INTRODUCTION 519 



upward in the general Miocene uplift, with some minor crumplings at 

 various points. It is not always apparent on what sort of surface these 

 beds lie, though it is probable that they lie on irregular surfaces; at the 

 margins of the older mass we find considerable thicknesses of conglom- 

 erates, the so-called "Agno beds'^ (from the river of that name) being 

 typical. The folding in the northern part of Luzon apparently has been 

 a gradual and gentle bending of the strata. In Tayabas Peninsula the 

 flexures are sharper. In the Zamboanga Peninsula the strata have been 

 so intensely compressed that schists are the result. These schists have 

 been considered by some to be as old as the Paleozoic, but there seems to 

 be no good reason for not referring them, in part at least, to the Tertiary. 

 The central portion of Mindanao consists of gently folded sediments. 



The major axis of folding in the Philippines is, in general, north and 

 south; along the outside (eastern) arc of the islands it is northwest and 

 southeast; on the inside (western) northeast and southwest, but in cen- 

 tral Mindanao, in the Cotabato Valley, the axis of folding is more nearly 

 east and west. 



Metamorphic rocks, more or less pronounced, occur in various parts of 

 the islands. In the province of Ilocos Norte there is a considerable oc- 

 currence of schist, and in Ambos Camerines there is schist and gneiss 

 along the border of the granite intrusion referred to above. Schists have 

 been found in one locality along the central cordillera of Cebu, at various 

 parts of Palawan, on the Zamboanga Peninsula, in the province of Bukid- 

 non, on the Surigao Peninsula and just east of the Gulf of Davao, Min- 

 danao; at one point on the Tayabas Peninsula, and on the Caramoan 

 Peninsula, southeastern Luzon. These schists, for the most part, appear 

 to be metamorphosed sediments. 



Eecent alluvium from the mountains deposited on coral shelves results 

 in a greater or less development of coastal plains around all the islands. 

 The coastal plains, for the most part, are negligible, but some of the in- 

 termontane plains are very important. The northern three-quarters of 

 the central plain of Luzon is largely alluvium, as is also the case in the 

 Albay plain and the great valley of the Cagayan, Agusan, and Cotabato 

 rivers. The central plain of Panay also shows a very considerable accu- 

 mulation of detrital material. 



Around Manila we have, in addition, a great area of pyroclastic ma- 

 terial which is cut through by the Pasig Eiver. From well-logs and river 

 sections this is known to be at least 100 meters in thickness. 



Each of these formations has resulted in a certain definite type of to- 

 pography which naturally follows from the weathering and erosive agen- 

 cies in any part of the world. In the Philippines, however, we have these 



