160 IDDINGS. 



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

 magnetite. The groundmass is holoerystalline, composed of consertal anhedrons 

 of feldspar, in part probably orthoelase, with some quartz. The rock is somewhat 

 altered in parts, and contains calcite. 



The hornblende-andesite from Sta. Inez iron mines, Rizal Province, is typical 

 of numerous others in these Islands. It is dopatic, seriate, mediophyric. with 

 abundant euhedral phenocrysts of labradorite (Ab^An : ,) and many small pheno- 

 crysts of greenish-brown hornblende with dark border. There are small magnetites, 

 and a few colorless apatites, but no phenocrysts of pyroxene. The groundmass is 

 holoerystalline, and consists of equant anhedrons of alkalic feldspar and quartz, 

 with euhedral rectangular plagioclase, some minute anhedrons of magnetite, and 

 brownish granules that are probably pyroxene. Megascopically it appears to be 

 distinctly crystalline, and represents a transitional variety between holoerystalline 

 andesite and diorite-porphyry. Similar rock occurs in the Batanes Islands ; in 

 this the groundmass contains euhedral and subhedral quartzes that are micro- 

 poikilitic. 



Horriblende-Motite-andesite is found among the river gravel at Montalban. It 

 is sempatic, seriate, and mediophyric. The plagioclase phenocrysts are euhedral, 

 zonally developed, and approximately AbtAn,. The biotite is brown; the horn- 

 blende in this ease is altered. There is magnetite and colorless apatite. The 

 groundmass is microgranular, and consists of rectangular euhedral plagioclase 

 with consertal anhedral feldspar, and micropoikilitic quartz. 



Dacites are not as common as the andesites and the few specimens 

 collected are in part noticeably porphyritic, in part megascopically non- 

 porphyritic. 



One from Benguet is sempatic and hyatal, mediophyric; that is, there is a 

 sharp contrast between the size of the smaller phenocrysts and the grain of the 

 groundmass. There are many rather large phenocrysts of plagioclase (Ab,An,) 

 with slightly rounded exhedral forms, distinctly zonal ; fewer and smaller 

 phenocrysts of green hornblende, brown biotite, and still fewer of subhedral, 

 rounded quartz; and some magnetite. The groundmass is microcryptocrystalline. 



A similar rock from Mindanao has the same habit ; but more phenocrysts, which 

 are somewhat smaller, and more of them are ferromagnesian minerals. The 

 somewhat rounded subhedral quartzes in some cases partly inclose euhedral 

 brownish-green hornblende at the extremities of "bays" or pockets of groundmass, 

 showing that these pockets and the rounded form of the quartz crystals were 

 produced at the time of the crystallization of the quartz, and did not result from 

 subsequent melting or solution of euhedral crystals. The groundmass is holoerys- 

 talline, with consertal, equigranular fabric; composed of anhedrons of alkalic 

 feldspar, in part probably orthoelase, and of quartz. This rock is a more quartzose 

 hornblende-andesite. 



A similar type of dacite occurs on Masbate. The phenocrysts are similar, but 

 the groundmass is slightly coarser, with much euhedral and subhedral crystals 

 of quartz in a matrix of equant anhedrons of alkalic feldspar. These are also 

 microcrystalline chlorite and epidote. 



On Corregidor Island massive dacite occurs which is white and dopatic, and 

 seriate, mediophyric. The phenocrysts are glassy plagioclase, quartz and biotite, 

 megascopically black and euhedral. The groundmass is holoerystalline and seriate 

 microporphyritic, consisting of small phenocrysts of prismoid plagioclase. probably 



