PHYSIOGRAPHY OF THE PHIMPPINE ISLANDS: II. 9 



one built up in the old explosion crater, while on the southern side there 

 is a hill of tilted basalt which may be the remains of a still older crater. 

 The present crater is horseshoe shaped, being broken down on the north- 

 ern side where the last lava tlow, a stream of basalt, has emerged. Numer- 

 ous, small fissures are seen in the crater and one cuts across its southern 

 wall. These fissures are probably the result of earthquakes. Nearly all 

 the lava flows are basalt, but the earliest seem to be andesite, although no 

 petrographic study of them has as yet been made. 



The end of the flow of basalt from the present crater is exposed in a 

 sea cliff on the northern shore of the island, and this tells in some detail 

 the story of the last eruption of Mount Iraya. The mountain had been 

 quiescent for a period long enough before the eruption to allow a con- 

 siderable stream valley to be cut through the bedded deposits of volcanic 

 debris which form the cliffs of this neighborhood. The renewal of its 

 activity was marked by considerable explosive force which probably blew 

 away part of the northern side of the present cone and nearly filled 

 the valley with a mass of angular fragments of volcanic material. 

 The latter part of this explosive phase was marked by the j)resence of a 

 number of basaltic bombs. Finally, a stream of basalt several feet thick 

 flowed down this valley, completely filling it. Since then the lava flow 

 has itself been buried under the mass of loose material constantly creeping 

 down the slopes of the mountain. 



The nearly flat region aroimd Santo Domingo and stretching across 

 the island owes its form to the piedmont wash from Mount Iraya. The 

 hyperbolic curve of the mountain must at one time have been continuous 

 from sea level to summit, but marine erosion has cut off the lower end, 

 leaving sea cliffs varying from 1-5 to 60 meters in height truncating the 

 piedmont plain and of much greater height where the waves have en- 

 croached upon the actual slope of the mountain in the extreme north- 

 eastern part of the island. (Fig. 3.) 



Pig. 3. 



