MINDANAO AND SULU : II. PHYSIOGRAPHY. 359 



THE APO VOLCANIC CLUSTER. 



Mount Apo is the highest peak in a volcanic cluster on the west side 

 of Davao Gulf. This peak, although higher, is not nearly as large as 

 the old crater to the northeast. As I have said elsewhere, I seriously 

 doubt if the present Mount Apo ever has been an active volcano. The 

 old mountain, the two highest points of which are called Culelan and 

 Pumantigan, was doubtless the center of a very great disturbance in 

 former times, as the nature of eruptive material indicates an explosion 

 or series of explosions which must have devastated the country for many 

 miles. The photograph (frontispiece) was taken from Digos. 



I have no first-hand information concerning Catarman, the only active 

 volcano which might be considered as belonging to Mindanao, it being 

 on the little Island of Camiguin off the Misamis coast. An extensive 

 account of this volcano and the eruption of 1871 has been written by 

 Father Maso, S. J. 13 



SHORE FEATURES. 



Coral reefs. — There are a number of phenomena along the shore 

 which vitally affect the life and customs of the inhabitants of the island. 

 Coral reefs are perhaps the most conspicuous of these features. The 

 latter are found in scattered patches all around the island. However, 

 the growth is not great on exposed coasts like the stormy eastern one, 

 but in sheltered bays like Sibuguey and Dumankilis it is exceedingly 

 difficult to navigate because of the reefs. They act as danger spots to 

 the navigator and as a protection to the people on shore, for besides 

 making it difficult for invaders to land they break the seas in the time of 

 great storms. A remote benefit due to reefs, remote because of the 

 exceedingly long period of time which must elapse before it can be 

 realized, is that they aid in the growth of the land. The coral polyps 

 grow up to a limiting line which is the level of the sea, and upon this 

 platform is deposited the detritus borne down from the mountains. In 

 time this will be dry land. If the whole body of the land is rising, as 

 is the case with much of Mindanao at the present time, this growth 

 proceeds with comparative rapidity. The prominent part that reefs 

 have played in the formation of Mindanao can readily be realized when the 

 raised reefs in many parts, particularly those found following the valley 

 of the Eio Grande, are seen. 



Terraces. — One of the most striking of all physiographic phenomena, 

 the significance of which is not always appreciated by the layman, is 

 the terracing along the seashore and on the sides of the valleys. 



"Maso, Rev. M. Saderra, S. J. Volcanoes and Seismic Centers, 208-215. Phil- 

 ippine Census. 



