RECONNAISSANCE OF MINDANAO AND SULU : III. 385 



magnesian phenocrysts. There are occasional holocrystalline patches to 

 be seen in the rock. 



Still another hand specimen is a hard, dense andesite which doubtless 

 came from an older part of the mountain. 



This brief summary gives our present knowledge of the geology of 

 Mindanao. We have as yet only a reconnaissance of this vast region. It 

 is hoped and fully expected that it will not be far behind the other parts 

 of the Archipelago when it is once opened by energetic men who will 

 unearth the riches so long buried. 



THE SULU ARCHIPELAGO. 



A host of small islands, rising above a bank which extends all the 

 way to Borneo, lie southwest of the Zamboanga Peninsula and in the 

 same tectonic line. The largest of these are Sulu, Tawi Tawi, Pangu- 

 taran, and Siasi. We have very little detailed knowledge of the geology 

 of this group, chiefly because this is the home of a large part of the Moros 

 with whom the Government, both Spanish and American, have always 

 had trouble. There is very little published information concerning this 

 region, and only two geologists have left us notes on the subject. Mon- 

 tano, whom I have already mentioned, devotes one paragraph to this 

 Archipelago ; it reads as follows : 



The Archipelago of Sulu extends from Borneo to Mindanao in a chain of 

 islands situated upon the summit of a submarine bank. Without doubt several 

 of these islands with a slight elevation consist in great part of banks of corals 

 which gradually reach the surface of the sea, and which, becoming covered with 

 detritus of various kinds, are covered with vegetation. This is especially true 

 of many islands, and notably of that of Sulu, which gives its name to the 

 Archipelago. In the Island of Sulu I have not observed signs of a coral formation 

 except upon the coast and it occurs here homogeneous and formed of a mixture 

 of madrepores and mollusks. It seems to rest upon sand. Its upper level is 

 raised about 2 meters above the highest tides. The mass of the island seems to 

 be of volcanic structure. Some deep trenches opened in the hills near the Spanish 

 village of Sulu (Jolo) show numerous blocks of lava containing pebbles. Some 

 larger blocks of the same rock are scattered over the plain and are seen in 

 abundance in the beds of the streams as well as near the ravines made by the 

 .rain which comes down from the summits of the island. Especially characteristic 

 is the great development of weathered material covered with cogon prairies or 

 forest which conceal the structure of the formation. 



Guillemard,^' in his Cruise of the Marehesa, has written a very in- 

 teresting general account of the crater lakes of Cagayan Sulu. He 

 speaks of a number of circular basins which are filled with water and 

 which he thinks represent tlie craters of extinct volcanoes. Beyond 

 this he gives no geologic data, but he has published a map of the island 

 with additions and corrections by Lieutenant Powell, Eoyal Navy, and 



" The Cruise of the Marehesa to Kamchatka and New Guinea. 2 ed. London 

 (1889), 179-188. 



