188 ^^16 Philippine Journal of Science i»« 



The pold-bearinp alluvium is found in preater or lesser degree upon all 

 the sedimentary formations enumerated, but principally on the conglomerate. 



Here, as in northern Luzon, the dioritic-syenitic rocks are the gold- 

 containers. The alluvium contains large quantities of mica, and according 

 to Minard three native metals: gold, platinum, and lead. Daubree noted 

 the presence of zircon in the washed sand, which again points to syenite 

 as the probable origin. 



So far as concerns the relative ages of the diflferent recent eruptive rocks 

 it is impossible to determine anything with certainty. The eruption of 

 all of these rocks may have taken place during the period of formation 

 of the tuffs; and there is no evidence that the eruption of volcanic masses 

 did not continue even after the elevation of the reef^. The still active 

 solfatara of Aknal and the hot springs of Bontoc are the last stages of 

 this wonderful phenomenon, which, perhaps, reached its greatest activity 

 in the vicinity of Mount Data. 



We are, therefore, led to conceive of the west part of northern Luzon as 

 having consisted at one time of a mountain range of crystalline schist, 

 which was elevated by mighty diabase and syenite eruptions, the fragments 

 of which later were stratified into great sandstone and breccia-complexes. 

 A long period of time perhaps elapsed between the deposition of the Agno 

 beds and the renewed manifestation of volcanic activity. With the later 

 vulcanism there were deposited large masses of tuff on the west coast; 

 there must have been a contemporaneous subsidence of the sea floor to 

 permit the deposition of such a great thickness of detritus. During the 

 intervals of quiescence, even though of short duration, the corals could carry 

 on their reef building. Vegetable remains transported from the mainland 

 formed the plant-bearing marls. Even before the end of this period of 

 subsidence it appears that volcanic activity had ceased. On the extensive 

 submarine platform of tuffs the coral polyps built their structure and 

 formed the coral reefs along the mainland which we now recognized in 

 the Sierra Tovalina and in the detached peaks on the left bank of the Agno. 

 Farther out in the sea atolls were formed, one of which we still find pre- 

 served in an excellent state in the valley of Benguet [Trinidad Valley]. 



An accurate knowledge of all of these individual reefs would enable us 

 to make a correct estimate of the form of the mainland in those times, which 

 apparently consisted of narrow islands having a north-south direction. 

 Finally there began the period of elevation, which still continues, and 

 which, judging from the age of the reefs, must pertain to a geologically 

 recent period. The effect of this process of elevation was to join together 

 the different islands as a related whole, in which we can easily distinguish, 

 through the form of the land and the consequent direction of flow of 

 the rivers (Abra, Agno, and Pan), a row of folds with north-south axes 

 attaining the imposing height of 4,000 feet. This is unquestionably a 

 very remarkable change of level within so short a time. 



The Abra eroded its bed in the former channel between the mainland 

 and the present coast range, the Sierra Tovalina, and utilized the large 

 transverse cleft at Vigan to reach the sea. The two parallel-flowing rivers. 

 Pan and Agno, may have flowed into the ocean-covered plains of Pampanga 

 and Pangasinan at Niyong and Sto. Niiio in very recent geological time. 

 Only after these plains had been lifted out of the ocean, an event that may 

 have been one of the last in the geologic history of Luzon, did they swerve 

 westward and follow the gentle slope of the plain. 



