﻿BAGUIO MINERAL DISTRICT. 225 



A comparison will show that although the silica remains fairly con- 

 stant in amount, there is very little stability in the percentages of the 

 other constituents excepting in the soda. This fact is not extraordinary 

 and, indeed, it might be expected from variations in the magma in a 

 distance of 50 miles, and from the subsequent alterations to which the 

 rock has been subjected. 



The exposure of the diorite in the Antamok Eiver is continuous from 

 Pakdal to a point a mile below Antamok, where a granitic intrusion (to 

 be described later) comes in. The valley of Gold Creek is also in this 

 rock, from its junction with the Batuaan to the headwaters at Pakdal. 



On the north side of the area diorite again crops out where the drainage 

 north and west of Baguio has cut deeper into the surface, and in various 

 places in the Bued Eiver canon, where later sedimentaries and extrusives 

 have been cut through, good exposures of the diorite are laid bare. 



From below Camp IV, or Kias, on the Benguet road, the diorite 

 does not outcrop, later extrusives and sedimentaries taking its place in 

 the river bed, and it is evident from the occurrence of a massive intrusion 

 of more granitic character to the east of this, that from this point on, 

 the dioritic exposure leads off to the east toward the Cordillera Central, 

 or in other words that on the topography of this basal mass, this south- 

 ernmost point represents the limit of exposure of any great elevation. 

 From here to the south, the diorite is evidently considerably lower and 

 as may reasonably be supposed, is buried in the plains region between 

 Benguet and Manila by a great thickness of recent sedimentaries. 



INTBUS1VES. 



About a mile to the south of Antamok, on the river of the same name 

 occurs a massive exposure of a rock which here is classed as an intrusive, 

 but which may possibly be but a phase of the diorite. On the Antamok 

 and Batuaan Bivers, both of which cut across the contact between the 

 diorite and the intrusive, the contacts are obscured by heavy talus and 

 wash, and the relative ages of the formations, if they differ, can not be 

 determined. The float on the Antamok Biver seems to show all varia- 

 tions in appearance from the one rock to the other. The rock may be 

 classed in the field as a granite or quartz diorite. It is a hard, light- 

 green massive rock showing to the eye quartz, feldspars, and a dark-green 

 hornblende ( ?) arranged in a granular holocrystalline structure with 

 particles of a light-green secondary mineral scattered throughout the 

 feldspars. Under the microscope the minerals exhibited are quartz, 

 plagioclase, amphibole and magnetite. The quartz does not form a 

 conspicuous constituent and it occurs as an interstitial filling. The ex- 

 tinction angles measiired on the plagioclase show it to be oligoclase in 



