74 ADAMS. 



of stone should be classed as volcanic bombs. Near Porac lie discovered 

 a dolerite tuff formation, which he described as extending up onto the 

 flanks of the cordillera to an elevation of 1,000 meters. He doubted if 

 the whole formation had been deposited in water. The part of this tuff 

 formation adjacent to the plains near Camp Stotsenberg is probably 

 water laid. Both Porac and Camp Stotsenberg lie well to the north of 

 the area which is mapped in this -report, but the formation obtained at 

 these localities proves that the tuff formation extends under the alluvium 

 and it is probable that it will be encountered in drilling wells near the 

 western border of the plains. The exposures of the tuff near Camp 

 Stotsenberg differ so materially in their general appearance from the 

 tuffs of the eastern border of the central plains that they can not be 

 correlated on lithologic grounds. 



Alluvium. — The part of the central plain included in the geologic 

 map is very largely occupied by the delta of the Pampanga Eiver, which 

 forms a network of channels dividing the delta into many islands. 

 These island areas are but little above sea level and many of them are 

 jnartially overflowed at high tide; those lying lower are occupied by 

 groves of mangroves and nipa palms. West of the delta the alluvium 

 rises toward the mountains and has the form of an aggraded plain. The 

 part of the plain which receives drainage from the western cordillera 

 may be distinguished by the presence of the plagioclase sand and frag- 

 ments of andesitic rocks which give rise to the sandy soil. The eastern 

 border of the delta system is limited by contact with the tuff formation, 

 and the sediments which reach it from the eastern cordillera are car- 

 ried by streams which flow in deep, well-defined channels. The larger 

 streams which head in the mountains carry sand and gravel. There is a 

 deposit of gravel at Caloocan which probably represents a former delta 

 deposit of the Tinajeros Eiver, which now flows to the north of the town 

 at a lower level. This bed of gravel, as shown by the records of wells at 

 Caloocan, is about 30 meters thick. The sediment brought by the Pam- 

 panga Eiver to the head of the delta is nearly devoid of gravel. 



While the alluvial deposit covers a large part of the central plains as 

 a thin veneer, underneath it there is a series of beds which are marine, 

 or at least deposited in brackish water. These beds are encountered in 

 drilling wells and may be distinguished by the presence of numerous 

 shells and marine silts and muds. At Tarlac, which lies near the eastern 

 border of the plain and a little more than halfway from Manila to 

 Dagupan, shell beds are found at a depth of about 4 meters. Details of 

 their occurrence are given by Centeno, who states that shafts were sunk 

 to the beds in order to obtain the shells for burning lime. This oc- 

 currence, together with the records of the wells, would seem to prove that 

 marine sediments underlie a large part of the plain. It is probable 

 that a strait once extended from Manila Bay northward to Lingayen 

 Gulf and that the gradual elevation of the Island of Luzon transferred 



