THE PETROGRAPHY OF SOME IGNEOUS ROCKS OF THE 



PHILIPPINES. 1 



By Joseph P. Iddings. 

 (United States Geological Survey, Washington, D. C.) 



PETROGRAPHY. 



The rocks of the Philippine Islands, so far as known, are : 



1. The igneous rocks that form the volcanoes and principal mountain 

 ranges, and that consist of massive lavas, both extrusive and intrusive, and 

 to a larger extent, perhaps, of tuffs and aerial breccias. These rocks not 

 only form the cones and slopes of the volcanic mountains, but extend in 

 certain regions over the plains and valleys, and mingle with the sands and 

 coral limestones which make up the sedimentary rocks of the islands. 



2. Those igneous rocks which have solidified at some depth beneath the 

 surface, and in places have been considerably metamorphosed by shearing 

 and recr} r stallization, and in other ways changed ; and which have become 

 exposed through erosion. These rocks appear to be older than the recent 

 volcanic rocks, but may not be older than the earliest tertiary formations. 

 They may be exposed intrusions of the volcanic lavas that form the older 

 portions of the volcanic mountains mentioned in the first paragraph. 



3. The sedimentary deposits that have been derived from the igneous 

 masses by disintegration in some instances, and by degradation of tuff 

 deposits, or by direct sedimentation of such volcanic material at the time 

 of its eruption, together with such other sediments as may have been 

 derived from coral reefs, and other rocks. 



1. ROCKS OF THE VOLCANIC MOUNTAINS. 



The great bulk of all the volcanic rocks of the islands is andesite, for 

 the most part pyroxene-andesite, in which hypersthene and augite are 

 both present, the orthorhombic pyroxene often in great abundance. A 

 considerable proportion of the andesites carry hornblende besides pyrox- 

 ene, and are hornblende-pyroxene-andesites. A smaller number are horn- 

 blende-andesites without pyroxene, and fewer contain biotite in addi- 



1 This work was performed in the laboratory of the division of mines, Bureau 

 of Science, Manila. 



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