IX, A, 3 Dalburg and Pratt: Iron Ores of Bulacan 213 



therefore the limestone in question can be assigned. Corrobora- 

 tory evidence as to the age of this limestone was obtained by 

 study of a sample take from Bagum Barrio on Bayabas River, 

 in which fragments of Lepidocyclina were found and identified 

 by Smith as a Miocene species. 



The Binangonan limestone is conspicuous along the margin of 

 the Central Plain in this region. It is yellow to white in color, 

 crystalline in texture, and massively bedded in structure with 

 numerous vertical joints which give weathered exposures a 

 columnar appearance. An unusually fine-grained, bedded ex- 

 posure near Bagum Barrio has been exploited to some extent 

 as lithographic stone. The maximum observed thickness, about 

 100 meters, is exhibited in the precipitous upper slopes of the 

 hills southeast of Sibul. 



Beneath this limestone is the principal member of the Miocene 

 series, a succession of shales, tuff, and sandstone, which is ex- 

 posed in greater thickness at Camaching than in the southern 

 part of the field. Together with one or both of the limestones 

 between which it occurs, this member reappears in irregular 

 exposures to the east of the granite exposure. The strata in 

 the upper part overlying the volcanic agglomerate are indurated 

 and nonuniform in bedding, with small rounded pieces or tongues 

 of andesite along the bedding planes. Fragments of chalcedony 

 and silicified wood are common in this portion of the formation, 

 and warm mineralized springs issue from this horizon at several 

 places within the area mapped. 



These upper shales are encountered uniformly throughout the 

 area, but the beds below them present considerable variation. 

 In the northern part of the region the upper shales overlie 

 bedded andesite tuffs, flows, and clastic rocks with an aggregate 

 thickness from 1,500 to 2,000 meters. Along Bayabas River 

 farther south the beds next below the upper shales consist of an 

 upper zone of fine-grained, regularly bedded, calcareous shale 

 which is gray, brown, or red in color and a lower zone of tuff- 

 sandstone, fine-grained clastic rocks, quartz-sandstone, and 

 conglomerate. The whole Miocene series in the Bayabas River 

 section is less than 1,000 meters thick. The conglomerate is at 

 the base of the series, and is found at places immediately over- 

 lying the igneous basement of granite and older effusives. An 

 exposure on Santol Creek reveals such a relation, and in the 

 conglomerate are angular pieces of both the granite and the 

 older effusives. Undoubtedly the quartz in the quartz-sandstone 

 and conglomerate was also derived through erosion from decom- 



