GEOLOGY OF THE PHILIPPINE ISLANDS. 32] 



Pyroxene andesites. — These rocks, as a rule, are dark colored, usually 

 dense but often porous, porphyrinic, commonly with small phenocrysts. 

 Phenoerysts and groundmass occur generally in equal proportions. A 

 typical specimen collected at Sisiman, Bataan Province, on the north 

 side of the entrance to Manila Bay, is described by Iddings as follows : 



A dark-colored sempatie, mediophyric rock; that is, one having many small 

 phenocrysts, about as much in bulk- as the groundmass containing them. The 

 phenocrysts are mostly labradorite, approximately Ab 2 An 3 , with pronounced zonal 

 structure, the narrow outermost zone being distinctly alkalie. The shapes are 

 those of rectangular prismoid to equant crystals. In size they are seriate (that 

 is, of different sizes), from those of several millimeters to less than 1 millimeter. 

 * * * There are fewer phenocrysts of hypersthene and augite, the former 

 faintly pleochroic in thin section. Augite occasionally surrounds hypersthene. 

 The pyroxene phenocrysts are euhedral (well-faced), with the first and second pina- 

 coids strongly developed. They are generally smaller in size than the largest 

 feldspar. There is considerable magnetite in small crystals. Those inclosed in 

 pyroxene are smaller than others, not so inclosed. Some are inclosed in the 

 margin of the feldspar. The groundmass consists of mierolites crowded together : 

 rectangular equant (equi-dimensional) , also prismoid plagioclase feldspar, pris- 

 moid pyroxene, and equant magnetite; probably with a centimeter matrix of 

 colorless glass. 



Hypersthene is very common in many of the Philippine andesites. 



Hornblende-andesite. — This rock is found in all parts of the Islands, 

 forming the summit of Mount Apo and several peaks in the Zambales 

 Range of Luzon. Its habit varies from a rock having large phenocrysts 

 of feldspar, 10 millimeters or more in diameter, and smaller ones of 

 hornblende, to that in which the relations are just the reverse. They 

 are the "trachytes" of the older writers. The disintegration of these 

 rocks with the large, glassy plagioclases is the origin of the Orani and 

 Tarlac sands, the two best sands for constructional purposes available 

 ;for use in Manila. The layman usually mistakes this plagioclase feldspar 

 for quartz, whereas the sands contain little or no quartz. Professor 

 Iddings has described the hornblende andesite used in the Manila break- 

 water and which came from Sisiman Point, as follows : 



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

 labradorite, Ab,An 3 , euhedral (well-faced) and subhedral (with less perfect faces) . 

 They possess a narrow outer zone of distinctly more alkalie feldspar, which, 

 however, has noticeably higher refraction than the anhedral (without crystal 

 faces) feldspar of the surrounding groundmass. The hornblende is greenish- 

 brown, but is mostly paramorphosed into aggregates of magnetite and pyroxene. 

 There are few phenocrysts of pyroxene and relatively large ones of magnetite. 

 The groundmass is noncrystalline (complete crystal outlines), composed of con- 

 sertal (intergrown, anhedrons of feldspar, in part probably orthoclase, with some 

 quartz. The rock is somewhat altered in parts, and contains calcite and ( ?) 

 chalcedony. 



This is the variety in which the hornblendes are very large and the 



