320 SMITH. 



PETROGRAPHY. 



The rocks of the Philippine Islands consist of the general groups which 

 are common elsewhere. 

 These are as follows : 



1. The igneous rocks of the recent volcanoes and the worn-down stocks 

 of the older ones, including both extrusives and intrusives. Under the 

 intrusives we not only have massive lavas but aerial breccias, or rather 

 volcanic agglomerates and tuffs. 



2. The deep-seated igneous rocks, those which have been exposed only 

 through long continued erosion. There is every gradation between these 

 and the extrusives, and in some cases there may be little or no difference 

 in age. These older crystalline massives formerly were thought to be 

 very old, perhaps Archean, but there is no good reason for believing 

 the majority of them to be of greater age than the earliest Tertiary. 



3. The sedimentary rocks which have been derived by the degradation 

 and disintegration of the above-mentioned classes, namely, coral reefs, 

 chemical precipitations, etc. 



4. The metamorphic rocks such as gneiss, schist, serpentine, etc. 



5. The mineral veins. 



Under the first head we have the following principal types given in 

 the order of their predominance : 



(1) Andesites. 



Hornblende andesite. 

 Pyroxene andesite. 

 Hornblende-pyroxene andesite. 

 Olivine-bearing pyroxene andesite. 

 Hornblende-biotite andesite. 



(2) Basalts. 



(3) Dacites. 



(4) Leucitites. 



The andesites are by far the most common of all of these rocks and 

 the pyroxene-bearing variety is dominant. Next to it comes hornblende - 

 andesite. The andesites form the older parts of the volcanoes, while 

 the basalts generally constitute the later flows. 



A number of petrographers, chief of whom are Oebbeke and Iddings, 

 have described various collections of Philippine rocks, and the latter 

 has recently worked over the entire collection of the division of mines, 

 Bureau of Science. Therefore I shall include some of his descriptions. 

 However, as Iddings has made use of several new terms with which only 

 petrographers are familiar, I have taken the liberty of changing the 

 vocabulary in certain instances. These quotations, which are taken from 

 a longer and very technical paper already published, 14 are printed in 

 smaller type. 



u This Journal, Sec. A (1910), 5, 155. 



