GEOLOGIC RECONNAISSANCE OF SOUTHWESTERN LUZON. 



73 



In places it 



to the east, the alluvium overlies a water-laid tuff formation which in 

 turn extends to the foot of the eastern Cordillera. 



Inasmuch as this region consists very largely of low-lying land, there 

 are but few exposures in which the formations may be studied. However, 

 fortunately, a large number of wells have been drilled and the records 

 of these throw some light on the stratification of the alluvial deposits 

 and their relation to the underlying marine sediments which are not ex- 

 posed and the tuff which forms a belt on the east side of the region and 

 extends under the alluvium. No wells have been drilled in the western 

 part of the area and there the relations are not so well understood. 



Water-laid tuff. — This formation is a continuation of the tuff which 

 has a wide extension in the southeastern volcanic region. It is usually 

 clearly stratified and exhibits beds of variable thickness, 

 grades into clayey, somewhat shaley beds and it oc- 

 casionally contains a conglomeratic phase, especially 

 near the foothills of the eastern cordillera. It is 

 probable that a large part of the tuff deposits was 

 thrown out by the volcanoes of the southwestern 

 region, but certainly some sediments must have been 

 derived from the adjacent cordillera. 



The records of the wells which have been drilled 

 in the tuff show occasional beds of marine sands Fl< *- 3 'T7, Sh * I ' k s } eetb , 



(Squahdae?) found 



and some strata which are composed of waterworn 

 gravels and fine pebbles. Occasionally a log of 

 wood has been encountered in drilling, and plant 

 remains, fish teeth (fig. 3), and one mammalian 

 tooth have been found in the beds. The presence of the plant remains 

 has been recorded by many observers. The greatest depth at which a 

 log of wood has been found is in the Alabang wells south- 

 east of Manila, where on was cut by the drill at a depth 

 ofbetween 130 and 132 meters. The mammalian tooth 

 (fig. 4) Avas obtained from the Pasig well at a depth 

 somewhere between 81 and 85 meters. 



ISTo tuff formation is exposed on the western border fig. 4. — Mamma- 

 lian tooth (cf. 

 antelopes of the 

 Siwalik plio- 

 cene of India) 

 found at a 

 depth of be- 

 tween 81 and 

 85 meters in 

 drilling a well 

 at Pasig. 



in a railway cut in 

 water-laid tuff on the 

 crest of the ridge 

 between Bay Lake 

 and Manila Bay. 



of the central plains within the area discussed in this 

 report. It will be remembered that the marine conglom- 

 erates on the flanks of the western cordillera are cor- 

 related with the tuff deposits of the southeastern volcanic 

 region, but these conglomerates have not yet been seen 

 adjacent to the central plain. A^on Drasche saw, at 

 Porac to the north of Floridablanca, a tuff formation 

 in an excavation that had been made for laying the foundations of 

 a church. It there consists of layers of sand with fragments of rock, 

 similar to those commonly presented in the western cordillera, and clay 

 beds interstratified. Von Drasche was of the opinion that the fragment's 



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