vni, a, 5 Pratt and Smith: Petroleum Resources 335 



Volcanic agglomerate. — In the northeastern part of the area 

 shown on the map several exposures of andesitic agglomerate 

 were encountered. Two outcrops were found at different hori- 

 zons in the Vigo shale and one in the base of the Canguinsa 

 sandstone. 



The most extensive outcrop is a conical hill about 1 hectare 

 in area and 50 meters high. The form is not unlike that of 

 an old volcanic plug, but may be due entirely to the work of 

 erosion. The material is principally agglomerate; but appar- 

 ently massive andesite is to be seen in the central part of the 

 exposure, while an intermediate zone consists of andesite in which 

 the fragments occur in a stony crystalline matrix. The other 

 exposures are also conical in form, but are much smaller and 

 consist entirely of agglomerate, angular fragments of andesite, 

 varying in weight from a fraction of a kilogram to 10 kilograms, 

 embedded closely in andesitic tuff. There is a suggestion of 

 bedding in the agglomerate in the base of the Canguinsa sand- 

 stone on Vigo River. The outcrops are gray to dark brown, 

 and are weathered in a manner that leaves the fragments pro- 

 truding irregularly from the matrix. The agglomerate appears 

 to be interbedded in the shale, but the contacts are obscure and 

 it is not certain that it does not lie upon an eroded surface 

 of shale. 



Near the village of Bato, bowlders of fragmental rocks, prob- 

 ably volcanic agglomerate, were observed in which sedimentary 

 types — sandstone and shale — are most prominent, but are ac- 

 companied by andesite. A thin section of fragmental andesite 

 from this agglomerate was examined under the microscope. The 

 texture is decidedly porphyritic with a large proportion of pheno- 

 crysts, consisting of dark green to brown hornblende and plagio- 

 clase feldspar crystals. In the subordinate groundmass oc- 

 casional crystals and fragments of magnetite are scattered. 

 The petrographic character indicates an extrusive, certainly not 

 a plutonic and probably not an intrusive, rock. A more homo- 

 geneous specimen of andesite taken from the main exposure on 

 Tangob Creek shows similar characteristics. It is porphyritic, 

 one of the hornblende phenocrysts measuring more than 1 centi- 

 meter in length. The hornblende crystals are more abundant 

 than in the previously described rock, and show well-defined 

 reaction rims. 



The several outcrops of volcanic agglomerate occur along a 

 line roughly parallel to the general strike of the sedimentary 

 beds. They are of small area, the distances between them are 

 relatively great, and there is little reason to believe that they 



