■PHYSIOGRAPHY OF THE PHILIPPINE ISLANDS: II. 19 



marine erosion must be a very significant feature. It is to be regretted 

 that there are no soundings on the Pacific side of Batan, as it wonld tend 

 to clear up this point if we knew whether this submarine shelf were 

 deepest on that side. Naturally, where open to the Pacific, the waves 

 would have greater force than on the western side, hence it is to be 

 expected that if the typhoon waves of the China Sea are efEective to a 

 depth of 20 fathoms, those of the Pacific should cut a deeper bench. 



MAKINE EROSION. 



In all probability Ibujos and Desquey were one island within recent 

 geological time, as was the case with Siayan and Mabudis and also 

 Y'Anii and May sanga. It is not improbable that Isbayat may have 

 been separated from the other limestone islands by marine erosion, 

 and that the four Siayanes Islands may at one time have formed one 

 island. If this is so, then the amount of land lost through marine erosion 

 must nearly equal the present area of the islands. 



The land, at least Batan Island at the present time, is essentially 

 stationary. This is shown by the high sea cliflrs of unconsolidated 

 material around Mount Iraya. A sea clifE represents the shoreward 

 limit of eifective action of storm waves. The action of waves upon the 

 coast, in addition to the formation of a clifl^, tends to build out a sub- 

 marine shelf, partly through cutting away the land and to a less degree 

 by depositing the material carried out by the undertow. The relation 

 of this shelf to a receding cliff must always be such that the shelf must 

 be kept worn down to such a depth by the abrasive action of material 

 carried out by the undertow, that the waves will be able to do effective 

 work against the cliffs. The analogy to stream conditions is very close. 

 Under normal, stationarj' conditions of the land the imdertow near the 

 coast acts in time of storm as a degrading stream, the force of the 

 waves hurling pebbles against the face of the cliff, loosens great blocks 

 which are later broken up to a size suitable for transportation. These 

 are carried seawards by the undertow which has sufficient force to keep 

 the wearing down- of the beach and shelf in such relation to the point 

 where the waves are tripjDed, that the latter are able to do their most 

 effective work on the cliff and to keep the undertow constantly supplied 

 with new material. When the undertow reaches deeper water it neces- 

 sarily has less velocity and consequently less carrying power. The ma- 

 terial borne out is therefore deposited, the coarser nearer shore and the 

 finer farther out, just as a stream degrading near its head deposits its 

 material wlien its velocity and can-ying power become less. Since the 

 carrying power of the undertow varies as the 3/2 power of its volume 

 and the sixth power of its velocity," it is clear that only during times 



''Gilbert, G. K.: U. S. G. S. 5th An. Rep. (1884). 80. 



