66 ADAMS. 



beyond tbe area discrussed in this report, he ascended the foothills of the 

 cordillera which form a ridge to the west of Porac. He colored the 

 area corresponding to the axis of this ridge as dolerite and surrounding 

 it he showed a border of doleritic tuff. The remaining descriptions of 

 the western cordillera by Von Drasche are accompanied by only a route 

 map. Becker has recently studied some rocks from the region which 

 Both classed as dolerites and, with the exception of a dike of dacite on 

 Corregidor Island, has pronounced them andesites. In this report the 

 igneous rocks of the portion of the western cordillera which is included 

 in the geologic map are classed as "principally andesites." The larger 

 alluvial areas are shown and the occurrence of marine conglomerates 

 lying on the flanks of the andesites is partially indicated. 



Besides descriptions of the occurrences of various types of igneous 

 rocks, there are, in the literature concerning the northern part of tbe 

 western cordillera, references to older sedimentaries (slates), limestones, 

 and a tuffaceous foraminiferal marl. These formations will now be 

 discussed under separate headings. 



Older sedimentaries.— Abella says, in describing the geology of the 

 vicinity of the hot springs at Canan which are situated 3-J kilometers 

 west of O'Donnell and to the north of the limits of the geologic map 

 accompanying this report: "The land in which all these springs appear 

 is of an alluvial nature, but of little thickness in the vicinity of Canan, 

 since in the bed of the Malibi and the lower slopes of Cosipen and 

 Marangla there appear strata of compact clayey slates with an ancient 

 appearance striking ENE. and WSW. with dips of 25° NNW. This 

 slate formation is cut by dikes of beautiful sanidine-trachyte with a 

 porphyritic texture which appear on one side in slopes of the Hills 

 Marangla and Cosilen and on the other between the town and the 

 Capatian Biver forming the volcanic line of the Hills Dayagdag, Taogan, 

 and Patlin." 



This is the only mention of a locality of older sedimentaries in the 

 Zambales Mountains which has been noted. However, it should be 

 remembered that Von Drasche says that the Porac Biver brings down 

 hard, flinty slates. 



Igneous rocks. 2 — The specimens of igneous rocks which were collected 

 from the western cordillera during the progress of field work are 

 principally andesites. There are some basalts and in one locality a 



2 The writer is not a competent petrographer but in the course of his field work 

 collected hand specimens of the various rocks. The microscopic determinations 

 have been made by Dr. W. D. Smith and Mr. H. G. Ferguson, whose assistance 

 and cooperation is gratefully acknowledged. In this number of this Journal a 

 petrographic description of some of these rocks will appear by Prof. J. P. 

 Iddings, who has kindly advised as to the petrographic part of this report. 



