GEOLOGIC RECONNAISSANCE OF SOUTHWESTERN LUZON. 77 



Becker, in summing up what was known concerning the older rocks 

 of the Philippine Archipelago, said that there is no inconsistency in 

 Abella's generalization that the dioritic rocks are the oldest in the 

 Philippines if the schists of similar composition are included under the 

 head of dioritic rocks. In the reports by Eveland on the Mancayan- 

 Suyoc mineral region and the B.aguio mineral .district, the basal rocks 

 are stated to be diorites; thus concurring with Abella's views concerning 

 the central cordillera of Luzon, on the western slope of which these 

 districts are located, far to the north of the area here under discussion. 

 However, there are no grounds for this conclusion. The oldest rocks so 

 far as now known are a complex containing gabbros accompanied by 

 periodites and pyroxenites besides some diorite and granite. 



Andesites. — In the northern part of the region under discussiou, 

 andesitic pebbles are very commonly found among the stream gravels. 

 and many of the mountains have probabty been formed by the eruption 

 of andesites. The andesites in Jalajala Peninsula, which the writer has 

 seen, were described by Both as dolerites. The northward continuation 

 of this area was crossed by Ickis, who indicated its borders in his section 

 from Infanta to Tanay. Ickis also described an area of andesites further 

 to the east near Infanta and separated from the one just mentioned by 

 a belt of sedimentaries. On the geologic map accompanying this report 

 these areas are indicated approximately, and also some additional ones 

 which were seen by the writer in traveling from Antipolo to Santa Inez. 



Basalts. — A greenish, ophitic basalt outcrops in the vicinity of Ango- 

 no, and extends northward passing to the east of Taytay, and is exposed 

 in some of the railway cuts on the line between Taytay and Antipolo. 

 In places the rock has a fine-grained texture and is frequently brecciated 

 and in places is amygdaloidal. The outcrops indicate an eruptive mass 

 trending a little to the west of north, the basalts and the limestone belt 

 further to the east being approximately parallel with it. To the south- 

 east of Angono this rock is overlain by later basaltic tuffs and breccias. 

 The best known occurrence of basaltic rocks in the part of the eastern 

 cordillera covered by this report is situated in the peninsular area on 

 which the towns of Binangonan and Morong are located and in Talim 

 Island, where they have been extensively quarried. Becker has published 

 petrographic descriptions of typical basalts, which he collected on Talim 

 Island. Hochstetter described Punta Gunong Bajang and Punta del 

 Diablo by Binangonan as formed of the most remarkable obsidian columns. 

 An examination of these places by the writer failed to verify his state- 

 ments, but basalt showing flow structure was found. Both, evidently 

 following Hochstetter to some extent, considered the "doleritic lavas" of 

 Talim Island and those in Jalajala Peninsula, together with the oc- 

 currence of "obsidian" near Binangonan, indicating a grand volcanic 

 center. Hochstetter's ideas concerning this is expressed in Von Drasche's 



