appears to be orthoclase. The zonally developed feldspars and those 

 exhibiting albite twinning are proved by their optical properties to be 

 alkalic plagioclase; in quantity these two classes of feldspar are about 

 equal. Titanite occurs usually as small anhedrons, but also in the well- 

 known wedge-shaped form, and anhedral amphibole with a tendency to 

 fibrous structure is present. The epidote also appears to be secondary, 

 and together with the amphibole probably resulted through the decompo- 

 sition of primary minerals, possibly biotite or pyroxene. The titanite, 

 epidote, and amphibole are usually closely associated, and with them are 

 small flakes of magnetite and hematite. 



The rounded appearance of the quartz noted in the petro- 

 graphic sections is much more pronounced close to the margins 

 of the granite exposure, and is evident in hand specimens. Thin 

 sections of the marginal rock from. just below the Constancia 

 ore body on Maon Creek, examined by Smith, showed corroded 

 and rounded quartz phenocrysts in a fine groundmass which 

 appeared to be fragmental or possibly effusive in character. 

 Another sample from the north end of the granite area near 

 Calumpang Smith found to consist principally of fragments 

 of quartz and feldspar. 



It appears that the granite has been subjected to some process 

 by which a brecciated or disintegrated border zone has been 

 recemented to form a rock much like the original granite in 

 appearance, but with a clearly fragmental texture and a marked 

 predominance of quartz over feldspar in abundance and in grain 

 size. 



