410 FERGUSON. 



It apparently extends north and south for something over 100 meters, 

 has a width of about 50 meters and a thiclmess of probably 20 meters, 

 apparently resting on andesite. On its northern boundary it seems 

 to be cut off by a small and barren quartz vein having a strike of 

 approximately north 80° east. The only explanation which can be 

 offered to account for the presence of the limestone here is that it is 

 a remnant of a larger mass which once formed a capping over a great 

 part of the region, and owes its peculiar topographic position to faulting. 

 This explanation is unsatisfactorj'^, for if this were the case we should 

 expect to find either remnants of the capping at higher levels or outcrops 

 of the same rock in the sediments of the Miocene formations on the 

 opposite side of Port Barrera. Perhaps when the geolog}^ of the main 

 southeastern range is studied more in detail, a better knowledge of 

 the relations of this limestone to the other formations may throw some 

 light on its occurrence at this point. 



Since the more recent sedimentary rocks mapped as the Lanang and 

 Port Barrera formations do not outcrop in the mining district proper, 

 a short description will suffice. Both belong to the Miocene coastal 

 plain, which rests upon the old land surface of the Aroroy district. 



The Lanang formation. — A rather coarse conglomerate, composed 

 largely of basalt pebbles, occurs south of the junction of the Lanang 

 Eiver and Kabakalan Creek and in the hill forming the center of 

 the curious abandoned meander of the Lalang Eiver. The conglomerate 

 is evidently the basalt member of this sedimentar}^ series, the pebbles 

 being derived either from an undiscovered basalt outcrop on the hills 

 to the northward, or a basalt flow formerly capping the andesite, and 

 now removed by erosion. Above this, in the hills between the abandoned 

 meander and the mass of andesite forming the lower gorge of the 

 Lanang, conglomerate and sandstone occur, the former containing peb- 

 bles of andesite and dipping about 20° to the southwest. This basal 

 series in all probability curves around the andesite mass at the lower 

 gorge, but its continuation was- not followed. 



The Port Barrera formation. — On the northern shore of the westward 

 emba}"ment of Port Barrera (not shown on the large map) are outcrops 

 of a fine-grained, gray shale containing numerous small shells of Lower 

 Miocene age, irregularly interlaced with, small sandstone dikes. This 

 formation does not outcrop higher than about 4 meters above the ' level 

 of high tide, and seems to dip very gently somewhat south of west. 

 Doctor Smith considers this shale to be similar to that found above 

 the coal measures in Cebii and Batan, and, in the hope of finding similar 

 local seams, the shale was here prospected by drilling to a depth of 

 50 meters, but only shale was encountered. This evidence, together 

 with that furnished by the topography of the country, makes it clear 

 that Port Barrera originally was a river valley eroded in the soft shales 



