PETROGRAPHY OF IGNEOUS ROCKS OF PHILIPPINES. 165 



2. COARSER-GRAIXED IGNEOUS BOCKS. 



Syenite. — A medium to fine grained rock occurs about 1 mile east of the summit 

 of Mount Aroroy on Masbate. It is non-porphyritic and consists of rectangular, 

 prismoid to equant, anhedral feldspars, that are to some extent twinned in the 

 Carlsbad manner, are cloudy orthoclase, probably sodic. Their arrangement is 

 diverse. There is a subordinate amount of ferromagnesian minerals in smaller 

 crystals included in the feldspars, and also intersertal to them. They are chiefly 

 brown mica, partly chloritized, diverse in arrangement; some magnetite, and long 

 thin needlelike crystals of apatite, with also diverse arrangement. There is some 

 scattered chlorite, epidote and secondary quartz. 



Another variety of syenite on Masbate from the south side of the Guinobatan 

 River, south of Mount Bagadilla, is like the one just described in texture, grain, 

 and the character of the feldspars, but contains more ferromagnesian minerals; 

 biotite in prismoids, of blades; pale green augite, partly replaced by calcite; a 

 small amount of what appears to be serpentinized olivine; also magnetite and 

 apatite. The rock approaches shonkinite in composition, but has rather more 

 feldspar than ferromagnesian minerals. These syenites are undoubtedly related 

 genetically to the leucitic lavas and are described in connection with them for 

 this reason. Other syenitic rocks, chiefly albite, will be described after the 

 commoner phanerocrystalline rocks. 



There are rocks in the collection that are normal gabbros with inequi- 

 granular consertal fabric, sometimes called "granitic," and others with 

 ophitic fabric, also called "diabasitic." They grade through finer-grained 

 porphyritic varieties into holoerystalline pyroxene-andesites, and basalts. 

 They also grade into diorites and quartz-diorites by variations in the 

 mineral composition, just as the basalts grade into pyroxene-andesites, 

 and these into hornblende-andesites and dacites. The extremes of the 

 mineral variation, or differentiation in this region appear to be albitic 

 granite and albitic syenite on the one hand, and peridotite and possibly 

 pyroxenite on tlie other. The alteration of pyroxene into fibrous am- 

 phibole, uralite, and also into compact hornblende in some instances, 

 leads to the metamorphism of some of the gabbros into metadiorite, which 

 in such cases is usually recognized by the character of the feldspar and 

 the texture of the rock. 



There are not sufficient data at hand to determine the relative abun- 

 dance of the different magmas and so indicate the composition of the 

 average or "parent" magma. It appears that the coarser grained, in- 

 trusive rocks have attained a higher degree of differentiation than the 

 extrusive lavas, but this may not be the actual fact, and further study of 

 the region may modify this conclusion. 



The best known active volcanoes are erupting lavas, but are not ex- 

 tremely differentiated. The modern lavas are not all alike, and some of 

 the older ones are highly differentiated from the probable parent magma, 

 as, for example, the dacite on Corregidor and the leucite rock on Masbate. 



Gabbro. — The gabbros with consertal, inequigranular fabric vary from 



