158 The Philippine Jouryial of Science 1914 



erate which rest upon the eastern flank of the cordillera. It 

 can be quarried under favorable conditions at Pandan, a little 

 more than 2 kilometers from the coast. At this site a quarry 

 with a face 30 meters high on the average could be advanced 

 over an area of about 16 hectares. It has been noted that 

 the composition of the coast limestones would require very- 

 little modification in cement manufacture; as a matter of fact, 

 from 5 to 7 parts of tuff are sufficient for 100 parts of limestone. 

 On this basis of calculation, there is available several times 

 the quantity of tuff required for combination with the total 

 supply of hmestone at the mill site. Quarrying the tuff would 

 not be expensive because, although it is moderately hard, it is 

 easily shattered and broken up and there is no overburden to 

 be removed. 



In chemical composition the tuff is unusually constant. The 

 average analysis shows 70 per cent of silica, 12 per cent of 

 alumina, from 1 to 2 per cent of iron oxide, 4 per cent of 

 alkalies, and 2 per cent of lime as the principal constituents. 

 In 20 analyses of drill-hole samples from widely separated points, 

 silica ranges from 67 to 72 per cent, and in 10 of these it lies 

 between 69 and 71 per cent. In spite of the high content of 

 silica, the tuff is very easily fusible due perhaps to the large 

 content of volcanic glass which is present. 



Siliceous materials 48 and 49. — The shale-sand-conglomerate 

 series beneath the volcanic tuff rest upon • the Pre-Oligocene 

 complex of igneous and metamorphic rocks which are exposed 

 at the surface in the mass of the cordillera. Various basic 

 igneous rocks, a majority of which are of the deep-seated type, 

 as well as schists and gneisses are encountered in this part of 

 the section, but the predominant rocks are slightly metamor- 

 phosed elastics or arkoses, which appear to have been derived 

 from a closely adjacent land area — one consisting perhaps of the 



time subsequent to the deposition of the tuff represented by sample 47, or 

 it may be conceived of as an intrusion, the outer shell of which has been 

 rendered fragmental by movement during cooling. Although the tuff ap- 

 pears to lie at a horizon lower than that of the agglomerate in the Pandan 

 section, yet elsewhere in Cebu the same rocks occur in reversed stratigraphic 

 position; that is, the tuff overlies the agglomerate or breccia. Whether the 

 andesite is extrusive or intrusive, it appears in either case to be of local 

 origin, while the uniformity and extent of the tuff suggest that it is a 

 widespread formation. The chemical compositions are quite different, too, 

 the tuff carrying considerably more silica than the andesite. On the other 

 hand, the two classes of rock are inevitably closely associated, and in an 

 exposure near Iligan, a barrio of Toledo, there appears to be a continuous 

 gradation between the two types. 



