IX, A, 3 Dalburg and Pratt: Iron Ores of Bulacan 215 



IGNEOUS ROCKS 



Effusive rocks. — The effusive rocks can be divided at once into 

 an older and a younger series, the one antedating the sedimen- 

 taries and the other being probably contemporaneous with the 

 later stages of sedimentation. 



The later effusives are represented by the agglomerates and 

 massive andesites (flows ?) in the upper part of the Miocene 

 beds. The best exposure is found south of Sampaloc and Bagum 

 Barrio on Bayabas River, where a small lens-shaped area is en- 

 countered, the major axis of which trends north and south. 

 Whether the effusives at this point were spread out contem- 

 poraneously with the beds in which they occur or were forced 

 into these beds at a later date is uncertain. The agglomerate 

 structure, tuffaceous matrix, and other characteristics suggest 

 purely surface rocks, but a slight metamorphism and irregular- 

 ity of the overlying beds are evidences of a disturbing factor, 

 intrusionlike in its effect, which was active subsequent to their 

 deposition. There is no indication that the period of vulcanism 

 marks a break in the sedimentary processes, and it is clear that 

 the volcanic rocks came from an adjacent center of extrusion. 

 Under such conditions confused relations would be a natural 

 result. 



The agglomerate is composed of irregular fragments of por- 

 phyritic andesite, usually less than 20 centimeters in diameter, 

 embedded in a soft, decomposed, light-colored matrix. The 

 formation presents no appearance of bedding, but is closely 

 jointed. No petrographic determinations were made, but the 

 rock is classed as an andesite from megascopic examination. 

 The porphyritic fragments consist of small phenocrysts of 

 plagioclase feldspar in a preponderant dark-colored groundmass 

 which is almost glassy in many specimens. The exposures of 

 massive-appearing rock are more like the matrix than the frag- 

 ments in character. 



Probably some of the rocks in the area of effusives and in- 

 trusives which lies to the east of the ore deposits are to be cor- 

 related with the later agglomerates, but such occurrences cannot 

 be delimited without more complete data. 



The older effusives are typically much altered and thoroughly 

 impregnated with silica. The hills and ridges to the east of the 

 ore deposits are composed very largely of rocks of this class. 

 They are light colored, felsitic, and are usually fractured or 

 brecciated, the cracks being stained with iron oxide. A rep- 

 resentative older effusive from Mount Maypapa which lies just 



