PETROGRAPHY OF IGNEOUS ROCKS OF PHILIPPINES. 161 



andesine-oligoclase, and abundant, euhedral, bipyramidal quartzes, 2 in a matrix 

 of anhedral quartz and intersertal alkalic feldspar. 



A similar dacite occurs east of the limestone, in the waterworks gorge at Mfln- 

 talban, Rizal Province. It is perpatic and mediophyric with few white phenocrysts 

 in the specimen. The groundmass is coarser, and more granular, the euhedral 

 and anhedral microscopic quartzes are somewhat larger than the minute prismoids 

 of feldspar, and their outline is less sharply defined than those in the dacite from 

 Corregidor, there is much minute subhedral magnetite, and globules of pyroxene, 

 besides some chlorite. 



Megascopically nonporphyritic dacite., or possibly rhyolite, occurs in 

 several localities. Its composition can not be determined by the micro- 

 scope, because of the scarcity of the recognizable crystals. It is not 

 known whether the feldspathic constituents are distinctly alkalic, or calci- 

 alkalic. 



At Montalban the rock is microscopically porphyritic, seriate and dopatic, with 

 microscopic phenocrysts of quartz, subhedral and also embayed, with inclusions 

 of prismatic feldspar; besides phenocrysts of alkalic plagioclase. The groundmass 

 is mierocryptocrystalline, with chlorite and iron oxide. Another variety at 

 Montalban is green, and has the same kinds of microscopic phenocrysts. The 

 groundmass is altered, brecciated, pumiceous glass which is now mierocryptocrys- 

 talline or microspherulitic. There is much chlorite in the pores of the pumice. 



On Cebu somewhat similar rock occurs. It has the fabric of collapsed pumice; 

 that is, it consists of devitrified. welded, angular fragments of glass, now miero- 

 cryptocrystalline. In it are small fragments of quartz, altered feldspar, and a 

 few shreds of brown mica. 



One variety does not appear brecciated, but consists of a brownish-gray ground- 

 mass, microgranular with indistinct spheralitic radial fabric. The granules appear 

 to be minute anhedrons of quartz and alkalic feldspar with some magnetite. 

 The spherulitie rays appear to be prismoids of ill-defined alkalic feldspar. These 

 are small phenocrysts of sanidine. 



Basalts. — There are transitions between olivine-bearing pyroxene-ande- 

 sites and basalts rich in olivine, so there are basalts with the textures 

 found in andesite, and others with texture not developed in andesites. 

 No line can be drawn between these two groups of rocks; and petro- 

 graphers differ as to the classification of rocks intermediate between 

 basalts and andesites. It happens that the lavas of Mayon and Taal 

 Volcanoes belong in part to these intermediate varieties, which may be 

 called olivine-bearing pyroxene-andesites or andesitic basalts, while other 

 varieties of lava from these volcanoes are normal basalts, with abundant 

 olivine. 



Basalts with andesitic habit occur in Batanes. In two cases the rocks have 

 the composition of hypersthene-augite-andesite with small phenocrysts of colorless 

 olivine partly altered to iddingsite; the groundmass being holocrystalline. with 



2 These microscopic crystals of quartz were considered to be feldspar by Geo. F. 

 Becker in his description of this rock. U. 8. Geol. Surv. 21st Ann. Rept. (1001) 

 Part 3. 516. 



