( 



18 FERGUSON. 



compared with mainland conditions. The erosive power of streams is 

 dependent very largely upon the size of their drainage area. The one 

 of two streams receiving more water because it has a larger drainage 

 area, has the advantage over one with a slightly better grade, but a 

 smaller drainage area. Hence, we must not look for the same proportion 

 of work of stream erosion on small islands as would be.found on Luzon. 

 However, another point to be considered here, is that the present strip 

 of Sabtan upland is bounded on the west by a fault scarp and that the 

 western streams have scarcely begun their work. 



SINKING OF IBUJOS-DESQUET. 



The sinking of the land to the west of Sabtan must have taken place 

 toward the end of the uplift, for the western coast of Sabtan shows 

 no traces of terraces or other evidence of having' been submerged. Prob- 

 ably the period of greatest movement was contemporaneous with that 

 of greatest activity among the volcanoes to the east of the line of faulting. 

 Movement along this line is in all likelihold still progressing. This is 

 indicated by the prevalence of earthquakes in the Batanes, where the 

 records for 1906 show eleven shocks, as against three at the mouth of the 

 Cagayan Eiver (Aparri) and on the northwest coast of Luzon (Vigan).^* 

 In five years (presumably 1898-1902) only twenty- five earthquakes were 

 recorded in the southern part of Formosa (Taiku).^'' 



The downthrow of the western part of the older plateau could not 

 have depressed the land to a point below that at which coral can grow 

 (100 fathoms). Coral, formed upon this depressed shelf and later ele- 

 vation affecting both sides of the fault, with perhaps some reverse move- 

 ment of the fault blocks, has brought the western islands, Ibujos, 

 Desquey and Isbayat to their present elevation. 



DEPEESSION. 



It is not to be supposed that the period of uplift was uninterrupted. 

 The benches which give us our evidence of elevation are in themselves 

 indications of stationary periods and hence it is not improbable that, 

 although uijlift has been the dominant feature of the history of the 

 islands, there also may have been small periods of depression or tilting. 

 The persistence of the 20-fathon shelf around Sabtan and Batan and 

 the submarine contours of Santo Domingo Bay (Map No. 3) may 

 indicate a depression. On the other hand, this 20-fathom shelf may be 

 indicative of the amoimt of marine erosion accomplished during the 

 time that the islands have remained at their present level, showing the 

 depth to which wave action is effective in the region. The islands, as has 

 already been stated, lie in the track of the majority of the typhoons, lience 



"Algue, J.: Monthly Bulletins of the Observatory, Manila, 1906. 

 1= Davidson. .J. W. : Formosa, Past and Present (1903.) 



