16 FERGUSON. 



Furthermore, the earthquake records seem to show a closer coimectioii 

 between the Batanes Islands and the northeast of Luzon than with the 

 northwest.^'' However, these points of tectonic geology will be dis- 

 cussed more fully in a subsequent paper. 



PHTSIOGEAPHY. 

 FORMATION OF THE LAND. 



The first geologic action of which we have any definite record is the 

 building up of the agglomerate, by exjDlosions from somewhere to the 

 southward. At a moderate estimate the agglomerate formation has a 

 volume above sea level of between 1| and 2 cubic miles, and the amount 

 of erosion undergone by the islands shows that this figure represents a 

 small fraction of the original vohunes. That the agglomerate was not 

 built up in a single explosion is shown by the areas of stratified sandstone 

 and conglomerate which occur here and there throughout the formation, 

 showing that there were periods of quiescence of sufficient length to 

 allow streams to work during this period. Nothing can be said concern- 

 ing the region in which the explosions took place except that it was 

 of volcanic formation, for only lava pebbles are found in the agglomerate 

 and this lava is practically all andesitic. 



THE SABTAN UPLAND. 



The first record of the physiographic cycle is foimd in the upland 

 of Sabtan. This belt from 300 to 400 meters above the sea shows a 

 topography which if not that of a peneplain, is at least in advanced 

 "old age" and represents the cycle previous to the present. It is of 

 pre-Miocene age, since Miocene limestones are found on the eastern flanks 

 of the plateau. We have then at the commencement of the latest phys- 

 iographic cycle in pre-Miocene time, a low-lying land mass without 

 marked relief covering at least the area at present occupied by the islands 

 of Desquey, Ibujos, Isbayat, Sabtan, and Batan. 



PERIOD OF TIPLIFT AND EROSIOX. 



The next chapter in the history was one of uplift, accompanied by 

 jDauses in which the limestone terraces of Batan and Sabtan were formed, 

 and possibly by some oscillation. However, this uplift was too rapid for 

 the streams to keep up with it and the present topography of the agglom- 

 erate area is characteristically "young," most of the streams flowing 

 through typical box canons. 



A peculiar feature in the drainage conditions of Batan is the amphi- 

 theatral valley north of Itbod. This is a wide, open valley of irregular, 

 ellipsoidal shape, inclosed between ridges of agglomerate and cut up 



- I'Maso, Rev. M. Saderra. S. J.: Philippine Census (1905), 1, 246. 



