72 



ADAMS. 



the accompanying sketch map, fig. 2, the author has attempted a recon- 

 struction of the land area of southwestern Luzon as it was before the 

 emergence of the sediments found on the flank of the western cordillera. 

 The Pico de Loro Mountains were probably an island as were also those 

 of the Mariveles and Cinco Picos groups. The Natib Mountains formed 

 a peninsula from the mainland which included the Pinatubo group. 



Fig. 2. — Sketch Map Showing Probable Land Abeas Before the Emergence of 

 the Central Plain and the Plains of the Southwestern Volcanic Region. 



Corregidor Island was low lying and probably, as erosion progressed, it 

 was reduced to a shoal. The area corresponding to the central plain of 

 Luzon was a broad strait. 



CENTRAL PLAIN REGION. 



The portion of this region included within the area described in this 

 report is occupied principally by an alluvial and littoral formation. To 

 the west it borders on the igneous formations of the western cordillera; 



