RECONNAISSANCE OP MINDANAO AND SULU : III. 387 



glomerate. '\''ertical jointing in Mount Vigia simulates the columnar 

 Jointing of basalt; it is only upon a close examination that the forma- 

 tion is found not to be volcanic. I ascended Mount Vigia and found 

 it to be conglomerate from top to bottom. The matrix is a sandstone, 

 ver}' ferruginous in places, well stratified and dipping southwest at an 

 angle of approximately 15°. The boulders of the conglomerate are 

 andesite, varying in size from pebbles to masses weighing several tons. 

 Just at the edge of the town and but a few feet from the sea, at an 

 elevation of 12 to 15 meters, there is a small shell bank, partly coralline. 

 The shells are very recent. With the exception of Mount Vigia, which 

 owes its height to the resistent material in it, the island is very low. 



I made the following notes while passing close to the Island of Tawi 

 Tawi. 



I saw no signs of vulcanism on the island. There are few high points; in 

 fact, the sky line is much like that of northern Cebu. A forest fire in 1903 

 decimated the forest and now there is nothing but straggling timber and thick 

 undergrowth. (We skirted the northern side of the island). Tawi Tawi Moim- 

 tain may be volcanic, but old. A very white rock outcropping on the side of 

 this mountain is seen with a glass. It may be limestone. 



A few miles northwest of the town of Jolo are a number of islets 

 which have peculiar profiles (Plate V). These islands are very low, for 

 the most part with shallow' depressions in the interior, sometimes dry 

 and sometimes filled with water. They are largely composed of coral, 

 but on Marongas Island there is an exposure of conglomerate which is 

 cut by more or less vertical basalt dykes. These dykes vary from a few 

 centimeters to a meter in width. On either side of the dykes the rock 

 has been vitrified for several centimeters. An examination of the dyke 

 rock under the microscope shows a very fine-grained lava with distinct 

 flow lines and phenocrysts. The phenocrysts are olivine and plagioclase 

 feldspar. The streaks, which have a fluxion arrangement, consist of a 

 very dense cryptocrystalline mass in which innumerable minute mag- 

 netite grains occur. There is a rude parallelism of the crystals border- 

 ing these denser streaks. The rock is very vesicular in part. The hum- 

 mocks on this island and several others are simply due to the hard 

 conglomerate and dyke formations. 



The history of the Archipelago briefly stated is as follows: A sub- 

 marine bank, which may represent a buried mountain chain, rose slowly 

 from the sea. Corals grew upon this, and at a number of points, flows 

 of volcanic material penetrated and poured out over the country. The 

 period of greatest activity was probably in the Pleistocene. There is 

 little or no volcanic manifestation at present. The weathering of this 

 volcanic material has produced a heavy and very ferruginous deposit not 

 unlike the laterite of India. I know of no economic deposits being 

 worked anywhere in the Sulu Archipelago. 



