PETROGRAPHY OF IGNEOUS ROCKS OF PHILIPPINES. 169 



Diorites rich in hornblende occur in Cebu. One from the Island of Palawan 

 corresponds in general texture to the fine-grained norite already describd from 

 this island. Its fabric is consertal, anhedral, equigranular, or nearly so. It 

 consists of nearly equal amounts of labradorite and brownish-green hornblende. 

 Within the feldspar are small euhedral inclusions of hornblende and magnetite; 

 and within the hornblende anhedrons there are small anhedral inclusions of 

 feldspar and magnetite. 



Quartz-diorvtes occur in different parts of the Islands. 



At Antamok, Benguet Province, there is medium-grained quartz-diorite, with 

 inequigranular consertal fabric. It consists of plagioclase and considerable 

 brownish-green hornblende, anhedral with respect to each other, but euhedral 

 toward quartz and orthoclase. There is some altered biotite. In places the 

 orthoclase is intersertal to poikilitic, with inclusions of plagioclase and hornblende. 



Another quartz-diorite from Camarines Province has the same texture as that 

 just described, but there is less hornblende, and more quartz. The plagioclase 

 is probably zonal andesine, with the outermost zone more alkalic. 



Other quartz-diorites have been found near Talisay and in the Loboo Mountains, 

 in Batangas Province, on Masbate, on Lepanto, and elsewhere. 



Near Sara, Iloilo, there is quartz-diorite which is more or less porphyritic and 

 forms a transition between non-porphyritic fine-grained quartz-diorite and holo- 

 crystalline dacite. One variety is fine grained and slightly porphyritic; that is, 

 perpatic, the inconspicuous phenocrysts, about 5 millimeters in diameter and less, 

 being plagioclase. The groundmass, or chief portion of the rock, is inequigranular, 

 and consists of euhedral, rectangular prismoid and equant plagioclase, zonally 

 developed; with euhedral, long prismoids of brownish-green hornblende, and con- 

 siderable magnetite; together with anhedral poikilitic quartz and orthoclase, which 

 form about 15 per cent of the whole rock. There is a small amount of sphene, 

 intersertal with respect to the plagioclase, a very uncommon mode of occurrence. 



Another variety of this rock, which appears megascopically to be medium- 

 grained and non-porphyritic, is in fact dopatic mediophyric ; that is, there is 

 much groundmass between the medium-sized phenocrysts. It is hyatal. The 

 phenocrysts are euhedral, rectangular prismoid plagioclase, subhedral and some- 

 what rounded quartz; euhedral prismoid hornblende and relatively large subhedral 

 magnetite. The groundmass is micrographic, quartz and clouded alkalic feldspar, 

 probably orthoclase. There is some secondary quartz and zeolite filling what were 

 possibly microlitic cavities. 



On Masbate there are porphyries intermediate between porplryritic 

 quartz-diorite and holocrystalline dacite. 



One variety, from the Guinobatan district, Aroroy, is mediophyric sempatic and 

 seriate, with wide range in the sizes of the phenocrysts. These of plagioclase are 

 euhedral, with jagged outline due to the interference of the crystals in the 

 groundmass. They are notably zonal. The ferro-magnesian phenocrysts have been 

 completely chloritized, and were mica, and possibly hornblende. The groundmass 

 consists of small plagioclase and a matrix of much quartz, both euhedral and 

 anhedral, with some orthoclase intersertal between the quartzes. There is 

 magnetite in small amount. 



Another porphyry from the west of Buyuan Bay, Masbate, is much like that 

 just described. Similar rocks occur in Benguet. 



