﻿154 SMITH. 



looked in vain for some sign of an old vent, some extinct crater, which 

 would throw light on the point of origin of this formation. A close 

 examination of the ridge itself showed some crater-like depressions, 

 always incomplete, hut I finally decided that all of these forms could 

 be produced in the ordinary processes of erosion. 



Next, I turned to the coast. Here I found ash beds, sedimentaries 

 and lava flows, but no sign of an extinct crater. After the study of 

 this portion of the country, the Babuyanes Islands and their circular 

 arrangement, highly suggestive of a drowned crater, occurred to me. 

 The distance, and the fact that elevation and not subsidence has been 

 the rule in this region, precluded my making use of their existence to 

 explain the mystery. Later I examined the roughly circular, flat-bot- 

 tomed valley in which Nagpartian lies. It is not at all improbable that 

 this valley, now filled with sediment, may at one time have been the 

 vent, or it may have contained one or more vents from which all this 

 eruptive material issued. Several different colors and textures of rock 

 were found among the bowlders, but the}' are all petrographically essen- 

 tially the same. 



In the hand specimen, the rock is grayish to reddish, hard, somewhat 

 vesicular, fine grained, with an aphanitic groundmass containing feldspar 

 phenocrysts of 1 to 2 millimeters in length. 



Microscopic. — The slide contains plagioclose feldspar and augite in 

 a fine, andesitic groundmass; that is, a mass consisting of minute, lath- 

 shaped feldspar crystals, so arranged as in places to show a distinct flow 

 structure. The angles of the feldspar phenocrysts were found to vary 

 from 2G° to 31°, and, as these were symmetrical angles measured on the 

 albite twinning, they indicate that labradorite is probably the particular 

 one of the series constituting this rock. Occasional Carlsbad, Baveno and 

 Pericline twins were noted; zonal growth is quite common. (PL VI, 

 fig. 6.) The other phenocrystic constituent of the slide is augite, 

 often exhibiting good basal sections with prismatic twinning. In pris- 

 matic sections extinction angles no higher than 40° were noted. Mag- 

 netite accompanied by hematite stains occurred almost entirely in some 

 slides ingrown with the feldspars, in others it was confined to the ground- 

 mass. The writer has examined almost identical rocks from Baguio, 

 Benguet Province. The similarity to the recent volcanics of western 

 America, Alaska and Japan and in fact of many parts of the Philippine 

 Archipelago is very noteworthy, making it appear to be quite evident 

 than the great Pacific Arc, or at least the northern part of it, is one 

 petrographic province as indeed Mr. Becker 8 has already suggested. 



8 Becker, G. F. : Geology of the Philippine Islands, U. 8. G. 8. 21st An. Rep. 

 (1900), 518. 



