PETROGRAPHY OF IGNEOUS ROCKS OF PHILIPPINES. 159 



A somewhat similar rock occurs at Nagcarlang in Laguna Province; it is, 

 however, dopatic, that is, it is richer in groundmass than in phenocrysts. The 

 euhedral phenocrysts of labradorite (Ab 3 An 4 ) have some zonally arranged inclu- 

 sions: there is much euhedral to subhedral, strong brown hornblende, without 

 dark border; in one case there is a shell of small pyroxene crystals. The pyroxene 

 is augite in euhedral and subhedral crystals. The groundmass is brown glass 

 crowded with euhedral microlites of rectangular, equant, and prismoid plagioclase, 

 prismoids of pyroxene, a small amount of hornblende, and magnetite. 



Hornblende-pyroxene-andesite from Mount Batalao, Batangas Province, has 

 red-brown hornblende with narrow dark borders, which occur also surrounding the 

 pyroxene. The groundmass is holocrystalline and microgranular ; composed of 

 indistinctly outlined, equant anhedrons, mostly feldspar. 



On Masbate, 1 kilometer southwest of summit of Mount Bagadilla, there is 

 a variety with holocrystalline groundmass having the same fabric as the pyroxene- 

 andesite already described ; it is porphyritic, with small anhedral plagioclase in 

 a matrix of consertal anhedrons of more alkalic plagioclase, pyroxene, magnetite, 

 and some quartz. This matrix is intersertal between the small euhedral crystals. 



An olivine-bearing variety occurs on Mindanao. The hornblende is brown, the 

 pyroxene is augite; there is a little colorless olivine, and possibly a little biotite. 

 The groundmass is pumiceous and glassy, crowded with microlites of feldspar, 

 pyroxene and magnetite. 



Hornblende-andesite, with little or no pyroxene, is of widespread oc- 

 currence. Its habit varies considerably, some varieties are magnophyric 

 with abundant large phenocrysts of plagioclase about 10 millimeters in 

 diameter, and smaller ones of hornblende. These are the rocks that were 

 formerly called "trachytes." 



At Olongapo, Zambales Province, there is hornblende-andesite which is sempatic 

 and seriate magnophyric. There are abundant phenocrysts of plagioclase and 

 hornblende; few of pyroxene, and a few of quartz. The plagioclase is pronouncedly 

 zonal with recurrent zones. The center is highly calcic, at least AbjAn.,, bytownite. 

 Successive zones are distinctly marked, with alternation in composition as shown 

 by extinction angles; becoming generally more alkalic toward the margin, but the 

 outermost narrow zone is strongly calcic like the central part of the crystal. 

 There are many inclusions, usually clustered in zones. The hornblende is greenish- 

 brown, contains many inclusions, often zonal, also central in some cases, with 

 the outer portion of clear hornblende, and a narrow dark border. Paramorphs of 

 hornblende are common. Rarely augite is inclosed, or partly surrounded by 

 hornblende. The quartz phenocrysts are rounded. There are rounded colorless 

 crystals of apatite. The groundmass consists of euhedral microlites of plagioclase, 

 equant and rectangular, with small amount of matrix of alkalic feldspar, or 

 glass base. 



The light-colored rock at Sisiman 'which is used in the Manila Break- 

 water, is hornblende-andesite. 



It is sempatic, seriate and mediophyric. The most abundant phenocrysts are 

 labradorite (Ab.An-), euhedral and subhedral. They possess a narrow outer zone 

 of distinctly more alkalic feldspar, which, however, has noticeably higher refraction 

 than the anhedral feldspar of the surrounding groundmass. The hornblende is 

 greenish-brown, but is mostly paramorphosed into aggregates of magnetite and 



