4 FERGUSON. 



more mature erosion of a complex of older sedimentary, igneous, and 

 pyroelastic rocks without any evidence of recent elevation; on the 

 contrary, it is not nnlikely that there has been some depression in 

 recent times. 



Tlie country lying directly east of the bay forms the Aroroy mining 

 district, and has been mapped in detail. In the northern part of the 

 district, near the barrio of Aroroy, there are scattered outcrops of a 

 quartz diorite. Further south on Aroroy and Bagadilla Moixntains oc- 

 cur metamorphosed sediments together with basic effusives. On either 

 side of the canyon of the Guinobatan Eiver are two small ledges of a 

 dark, tine-grained limestone which forms a capping on two of the points. 

 Between Ivalakbao hill and the hills near the Lanaiig Eiver the prevail- 

 ing rock is an andesitic breccia, the beds of which dip at low angles to 

 the southwest; some of the hills, however, such as Panique hill and two 

 small ones near the bay are composed of andesite and may be volcanic 

 stocks. The high range of hills cut by the Lanang Eiver is also andesite, 

 while to the south basalt and conglomerates with basalt pebbles come in. 

 The topography of the region shows, even more clearly than the western 

 side of the ba}', the rectangular system of lineaments. Mounts Aroroy, 

 Bagadilla, Kalakbao, and C'ogran show a most marked northwesterly 

 trend. These hills contain practically all the mineral ^'eius of the dis- 

 trict, and owe their prominence to the greater resistance offered to 

 erosion by the quartz of the veins. The vein system shows very clearly 

 the presence of a northwestern line of weakness, as all but two or three 

 of the veins strike in a northwesterly direction. They all give evidence 

 of frequent opening and recementation with several periods of minerali- 

 zation. Furthermore, the three principal streams of the district, the 

 Guinobatan, Panique, and Lanang, all follow a northwesterly course, as 

 do the two brooks which empty into the bay near Aroroy. The south- 

 westerly lineaments are also very strongl}' marked. Principal among 

 these are the rather broad valley of Balagting Creek, Mount Yil-lon, 

 and the two gorges of the Lanang Eiver. 



Mount Vil-lon is the highest mountain in the district, reaching an 

 altitude of 400 meters. It has a clear and sharp northeasterly trend and 

 breaks off precipitoiisly at Monument Eock, near the Guinobatan Kiver, 

 tlnxs differing from the other hills of the district which all trend to the 

 noi-thwest. The rock is an andesitic porphyrj- and the mountain may 

 be a volcanic stock or more probably an intrusive mass. 



A i^eculiar feature of Mount Vil-lon is that, while it lies on the strike 

 of the principal veins of the Aroroy district, diligent prospecting has 

 failed to reveal any mineral veins. Veins occur on the northeast side 

 of the mountain, but in an irregular network and no longer showing 

 the dominant northwesterly strike. 



Nearly all the mineral bearing veins of the district occur in the com- 



