156 IDDINGS. 



tion to the ferromagnesian minerals already named. Some pyroxene- 

 andesites cany small amounts of olivine, and form transitions between 

 andesite and basalt. 



Basalts with a variable amount of olivine are abundant, and constitute 

 some of the more prominent and the more active volcanoes. On the other 

 hand dacites, and possibly rhyolite, are rare, so far as known, and occur 

 in relatively small bodies. 



Certain basaltic rocks characterized by rather alkalic feldspar, and in 

 one case by altered leucite, are found in several localities. 



Pyroxene-andesite . — In most instances these rocks are dark colored, 

 dense or porous, less often vesicular : porphyritic with many small pheno- 

 erysts, that is, they are mediophyric. The relative amounts of pheno- 

 crysts and groundmass vary somewhat in different cases, but the great 

 majority have nearly equal amounts of phenocrysts and groundmass 

 (sempatic), or have rather more groundmass (dopatic). Several of the 

 freshest varieties collected may be described as follows : 



From Cochinos Point and Sisiman, Bataan Province, there is a dark-colored, 

 sempatic, mediophyric rock ; that is, one having many small phenocrysts, about as 

 much in bulk as the groundmass which contains them. The phenocrysts are 

 mostly labradorite, approximately Ab 2 An 3 , with pronounced zonal structure, the 

 narrow outermost zone being distinctly alkalic. The shapes are those of rectan- 

 gular prismoid to equant crystals. In size they are seriate; that is, of different 

 sizes, from those of several millimeters to less than 1 millimeter. They are well 

 twinned in Carlsbad, albite and pericline manner. They contain many micro- 

 scopic inclusions, usually in the central portion of each crystal. There are fewer 

 phenocrysts of hypersthene and augite, the former faintly pleochroic in thin 

 section. Augite is twinned in some cases, and occasionally surrounds hypersthene. 

 The pyroxene phenocrysts are euhedral, with the first and second pinacoids strongly 

 developed. In size they are generally smaller than the largest feldspars. 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 microlites crowded together; rectangular 

 equant, also prismoid plagioclase feldspar, prismoid pyroxene, and equant mag- 

 netite; probably with a cementing matrix of colorless glass. 



Almost identical rocks occur at Corregidor Island and Mount Arayat, 

 Pampanga Province. In one rock from Corregidor the magnetite has 

 crystallized in curved, skeleton-like form within a cluster of hypersthene 

 and augite crystals, as when quartz forms graphic intergrowth with 

 orthoclase; indicating synchronous crystallization of the magnetite and 

 pyroxene. 



Pyroxene-andesites having the same composition and habit as that 

 described from Cochinos Point occur in Albay Province, Luzon, on Cebu, 

 and elsewhere in the Islands. 



A pyroxene-andesite of the same general type as those just described, 

 that is, sempatic and mediophyric, with the same kinds of phenocrysts, but 

 with a distinctly glassy groundmass, occurs at Siniloan, Laguna Province. 

 The groundmass consists of brown globulitic glass, which is slightly more 



