﻿BAGUIO MINERAL DISTRICT. 



227 



EXTRUSIVES. 



The occurrence of comparatively recent eruptive rocks over a large 

 part of the Baguio district is particularly noteworthy. North of the 

 areas of quartz diorite, all the elevations are capped with extrusives of 

 various types, but all show relations which point to one petrographic 

 province for the series. Andesites and allied rocks prevail. 



An exposure of limited extent within the area studied on the ridge 

 east of the Antamok and east of the river is of a light gray-green volcanic 

 rock which can not be classified in the field, more closely than as an 

 augite-leucocphyre, although even this porphyritie development is rare 

 and the rock generally is almost a felsite. Under the microscope, sections 

 exhibit an orthophyric structure, the rock being composed of plagio- 

 clase (almost invariably fragmental) augite, and magnetite. 



Determination of the feldspar is difficult, but measurements on one 

 Carlsbad twin gave angles of extinction corresponding to oligoclase and 

 while much sbattered, the crystals are truly idiomorphic. Pyroxene is 

 present in small amount throughout the slides, with sufficient indication 

 of idiomorpbism to justify the use of the term, and from the structure 

 it is evident that crystallization of the augite has taken place later than 

 that of the feldspars. The rock is much decomposed, breaking down 

 into epidote and magnetite; chlorites are present in small amount, closely 

 associated with the feldspars and augites. All sections show clearly 

 that the rock is an extrusive (andesite), but that during crystalliza- 

 tion or subsequently, great strain has developed, so that the rock is almost 

 fragmental. 



Two analyses 7 of this rock are given : 



Analyses of the andesite. 



Constituent. 



II. 



Moisture at 110° (11— 110°) 

 Loss on ignition (M+110°) 

 Silica (SiOo) 



Alumina (A1 2 3 ) 



Ferric oxide (Fe 2 3 ) 



Ferrous oxide (FeO) 



Calcium oxide (CaO) 



Magnesium oxide (MgO) — 

 Potassium oxide (K»0) _„ 

 Sodium oxide (Na»0) 



Total 



Per cent. 

 0.41 

 2.23 

 53.92 

 18.68 

 4.27 

 2.65 

 8.85 

 2.93 

 2.66 

 2.92 



Per cent. 

 0.52 

 2.04 

 54.10 

 19.01 

 4.32 

 2.45 

 8.90 

 3.02 

 2.98 

 3.08 



99.55 



100. 42 



7 Made by Paul J. Fox, Chemical Laboratory, Bureau of Science, Manila, 190G. 



