PHYSIOGRAPHY OF THE PHILIPPINE ISLANDS. 9 



region as it rises in sliarp cliffs from the plain at its foot to a height 

 of perhaps 200 meters above the plain. To the westward its slope is 

 gradual. 



About 3 kilometers south of the Mabui Elver the land, which has 

 been gradually rising from an elevation of 160 meters at the Mabui 

 Eiver to 240 at this point, suddenlj'' falls off in a veiT steep slope to a 

 broad, flat plain, having an average elevation of alDOut 130 meters, the 

 bounding escarpment running first to the southeast and then northeast. 

 The plain is drained by two parallel streams, the Butuan and Mombog 

 Elvers, which flow to the westward through the escarpment already 

 described, although the princij)al branch of the Butuan has a southwest 

 course parallel to the cliffs bounding the plain. The peculiarity of the 

 courses of these rivers again suggests superposition. One would hardly 

 expect to find two such dee]), narrow canyons close together when more 

 reasonable avenues for the drainage of the plain lie to the northwest 

 around the end of the escarpment into the Mabui Eiver or southwest 

 into Nin Bay. A sharp pinnacle of limestone, Butuan Hill, stands in 

 the center of the plain. This peak is flat-topped, rising in a sharp clifE 

 about 70 meters above the plain; its base is elliptical, the axes being 

 about 400 and 150 meters in length. The limestone has been largely 

 dissolved and large caves have been formed. As several skeletons have 

 been found in these places, it is evident that they once served for 

 burial. 



This limestone peak and the conglomerate to the west mark the 

 present limit of a marine sedimentarj^ series which extends northward 

 to San Agtistin and the western side of Port Barrera. The country 

 south from the Lanang Eiver, consisting of well-worn hills of volcanic 

 and pyroclastic rock mth occasional fluviatile sediments, clearly repre- 

 sents the old land upon which these sediments were laid down. That 

 the sediments probably once covered a larger area is indicated by the 

 peculiarities of the courses of the various streams, particularly the 

 Lanang Eiver and Butuan and Mombog Creeks. The present boundary 

 of the series is well defined by a trough running northward from the 

 Butuan plain to the head of Port Barrera. Possibly the escarpment 

 bounding the plain on the north represents the old shore line and Butuan 

 Peak is an isolated coral reef. Butuan plain itself drops off to the 

 south in another escarpment similar to, although smaller, than the first. 

 To the west the sedimentary escarpment, now turning off to the south- 

 west, overlooks the plain, while on its eastern side the upiper jDlain, 70 

 meters above, stretches away in a southeasterly cliff. In both the upper 

 and lower plains the underlying rock is basalt, strongly magnetic. The 

 lower plain is broken by numerous small hills and grades gently down 

 from an elevation of about 40 meters to the mangrove swamps surround- 

 ing- Port Mandaon. 



