412 FERGUSON. 



into the mangrove swamp which, is developed at the mouth of the river, 

 but the northern one flows through the paddies, parallel to the main 

 stream for about one kilometei", to a point where a small difference in 

 grade allows it to join the Lanang. The upper portion of the Guinobatan 

 Valley contains a small gravel plaiu just above the gorge, and ia the 

 gorge itself a well-marked gravel bench occurs a few meters above the 

 river level. This portion of the river probably furnishes the most 

 promising location for placer mining to be found in the district, but it 

 has never been prospected properly. The broader flood-plain of the 

 Guinobatan below the gorge was the site of the operations of the dredge 

 of the Oriental and Masbate Company. It is reported that marine clay 

 was encountered at a depth of from five to eight meters. This is not 

 improbable, considering the changes of level which the district has 

 undergone in comparatively recent times. 



Except for the swamp at the mouth of the Lanang Eiver, the largest 

 areas of mangroves are not shown on the map, but are found along the 

 southern shores of Port Barrera. Whitford,^" in his report on the 

 timber of the district, estimates the area of mangrove swamp at about 10 

 square kilometers. 



Summary of geologic history. — The Kaal formation, the oldest known 

 in the district, is entirely of sedimentary origin and must have been 

 derived from an earlier land mass of which no trace exists in the district. 

 At some later time the intrusion of a large amount of quartz diorite 

 took place, only a small portion of which is exposed in the area covered 

 by the map. Following this, the land was exposed to long-continued 

 erosion which laid bare the mass of the quartz diorite intrusion. The 

 uplift, which bowed up the southeasterly anticline which to-day forms 

 the backbone of the island of Masbate probably came in late Eocene times. 

 A period of vulcanism accompanied this uplift, during which the dike 

 flows and p}Toclastic rocks which constitute the Panique series were 

 formed. The principal period of vein formation followed close upon 

 this. The universal northwesterly trend of the veins is evidence that 

 the Assuring was resultant upon the continued bending up of the anticline. 

 Even later than this period of vein formation, igneous action again took 

 place, as is shown in the dike on the Gold Bug claim. The leucite tephrite 

 is in all probabilit}'- of the same late date. A little later, or perhaps 

 contemporaneous with the intrusion of these dikes, there was faulting 

 in an east and west direction in the region of the present Guinobatan 

 Eiver. Mineralizing solutions filled these fissures giving the minor 

 series of east and west veins which cut the older and more important 

 series. Probably somewhat earlier than this time the submergence took 

 place w^hich resulted in the deposition of the ilountain Maid limestone, 



'^ Min. Resources P. I. Bur. Sci. Div. Min. (1910), 72. 



