﻿BAGUIO MINERAL DISTUICT. 223 



the porphyritic nature of the rock, which has amphiboles sometimes 

 measuring 15 millimeters in length, is very evident. In connection with 

 the development of the amphiboles, the almost universal quartzose 

 nature of the rock is less evident or entirely wanting — no quartz can 

 be seen with the eye or with a hand lens. Under the microscope the 

 normal phase of the basal diorite presents a granitic or a pan-idio- 

 morphic granular structure. In almost all the sections examined much 

 saussuritization has taken place and the resultant minerals are variable 

 in different localities. The structure is plainly that of a plutonic rock 

 and there is evidence of slow cooling, the first crystallization taking- 

 place being that of the feldspars. Certain sections show porphyritic 

 facies and the mass is evidently variable. The plagioclase feldspars 

 show polysynthctic twinning, and in these sections they are probably 

 near the lime end of the series, calcite being pronounced. Possible 

 intergro-wths of feldspars are noticed. Measurement of extinction an- 

 gles on different sections in some cases gives labradorite (observed on 

 Carlsbad twins) ; other sections at right angles to the albitc twinning 

 gave 31° to 33° as extinction angles. A slide occasionally shows anor- 

 thite, and a sample of the rock from the valley of the river Sili exhibits 

 andesine. 



The feldspars are much decomposed, and calcite, zeolites, epidote, and 

 chlorite are present in varying amounts. The ferro-magnesium min- 

 erals generally are too much decomposed to show more than .outline of 

 faces, but from occasional fresher sections they are clearly determined to 

 be pyroxenes. Actinolite has evidently been a constituent of prominence, 

 although hornblende more nearly represents the average composition. 



Magnetite is present in large amount, over 10 per cent being present in 

 some sections, and it, and chlorite developed in light-green flakes and 

 string's throughout the slides, are most prominent. 



Although it is apparently a basal mass the diorite has been through 

 other processes than mere crystallization from the magma. The micro- 

 scopic aspect of samples from certain localities seems to point to a granu- 

 lation and recrvstallization of the original rock, and in view of the neo- 

 volcanics which are so prominent over the area, some metamorphism is 

 to be expected. There is enough evidence in the numerous slides exam- 

 ined to make it certain that, although basal in position, the diorite is 

 not, strictly speaking, representative of a deep-seated igneous mass. It 

 shows variation of structure and composition identical with those exhib- 

 ited by an extrusive and although subsequent alteration may be, and 

 probably is, responsible for many of these variations, it seems to be a 

 more justifiable hypothesis that the diorite, although now it is strati- 

 graphicallv the basal mass of Luzon, at the time of its origin was, in 



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