﻿NOTE ON THE OCCURRENCE OF RHYOLITE IN CEBU. 



By Henry G. Ferguson. 

 (From the Division of Mines, Bureau of Science, Manila, P. I. 



This rock, collected by Mr. Warren D. Smith during his investigation 

 of the coal fields of Cebu, 1 seemed to be of interest because of the difficulty 

 in determining whether it should be classed as an igneous rock or a sili- 

 ceous precipitate. 



The field relations are not at all clear. The rock occurs in scattered 

 outcrops along the side of the Muao River valley, and near the Bureau 

 of Science bench mark (B+S on the map accompanying Mr. Smith's 

 paper). "Greenstone" and andesite outcrop in the stream bed, and 

 limestone forms the capping of the hill. The contacts are everywhere 

 masked by talus. 



CEBU NO. 57. 



Hand specimen. — The rock in the hand specimen is whitish, compact and 

 extremely fine grained. It is rather light, four determinations of specific gravity 

 giving: 2.20, 2.26, 2.10, 2.17, average, 2.18. It is sufficiently soft so that it 

 can be scratched with a knife and gives no effervescence with hydrochloric acid. 

 In one part of the specimen there is a well-marked banding which consists of 

 small, slightly darker lines about 2 millimeters apart. A slight tendency to 

 break along this banding was noticed in making the thin sections. A few small 

 grains of magnetite appear in these dark bands. The rock is too fine grained 

 for any other minerals to be identified in the hand specimen. 



Another specimen bearing the same number seems to be a more weathered 

 phase of this rock. The color is light green instead of white, as in the unaltered 

 specimen. Beyond this the rocks seem the same. One grain of quartz (about 

 0.5 millimeter in diameter) was seen in the weathered rock. No banding was 



noticed. Specific gravity 2.14, 2.15, 2.15. 

 s S~//(- t ^ Microscopic. — The most easily recognizable min- 



erals are quartz and feldspar, occurring in minute 

 grains. The quartz is clear and glassy, the grains 

 as a rule have sharp edges ; and apparently, in 

 some cases at least, crystal faces are present. The 

 grains sometimes are broken and occasionally con- 

 tain inclusions. 



The feldspars occur in small, clear, fresh grains. 

 The edges are sharp, but there are no idiomorphic 

 crystals. Only one good determination could be 

 made and this gave basic oligoclase (extinction 

 angle of albite twins, 6° and index of refraction 

 lower than quartz). Probably the majority of 

 inclusions. the undetermined feldspars are orthoclase. . 



Pig. 

 grain ; 



b 



-a, Broken quartz 

 quartz grain with 



1 Smith, W. D.: This Journal, Sec. "A," Gen, Sci. (1907), 2. 



407 



