390 SMITH. 



The upper portion of the sandstone in wMch the coal seams occur is 

 a conglomerate of basic igneous rocks, the component pebbles of which 

 are ver}- much altered and serpentinized. Overlying this is a hard cal- 

 careous shale passing into a semi-crystalline limestone. 



Gravel panned on the Asiga, just above its junction with the Tubay, 

 showed no color, but this is probably due to the high velocity of the 

 current at this point. 



Above Santiago, the Tubay Kiver has a very slow current, and an 

 average width of about 30 meters. Xarrow channels between floating 

 islands of swamp growth alternate with miniature lakes 300 to 400 meters 

 wide. In general, tlie west bank is high and wooded while the east bank 

 is flat throughout and generally bordered by' fresh water swamp. 



The Tubav Eiver m.ay be considered as a channel draining Lake Mainit 

 into Butuan Bay. This lake is almost circular and measures 9.5 to 10 

 kilometers in diameter. Only two towns "are situated on its border, 

 Jabonga on the south and at the head of the Tubay Eiver, and Mainit 

 on the north. A short distance north of the town of Mainit is a 

 mountain rising prominently from the surrounding, comparatively level 

 country. To judge from its geologic formation and its s}Tnmeti-ical 

 slopes. Mount Mainit. or Saporga, as it is sometimes called, must be of 

 volcanic origin. 



Lake Mainit is frequently spoken of in the literature as a crater lake, 

 mainly I suppose, because of its location, outline, and proximity to the 

 extinct volcano Mainit. However, from my own observations I believe 

 that the lake was caused by a subsidence of the submerged area, con- 

 sequent upon the withdrawal of the material which went to make up 

 Mount Mainit and other volcanic masses to the north. I did not visit 

 the east shore, but Montano ^* reports the presence of hot springs 

 emanating from limestone strata, but I crossed the narrow ridge separat- 

 ing Lake Mainit from the Surigao Sea and there also I found nothing 

 but stratified rocks in their original or metamorphosed condition. Most 

 of them were ehlorite-bearing schists or alteration products of the same. 

 The original bedding planes are obscured by the new cleavage planes 

 induced°bv dynamic pressure. In general, the direction of the cleavage 

 planes is somewhat west of north, while their inclination is in both di- 

 rections east and west. The east flank consists largely of red and brown 

 slates finelv laminated, and in places interleaved with white marble. 

 :N"ear the top of the ridge the proportion of calcite increases, and some 

 fine pieces of marble are^to be observed. Inclusions of a green micaceous 

 mineral, probably chlorite, are very common. The slate is highly cal- 

 careous and is probably an alteration product of an impure argillaceous 

 shale. It has a very fine and distinct cleavage, and if sufficient material 



"Voyage aux PMlippines et en Malaisie, Paris (1886), 314. 



