vni. a, 4 Saderra Maso and Smith: Seismic Disturbances 223 



been noted. In the neighborhood of Lake Mainit the rocks are volcanic, 

 although a short distance east of this body of water is a belt of metamor- 

 phics. In the Matutan Range the rocks are largely extrusives of more 

 or less recent date. 



Line P — P passes through three epicenters and along very important 

 structural lines in the Archipelago. Beginning at the north, it passes 

 between the Islands of Catanduanes and Luzon, through Batan Island, 

 south through an epicenter near Biliran Island, and thence through an 

 epicenter located near the southern end of Leyte. From here it extends 

 through one in Butuan Bay, thence traversing very closely the structural 

 line of the Agusan Valley in Mindanao, and finally emerges from Mindanao 

 near the town of Mati. Very little, is known about the geology of the 

 parts of the Archipelago traversed by this line. In the Island of Batan 

 the rocks are largely sedimentary; however, their strikes do not coincide 

 with this line at all. 



The Agusan Valley in Mindanao is very clearly a structural 

 valley. What the condition of the rocks is, with depth, we do 

 not know, as the alluvial filling in the Agusan trough conceals 

 everything. This line is one of the most important in the 

 Archipelago, and has been described in previous articles. 28 In 

 the first of these Saderra Maso says : 



"We call this line the 'Line of the Agusan .River Valley' because the 

 portion of it which lies within the said valley has been the seat of the 

 greatest number of violent earthquakes which occurred during the last fifty 

 years." The first seismic district of importance of this line comprises the 

 large Gulf of Davao, which is 120 kilometers long and 50 to 70 kilometers 

 wide. The average depth of this basin is 800 meters, increasing, however, 

 toward southeast in such manner as to exceed 1,650 meters west of Cape 

 San Agustin. * * * To the west of the gulf rise the gigantic Apo 

 Volcano, the Matutun and several other cones of less importance, which 

 constitute the northern boundary of the volcanic zone extending, as it 

 seems, from Mount Apo as far as the Celebes, * * *. 



"The extensive valley of the Agusan River runs from south-southeast to 

 north-northwest, almost parallel to the east coast of Mindanao, * * *. 

 The entire bottom of the Agusan Valley consists of marine sediments con- 

 taining an abundance of recent shells: only at the mouths of the water 

 courses, which descend from the mountains bounding it east and west, 

 is found gravel containing well-worn pebbles of andesite and other igneous 

 rocks. Every geologist who has visited this part of eastern Mindanao 

 received the same impression, to wit, that its emersion from the sea is of 

 quite recent date, and its elevation is still increasing. Some of them 

 assigned the post-Pliocene period as the epoch of the formation of the 

 sediments found in the Agusan Valley, * * *. This valley and the Davao 

 Gulf appear to be portions of one and the same synclinal." 



Line Q — Q begins near Placer on the northeast coast of Surigao 

 Peninsula, and passes through an epicenter in Butuan Bay and the 



"Saderra Maso, Miguel, Bull. P. I. Weather Bur. (1910), 283; (1911), 

 225. 



" Italics are ours. — Authors. 



