GEOLOGY OF THE PHILIPPINE ISLANDS. 323 



when examined in thin section. In some cases,, though rarely, the original 

 leucite can be seen. 



I have seen very similar appearing rocks in northern Luzon, but in none 

 of them could I be positive that the small isotropic whitish areas were 

 leucites. The finding of this class of rocks in the Philippines is note- 

 worthy, as potash-bearing rocks apparently are of rare occurrence. 



Dacites. — The dacites may be thought of simply as andesites with 

 quartz. They are not common, as acidic rocks generally are not common 

 in the Philippines. I have seen only two good specimens, one from 

 Benguet and one from Corregidor Island at the entrance to Manila Bay. 

 For field and petrographic descriptions of this rock the reader is referred 

 to Becker 15 and Iddings, 10 respectively. 



The rocks of the second group are in the order of their abundance : 

 (1) Diorite, (2) quartz diorite, (3) metadiorites, (4) pyroxenite, (5) 

 gabbro, (6) peridotite, (?) granite, (8) syenite. 



1. Diorites. — The parent magma of the Philippines was such that 

 when it reached the surface and cooled quickly it produced an andesite 

 or dacite ; if it did not reach the surface, but cooled slowly at some depth, 

 a holocrystalline rock was formed of the same chemical composition, but 

 differing in fabric, and diorite was the result. Of course, there were 

 variations in the magma so that rocks of just the diorite or andesite 

 composition did not always result. However, it was the rule rather 

 than the exception. Therefore, we would expect to find the diorites in 

 the cordilleras where the streams have cut deeper into the core of the 

 islands. 



The chief characteristics of the diorites are the considerable amount 

 of hornblende, and the fact that the feldspar is a plagioclase. When 

 they contain quartz, they approach certain varieties of granite; they 

 also are closely related to hornblende gabbros. 



The true diorites may sometimes be confused with the metadiorites 

 which are abundant, but in which the hornblende is secondary and not 

 primary. 



Typical diorites have been found in Benguet, Tayabas, Cebu, Ambos 

 Camarines, and many other localities. 



2. Quartz diorites have been found in Benguet, Tayabas, and Ambos 

 Camarines. Of the Benguet rock (Antamok) Iddings says-: 



It is median grained quartz-diorite (grown together) fabric. It consists of 

 plagioclase and considerable brownish-green hornblende, anhedral (without good 

 faces) with respect to each other but euhedral (with good faces) toward quartz 

 and orthoclase. There is some altered biotite. In places the orthoclase is in- 

 tersertal ( intergrown ) to poikalitic with inclusions of plagioclase and hornblende. 



15 Geology of the Philippine Islands.. 21st Ann. Rep. ZJ. S. Geol. Surv. (1900), 

 pt. 3, 516. 

 10 hoc. cit. 



99514 2 



