166 IDDINGS. 



those with nearly eqnigranular feldspars to others with intersertal fabric, 

 due to the abundance of smaller crystals between the larger ones. There 

 is thus a transition to seriate porphyritic fabric, and the texture of 

 holocrystalline andesites. The similarity of the component minerals also 

 indicates a genetic relation between these coarser grained rocks and the 

 extrusive lavas in this region. Some of the freshest examples in the col- 

 lection will be described. 



An olivine-gabbro occurs in river gravel at Montalban. It is medium to coarse 

 grained, inequigranular, consertal; and consists of labradorite (AbA.n 3 ), pale 

 green augite, colorless olivine, and very small amounts of primary hornblende, 

 biotite and magnetite with some secondary minerals locally developed. The 

 proportion of feldspar to ferro-magnesian minerals is about 3 to 1. 



The labradorite is anhedral, without zonal structure, is somewhat twinned 

 according to the three common laws, albite, pericline and Carlsbad; and contains 

 a small amount of minute rod-like inclusions. The augite is anhedral; has a 

 distinct pinaeo/dal cleavage characteristic of diallage ; is twinned, and contains 

 rod-like inclusions. The olivine is also anhedral; in places it is intersertal with 

 respect to the labradorite, showing its later crystallization. There is a narrow 

 border of minute anhedrons, probably amphibole, between the olivine and labra- 

 dorite. The proportion of augite to olivine is about 6 to 1. Primary green 

 hornblende surrounds augite and magnetite in some places, and light brown biotite 

 in very small amount is associated with the hornblende. The magnetite is 

 associated with the augite, in juxtaposition and also intersertal between augite 

 anhedrons. The intersertal position of much of the augite, olivine, magnetite 

 and primary hornblende between the larger anhedrons of labradorite indicates 

 the synchronous crystallization of the ferro-magnesian constituents together with 

 some labradorite, as the last act of the crystallization of the magma, after much 

 feldspar had crystallized. In places this rock is altered, and secondary green 

 hornblende has been formed in narrow veins traversing altered feldspar and other 

 minerals. This shows the ordinary metamorphism to metadiorite. Olivine-gabbro 

 similar to that just described in texture and composition occurs in Albay Province. 



A coarser-grained olivine-gabbro of this type occurs in Nueva Vizcaya. The 

 labradorite is more abundant, and is about five times as much as the olivine and 

 scarce augite. There is no magnetite, but some secondary amphibole. 



At Nagcarlang, at the base of Banahao Mountain, Laguna Province, there is 

 gabbro having the same type of texture as that just described. It is without 

 olivine or magnetite, and consists of monoclinic and orthorhombic pyroxene with 

 some pale green secondary hornblende, and a smaller amount of labradorite. 



At Subig, in Zambales Province, there is gabbro with medium grain, inequi- 

 granular, seriate, consertal fabric, composed of approximately 5 parts of labra- 

 dorite and 1 of pyroxene, without olivine or magnetite. There are some stout 

 prismoids of labradorite surrounded by equant anhedrons of the same mineral. 

 Some of the pyroxene is irregularly shaped, relatively large anhedrons partly 

 surrounding labradorite, while other pyroxenes are intersertal to the feldspar, and 

 some are small anhedrons. All of the pyroxene is uralitized. 



A medium-grained gabbro occurs in Benguet. Its fabric is inequigranular 

 consertal, with rectangular subhedral prismoids of feldspar and pyroxene in 

 diverse arrangement. The feldspar and ferro-magnesian minerals are in nearly 

 equal proportions. The feldspar is labradorite (Ab 2 An 3 ), without zonal structure. 



