PHYSIOGRAPHY OF THE PHILIPPINE ISLANDS. 7 



Panique Creek and various small tributaries of the Lanang, and would 

 run along the northwestern flank of Mount Vil-lou. The headwaters of 

 Panique Creek and Kaal Creek also form a northeasterly trough. It is 

 quite probable that the cutting off of the vein series may be due to a 

 fault along the northwestern side of Mount Vil-lon. Since the vein 

 system is later than the andesite breccias^ Mount Yil-lon can hardly be 

 an intrusion of later date than the period of vein formation, and the vein 

 series being regular rather than radiating, can not owe its origin to 

 fractures formed around an intrusive mass. 



In the westerly stretch of the Lanang, between the two gorges, is an 

 abandoned meander of the river, its course having been shortened by 

 lateral erosion on the convex sides of the two bends. The hill between 

 the old and new' courses is composed of a firm conglomerate of basalt 

 pebbles, a common country rock of the surrounding region, and is about 

 50 meters above the river. The river must, therefore, have been at grade 

 at least 50 meters above the present level, and this implies an uplift. Its 

 course having been already determined for it, the cutting through of the 

 lower gorge allowed the stretch above to remain at grade, and as the 

 gorge was cut down its meander became entrenched and it could only 

 have been in comparatively recent times that the river, being nearly at 

 grade up to the upper gorge, cut through the neck of its old meander. 

 The deserted 'meander is filled with old river gravels and furnishes 

 promising dredging ground, as the Lanang and its tributaries cut the 

 Mount Cogran vein and several as yet undeveloped veins to the west of 

 Mount Cogran. 



The course of the Lanang gives the impression that the present stream 

 is the result of the union under peculiar circumstances of two streams 

 flowing northwest, being a case where captures have tended to lengthen 

 rather than simplify the course of the stream. The evidence of the 

 streams in the Aroroy district seems to point to a period of elevation 

 after the courses of the streams were in a large degree determined, 

 followed by a period of considerable depression when deep bays were 

 formed and the dark blue limestone found in the valley of the Guinobatan 

 at 150 meters' elevation was deposited. Uplift followed, and during 

 this period the readjustments of drainage of Balanting Creek and the 

 Lanang region took place. Since then there seems again to have been 

 some slight depression shown by the slight emba3'ment at the mouths 

 of rivers, and the extent of the mangrove swamps. 



The growth of the mangrove swamps has played an important part in 

 the formation of the land on this side of the bay. The mangrove tree 

 must grow with its roots in the water, and the network of roots catches 

 debris and soon builds up the land. As solid ground is built up, the 

 inland trees die and the swamp keeps advancing seaward, building up 



