200 "^he Philippine Journal of Science ins 



streams, attesting the presence of a limestone remnant some- 

 where farther upstream. As pointed out in discussing other 

 parts of the Archipelago this deposit is found most abundantly 

 on shale, and wherever it occurs it completely obliterates the 

 rocks beneath. 



STRUCTURE 



The generalized cross section will give a clearer idea of the 

 structure in the province than any amount of description. 



Considering northern Luzon broadly, we note on the extreme 

 east an anticline (presumably — I have no specific information 

 concerning the structure of the eastern cordillera) with the 

 Cagayan syncline, or trough, immediately to the west. From 

 this trough the sedimentaries rise gradually, with minor folds, 

 in a long monocline until they abut on the igneous complex of 

 the Cordillera Central. To the west of the cordillera is the long 

 tectonic trough occupied by Abra River. It is not yet proved 

 whether this is a rift valley or not, but certainly it has a strong 

 resemblance to one. As much of the country has been covered 

 with volcanic ejecta, it is difficult without more field work to 

 discuss the structure in many parts of the region. 



It is not unlikely that there was a broad, gentle arch of younger 

 sediments, chiefly limestone, over the Cordillera Central, judging 

 from remnants of this formation on the crests of some of the 

 highest ranges, but even here the evidence is not sufficient. Eve- 

 land * has already suggested this. 



In conclusion, we may say that the grain (to use a word em- 

 ployed by Professor Gregory, of Glasgow) of northern Luzon is 

 north and south, as we would expect from a glance at the align- 

 ment of its topographic features alone. Many irregularities in 

 strike and dip with faults complicate the study of the region. 



GEOLOGIC HISTORY 



It is a question whether any one has done enough geologic 

 work in this part of Luzon to give other than suggestions of 

 the geologic history. Von Drasche does not say enough about 

 the sediments in this region, undoubtedly because he did not get 

 far enough eastward to encounter them. Not only was there a 

 long period of subsidence, but this was interrupted by minor 

 oscillations of level as indicated by the rapid change in the 

 character of the sediments. He says nothing about the great 



• This Journal, Sec. A (1907), 2, 207. 



