406 FERGUSON. 



not difficult to decide that the}' are of much later date than the former, 

 and it seems probable that the quartz diorite had undergone considerable 

 erosion before the eruption of the Panique series. 



The Paniqtie. series. — -Volcanic rocks and their pyroclastic derivatives 

 cover by far the larger poiiion of the Aroroy district (Plate II, fig. 1) 

 and as they show great petrographic variety this formation might be split 

 into a far greater number of units. The different outcrops are so scat- 

 tered, and their limits so uncertain, that it would be exceedingly difficult 

 to show all the rock types on a map of this scale without confusion. The 

 following igneous rocks, included in this formation, have been determined 

 microscopically: Dacite, augite-andesite of several varieties, hornblende- 

 andesite of at least three distinct types, several t3rpes of basalt, and leucite 

 tephrite. The formation is divided on the map as follows : The andesites, 

 which are the common rocks of the district, the pyroclastic rocks, includ- 

 ing agglomerates and tuffs, the rather rare occurrences of sedimentaiy 

 rocks derived from the andesites, and the more basic rocks. In working 

 on the igneous rocks of the area it has been found difficult to show fine 

 distinctions, as they are so deeply decomposed in places as to make deter- 

 mination difficult, and in many cases a cei-tain amount of transportation 

 has been added to this decomposition, forming arkose-like rocks, the 

 "toba" of the Spanish geologists. 



The andesites. — Augite andesite is the commonest volcanic rock here 

 as elsewhere in the Philippines. The rocks of this type have been de- 

 scribed so often that a repetition seems unnecessary. Moreover, notes on 

 the augite andesite from Aroroy will be found in Idding's paper.^'' The 

 rocks of this type found here are, as a general rule, markedly porphyritic 

 and show prominent phenocrysts of augite and feldspar, the former being 

 the most prominent. A peculiar phase of the augite andesite is exhibited 

 on the shore of Port Barrera near Maguilanguilan Island, where the 

 rock is extremely porphyritic and carries phenocrysts of augite, the bases 

 of which are sometimes as much as 15 millimeters square. The feldspar 

 phenocrysts grade from andesite to labradorite, and are of smaller size 

 than the augites. Magnetite is present in varying amounts. The ground- 

 mass always shows the characteristic hyalopilitic texture with occasional 

 marked flow-structure. While the presence of large amounts of pyro- 

 clastic rocks derived from the augite andesite is evidence that a part at 

 least of this series is of extrusive origin, in only one place was any 

 structure found indicative of a surface flow. At one point in the western 

 part of the district the andesite was found to contain broken blocks of 

 the same rock, as if it had been brecciated at the bottom of a surface 

 flow. The lack of large exposures greatly reduced the chances of finding 

 other similar evidence. With the appearance of olivine, the augite 



^'Loc. cit., 157. 



