GEOLOGIC RECONNAISSANCE OF SOUTHWESTERN LUZON. 67 



dacite. Besides these extensive rocks there are gabbros and peridotites 

 and some metadiorites. 



The southern part of the cordillera in Bataan Province was mapped as 

 dolerite by Von Drasche following the statements of Roth. Becker, 

 who examined rocks found near Mariveles, classified them as andesites. 

 He says that they vary from the basaltic to the trachytic type, but they 

 are all essentially labradorite rocks without ophitic structure and that 

 for the most part they are pyroxene-andesites containing augite and a 

 dichroic, rhombic pyroxene, biit one gray rock quarried (at Sisiman) 

 for use in Manila is hornblendic. He says that he is rather explicit 

 about the rocks of this locality because Both calls the rocks of this 

 region dolerites. Further on he remarks that the rocks of the Semper 

 collections from Mariveles correspond with his own in most respects. 

 being labradorite rocks more or less glassy without ophitic structure, 

 and that portions of the Mariveles rocks examined by Oebbecke were 

 found to carry hornblende in addition to augite, and some grains of a 

 mineral, which is probably olivine, were detected. The Pico de Lore 

 Mountains south of the entrance to Manila Bay were classed by Both as 

 doleritic. The specimens obtained from this area are andesites. in 

 every respect similar to those of the Mariveles coast. 



The dacite which was discovered by Becker on Corregidor Island was 

 described by him. He says in part : "It is blindingly white and shows 

 to the naked eye besides feldspar only some quartzes and minute opaque 

 mica scales. . . . The bulk of the rock is made up of feldspar micro- 

 lites with just a sprinkling of magnetite and a little apatite, . . ." 



Von Drasche crossed the cordillera to the north of the area here under 

 discussion. In describing the structure along his route from O'Donnel 

 to Iba, he says : "It appears that the sierra consists principally of dioritic 

 and peculiar diabase gabbro rocks which often exhibit bedded structure. 

 These rocks are in intimate relation with ordinary gabbros and serpentines. 

 On the eastern slope of the southern half of the sierra one finds these 

 rocks overlaid by a thick, trachytic tuff formation which incloses numerous 

 fragments of trachyte. This tuff can be traced up to the watershed 

 (pass) at a height of 3,000 feet and to the east it is related to the plains 

 of Pampanga (central valley), the floor of which consists principally 

 of the weathered products derived from it. The crystalline rocks must 

 be broken through at numerous points by trachyte, since such rocks 

 are found in great numbers among the bowlders derived from the sierra." 



He describes especially what he called a sanidine hornblende-trachyte 

 found near Porac, and Abella the occurrence of a similar "sanidine 

 trachyte" with porphyritic texture which he found cutting older slates 

 near O'Donnel. Smith has shown this rock to contain chiefly plagioclase 

 feldspars and to form Mount Pinatubo. At Olongapo there is a small 



