MINERAL RESOURCES OF AROROY DISTRICT. 407 



andesite passes over into a basalt, while on the other hand the appearance 

 of hornblende in place of the angite gives the hornblende andesite. 



The hornblende andesites axe mapped with the angite andesites and 

 it is often difficult to distinguish between the two in the decomposed rocks 

 frequently met with here, owing to the decomposition of the femic mine- 

 rals. The largest areas of hornblende andesite are in the range of high 

 hills Just north of the Lanang Eiver, and in Mount Yil-lon. Specimens 

 from the former show small hornblende phenocrysts and rather more acid 

 feldspar than is found in the augite andesites, and a slightly larger pro- 

 portion of salic minerals. Quartz is present occasionally. In another 

 part of the area enough quartz is found under the microscope to bring the 

 rock into the rank of dacite. The Mount Vil-lon andesite contains similar 

 minerals, but is much more porphyritic, the phenocrysts so far exceeding 

 the ground-mass as to make it doubtful whether the rock should not be 

 classed as a diorite porphyry. A very peculiar type of hornblende andesite 

 is that occurring in occasional outcrops, probably dikes, in the two creeks 

 north of Mount Aroroy. Here the rock is characterized by an extremely 

 fine-grained hyalopilitic ground-mass containing minute rods of andesine 

 feldspar, small andesine phenocrysts, scarcely noticeable hornblende pheno- 

 crysts often reaching a length of 3 or 4 centimeters. 



Tlie pyrodastic rocTcs. — As will be seen from the map, the pyroclastic 

 rocks cover the greater part of the central portion of the area. The 

 greater proportion of these rocks are agglomerates, always derived from 

 augite andesite, suggesting that the period of extrusion was contempor- 

 aneous with the augite andesite phase of the volcanic series. The agglo- 

 merate contains large, angular blocks of augite andesite often exceeding 

 half a meter in width. On the outcrop, the weathering proceeds most 

 rapidly in the matrix, giving the rock an irregular appearance which 

 makes it easily recognizable. Small beds of andesitic tuff and volcanic 

 ash occur with the agglomerate, but these are comparatively unimportant; 

 in general it may be said that the whole are mapped as pyroclastic and 

 may be considered as covered by agglomerate. "WTiere contracts between 

 the different beds are found, as for instance on Balawing Point, the strike 

 is northwest and the dip rather gentle to the southwest. 



The sediments. — Waterlaid sediments derived from the augite andesite 

 are found in two localities, on Panique Creek and on the Gold Bug claim, 

 at the mouth of the gorge of the Guinobatan. There are alternations in 

 Panique Creek of a rather fuie-grained feldspathic sandstone, containing 

 in its coarser phases small pebbles of augite andesite and thin beds of fine 

 volcanic ash with fragments of augite crystals, separated by layers of 

 carbonaceous material, these never exceeding 3 or 4 millimeters in thick- 

 ness. The sediments here suggest temporary lacustrine conditions. The 

 strike is northwest and the dip 10° to 20° to the southwest. On the 

 Gold Bug claim the lower member of the series is a black shale heavily 



105046—4 



