﻿COMPOSTELA-DANAO COAL FIELD. 391 



UNCONFORMITY. 



Pre-Eocene ( ?) Basal diorite and conglomerate. 



STRATIGRAPHY. 



I have compiled in Table III a statement of the stratigraphic column 

 which gives, as nearly as I now know it, the relation of the different 

 formations in this part of the island. Beginning with the lowest, we 

 have the basal complex largely composed of diorite and, as a closely related 

 part of this, the basal conglomerate. Its extension where it comes to the 

 surface can be seen on the map, represented by the red single-cross-hatched 

 color. It occupies a' portion of its western part, in fact the map was not 

 extended farther because of it. A typical diorite is found in the winding 

 gorge of the Danao River, in every respect resembling the basal rock of 

 Masbate, Benguet, etc. A typical development of basal conglomerate is 

 to be seen in portions of the river course, while in others but little of 

 this can be observed. 



THE IGNEOUS BASE. 



For some time I was puzzled by the particular phase of igneous rock 

 constituting the hills on the left, or north side, of the Danao River. Here 

 the rock is more of a porphyry than holocrystalline. From all the 

 evidence I was able to procure the rock exposed on this higher ground 

 is merely a porphyritic facies of the diorite, as would be natural to 

 expect in the upper part of the mass, where the cooling had been more 

 rapid. A feature of this rock is the innumerable, minute, calcite veins 

 cutting through it without definite system. 



The following is a description of this rock from the region of Sili 

 Creek, not far from the coal measures : 



CEBU NO. 5 (DILWORTH). — DIORITE PORPHYRY. 



Hand specimen. — A dark colored, fine-grained, igneous rock. The only minerals 

 identifiable in the hand specimen are dark green plagioclase and rare specks of 

 magnetite. 



Microscopic {2 sections). — The rock is composed almost entirely of feldspar, 

 rather decomposed. There are several porphyritic crystals reaching a maximum 

 size of 2 by 1 millimeters, but the majority are small laths. About 5 per cent 

 of the feldspars seem to be orthoclase. No good determinations could be made 

 of the plagioclases, but six rather doubtful ones gave three of acid labradorite 

 and three of basic oligoclase. Many of the feldspars show good zonal structure. 

 Several, especially among the smaller laths, are bent. 



Biotite is present in all stages of decomposition, but is always well chloritized. 

 There are no well-defined plates. 



Quartz occurs in small and inconspicuous grains. It forms a very small part 

 of the rock and is accessory rather than essential. 



There are a large number of small crystals of titaniferous magnetite or limo- 

 nite, generally associated with the biotite. The presence of titanium is shown by 

 the dirt}' white titanite, which surrounds these crystals. 



Secondary minerals are: Kaolin and sericite, along cracks in the feldspars; 

 chlorite replacing biotite; patches of calcite; and titanite associated with the 

 magnetite. 



