﻿304 SMITH. 



of other rocks, chiefly igneous; we would call them graywackes. Some 

 rocks of pyroclastic origin are also doubtless associated with these. The 

 best place to see these graywackes is in the gorge of the Cot-Cot just above 

 Muao, where they can be seen dipping at a low angle to the east ; and 

 above them at approximately 300 meters (1,000 feet) elevation is the 

 andesite capping. 



Petrographic descriptions of both the andesite and the detrital material 

 made for me by my colleague, Mr. H. G. Ferguson, are given below : 



CEBU NO. 45. — ARKOSB. 



Hand specimen. — Extremely fine grained, grayish rock, the distinguishable 

 minerals of which are pink feldspars, quartz, magnetite and occasional hornblende 

 ( ?). The grains are all very small and approximately the same size. 



Microscopic. — The rock is much .decomposed, especially the feldspars. Feldspar 

 is the most prominent mineral, and is chiefly orthoclase, but one grain of albite 

 was found. There are no perfect crystals, but occasional crystal faces occur. 

 Quartz is rather rare in definite grains, but seems to fill spaces between other 

 crystals. Biotite occurs in small amounts, but in bent and frayed fragments 

 rather than plates. Occasional fragments of hornblende crystals are present. 

 Magnetite occurs in numerous, small grains, often partly altered to limonite. One 

 grain of topaz (?), a few minute grains of olivine (?) and numerous very 

 minute grains (diameter generally about 0.01 millimeter) of a mineral with 

 high refractive index and rather high double refraction, apparently titanite, are 

 the accessory minerals. 



The rock shows no definite structure. The grains vary in size,- but never 

 exceed 0.25 millimeter and perfect crystals are never found. The biotite especially 

 presents a fragmental appearance. I believe the rock to be an arkose, formed by 

 the decay of a traehytic igneous rock and with only slight transportation, as 

 there is no evidence of assortment of the minerals. 



CEBU NO. 70. — ANDESITE. 



Hand specimen. — The specimen is taken from the contact of two igneous rocks 

 of different texture, one a greenish-gray, aphanitic rock and the other more 

 porphyritic. consisting of greenish and glassy-white feldspars, in a purplish 

 groundmass. The contact is a shearing plane, stained with iron oxide. 



Microscopic (2 slides, both from the porphyritic rock). — Feldspars in idio- 

 morphic crystals are very prominent, but are all completely decomposed, chiefly 

 to sericite aggregates, and are also to a large extent replaced by calcite. One 

 doubtful ease of albite twinning was found, which gave extinction angles corre- 

 sponding to oligoclase. 



Biotite is present in considerable amount (possibly 5 per cent of the slide). 

 This mineral is interesting as it shows different stages of decomposition. In 

 part it has simply lost a portion of its iron content and become more hydrated. 

 showing in plane polarized light a greenish pleochroism and under crossed 

 nichols a much lower double refraction than normal biotite, the colors ranging 

 from first order gray to first order yellow. For a part of the mineral, however, 

 the alteration has been more complete and the biotite has gone over to chlorite; 

 sometimes both stages can be observed in the same crystal. Biotite has also 

 suffered somewhat from replacement by calcite. 



Other dark silicates if originally present have been altered beyond recognition. 



Magnetite is present both in large grains (largest 0.3 by 0.2 millimeter) 



