216 The Philippine Journal of Science im 



outside the eastern edge of the central part of the area shown 

 on the map is described by Rowley ^ as follows : 



The specimen is an aphanitic rock, variously tinted gTay, pink, yellow, 

 and brown, with areas, sometimes roughly banded, which resemble feldspar 

 crystals, but have a cherty appearance. In thin section the rock is seen 

 to be an altered porphyry, stained with iron oxide and composed almost 

 wholly of cryptocrystalline quartz. The outlines of the phenocrysts in- 

 dicate that they were originally feldspar, which has been completely 

 replaced by silica, cryptocrystalline to crystalline in character. The ground- 

 mass is likewise cryptocrystalline quartz. Anhedrons of mag-netite in 

 various stages of decomposition are scattered throughout the rock. Iron 

 oxide is so abundant as to make the rock opaque in part. 



A sample taken from the upper slopes of Mount Camanglao, 

 just east of the Hison ore deposit, is classified by Rowley 

 as a rhyolitic type composed largely of quartz and feldspar 

 anhedrons of microcrystalline to microgranular size. Silica has 

 replaced much of the original rock material. 



Green altered f elsites in the vicinity of the Hison and Constancia 

 deposits were classed by Smith as fragmentals, probably tuffs, 

 and Eddingfield found similar rocks from Santol to be silicified 

 tuff. 



The conspicuous feature of the older eff usives is their alteration 

 and replacement by silica. In their present condition they are 

 essentially iron-stained quartz. 



Intrusive rocks. — The numerous small exposures of fresh- 

 appearing rocks which are encountered along the perimeter of 

 the granite and in the base of the sedimentaries have been 

 spoken of as dikes, and it is believed that these rocks occur 

 chiefly as dikes, but the obscurity of geologic relations, due 

 to the lack of clear exposures, the extensive mantle of saprolite, 

 and the prevalence of impassable undergrowth renders it 

 impossible to trace their contacts accurately. 



There are several varieties of rocks which are classed as 

 intrusives because of their occurrence in fresh unaltered condition 

 in the decomposed granite and older effusives and in the sedi- 

 m.entaries. The most clearly dikelike exposure is encountered 

 on Maarat Creek (a small eastward-flowing affluent of Maon 

 Creek) just below the Santa Lutgarda ore deposits. This dike 



' Petrographic studies of rocks collected by us vjere made by the men 

 who at various times have i)erformed the petrographic work required by 

 the Bureau of Science. Warren D. Smith and Frank T. Eddingfield, of 

 this Bureau, and Randall A. Rowley, of the University of the Philippines, 

 have all contributed in this way to the present paper. In each petrographic 

 description the name of the petrographer is mentioned. 



