﻿BAGUIO MINERAL DISTRICT. 229 



along a series of three directions of cleavage leaving hard and compact 

 rounded boulders in the subsoil. 



Under the microscope, augite is the most prominent constituent, with 

 twinning parallel to the macropinacoids. The feldspars are plagioclase; 

 oligoclase, andesirie, labradorite and anorthite all occuring in at least 

 two generations, if not more. The groundmass is typically andesitic in 

 structure. Magnetite, in fine grains, is prominent in the groundmass. 



All of the younger rocks described above are of andesitic varieties and 

 in fact, with the exception of the two closely related rocks, the basal 

 diorite and quartz diorite no other igenous rocks have been seen in this 

 area, excepting andesite and related extrusives. There is a close petro- 

 graphic and geologic resemblance between this district and the region 

 around Mount Shasta (California), which has been described by Diller 

 in Monograph 15 of the United States Geological Survey, and in general, 

 between it and many of the petrographic provinces represented by the 

 Pacific coast cones of Mount Hood, Mount Kainier and others. Spurr 8 

 has also recently called attention to this "Pacific Zone" of andesitic rocks. 



SEDIMENTAEIES. 



Exposures of sedimentary rocks, although more prominent in the 

 immediately adjacent areas, occur in the Baguio district, and from these 

 a series can be constructed giving some fairly accurate idea of the geologic 

 history of this region. At the base of this series, resting on the diorite 

 rocks beneath, is a conglomerate of considerable thickness, made up of 

 the dioritic materials derived from the basal mass. None of the exposures 

 show a sufficiently large section to allow the taking of accurate measure- 

 ments, but the conglomerate certainly must be considerably over 100 

 feet in thickness. The beds consist of heavy conglomerate at the bottom, 

 with sandstones and clay above. No fossils have been found in these 

 beds, but the one-foot lignite seam in the upper measures and the con- 

 formability of the beds with the Baguio limestone directly above them 

 establishes the fact at least, that they are only a little earlier and older 

 than the fossiliferous limestone. The material has little uniformity of 

 size, and the bowlders vary from pebbles to rounded masses of many 

 tons' weight. No outcrops of the conglomerate base are visible in the 

 area, only the upper measures being laid bare in the upper end of the 

 Bued Biver gorge. However, below, in the same canon, for a distance 

 of a few miles, the whole series is admirably exposed. The formation 

 dips to the south on the southern end of the highlands and a few miles 

 to the northwest of Baguio, to the west, the beds curve around the dioritie 

 islet like a cape. The upper beds in the vicinity of Baguio are fairly 



s Spurr, J. E. : Geology of the Tonopah Mining District, Nevada, U. 8. Q. S., 

 (1905), 47. 



