PETROGRAPHY OF IGNEOUS ROOKS OF PHILIPPINES. 163 



phenocrysts of olivine and none of augite in the thin section studied. The ground- 

 mass consists of equant anhedral augite, much magnetite, less plagioclase, with 

 very little brown glass, scarcely recognizable as such. 



Another rock from this region is perpatic, with few small phenocrysts of 

 plagioclase, augite and olivine, in a groundmass like that in the second rock 

 described from Taal Volcano, but finer grained. 



Somewhat similar basalts with more olivine and nearly the same type of ground- 

 mass occur on Mount Mariveles and elsewhere in Bataan Province. 



Basalt from Mayon Volcano, Albay Province, is dopatic and mediophyric, and 

 highly vesicular or porous. The euhedral, rectangular, prismoid phenocrysts of 

 labradorite contain many inclusions of brown glass; the phenocrysts of green 

 augite are subhedral ; those of colorless olivine are subhedral to euhedral. The 

 groundmass is a dark brown globulitic glass with microlites of thin needle-like 

 prismoids of plagioclase, and anhedrons of augite and magnetite. 



Another rock from this locality, is dopatic, minophyric, with many phenocrysts 

 of augite and colorless olivine, and few of plagioclase. The groundmass is like 

 that of the rock from Mayon Volcano', just described. 



A basalt from the quarry near San Pablo, Laguna, is quite different 

 in texture from most of the others yet collected. 



It is saccharoidal, perpatic and minophyric, with small phenocrysts of olivine. 

 The groundmass is holocrystalline, with microlitic cavities. It is chiefly prismoid, 

 or bladed plagioclase in subparallel arrangement (flow structure), much anhedral 

 olivine and augite, in small crystals. The olivine and plagioclase are in part 

 anhedral with respect to each other, the olivine having begun to crystallize first. 

 Magnetite occurs as minute anhedrons within the olivine, but mostly as intersertal 

 and poikilitic anhedrons, partly surrounding, with rounded surfaces, plagioclase 

 and augite, as the last component, or the chief eutectic component to end the 

 crystallization ; the intersertal orthoclase being the last feldspathic constituent 

 to crystallize, probably synchronous with the magnetite. 



Basalts from Mindanao differ somewhat from those already described 

 from Luzon in being richer in ferromagnesian minerals, and in having 

 plagioclase slightly less calcic. 



One basalt from Mindanao is dopatic, mediophyric, and seriate; with many 

 subhedral phenocrysts of olivine, but slightly altered, and with inclusions of 

 magnetite. The groundmass consists of much anhedral augite, some anhedral 

 olivine, less magnetite, and prismoid plagioclase about equal to the ferromagnesian 

 minerals in amount. The composition of the plagioclase is not readily determin- 

 able, but it is as calcic as andesine. There is a small amount of colorless matrix 

 with lower refraction, which may be glass. 



A similar basalt from Mindanao occurs with smaller phenocrysts (minophyric) 

 of olivine and augite. The groundmass is similar to that of the basalt just 

 described, but is partly altered to a zeolite, or opal. 



Another basalt from Mindanao is porous, and like the last in habit. The few 

 small phenocrysts are olivine, labradorite (Ab 3 An 4 ), and pyroxene, in part hypers- 

 thene. The groundmass is chiefly pyroxene, with anhedral magnetite, and rod-like 

 crystals of what may be ilmenite. 



Still another basalt from Mindanao is perpatic, mediophyric, "with few pheno- 

 crysts of labradorite. The groundmass consists of euhedral prismoids of labra- 

 dorite, equant anhedrons of augite, and equant subhedrons of olivine, in an 



