MINERAL RESOURCES OF AROROY DISTRICT. 405 



latter, and it is reasonable to conclii<le that the Kaal is the oldest formation 

 here exposed. Of course, the intrusion by the diorite of a slate similar 

 to that of the main sedimentar}- mass is not of itself absolute proof, but 

 were the slate not older than the diorite it would be strange if somewhere 

 a basal conglomerate containing diorite pebbles, were not found. Little 

 can be said as to the absolute age of the Kaal formation. Xo fossils 

 have been found, but Judging from its position, its extreme contoi'tion 

 and considerable metamorpliism, it is my belief that it may be classed 

 provisional!}^ as pre-Tertiary,^° the earliest Miocene shales of the Port 

 Barrera formation. 



The Aroroy quartz diorite. — The quartz diorite mass mapped imder 

 this name is of comparatively limited extent in the Aroroy district, being 

 found only in the extreme northern part of the area. However, it widens 

 extensively to the eastward, being the principal rock met with, between 

 the Guinobatan River and Magdalena Bay. 



The details of the petrography of the igneous rocks of tliis series and 

 the more varied types to be foitnd among the voleanics of the Panique 

 formation are left for a future paper, the present article being more 

 particularly concerned with the economic geology. Dr. Iddings,^® in a 

 preliminary article on Philippine petrography, has described several of 

 the characteristic rocks from this district. It will be sufficient to mention 

 that the quartz diorite is generally rather fine grained, with granitic 

 texture, although coarse grained phases are common. The visible min- 

 erals are feldspar and hornblende: rarely biotite. Under the microscope 

 the feldspars prove to be andesine and oligoclase with rarely small grains 

 of what apparently is orthoclase ; the area pertaining to this mineral is 

 about 60 per cent of the slide. Quartz occurs in the interstices between 

 the feldspars. The quartz grains are often cracked and show cloudy 

 extinction due to strain. This mineral occupies rather more than 20 

 per cent of the area of the slide, and the hornblende rather less than 

 this. Magnetite occurs in scattered, minute grains. The striking feature 

 of the rock is the small proportion of femic minerals which in general 

 are present. 



Scattered outcrops of deep-seated igneous rocks occur, along the Gui- 

 nobatan Eiver and near the headwaters of Balangting Creek. These 

 rocks, for convenience, are mapped with the quartz diorite; under the 

 microscope they are found to include diorite, syenite, and gabbro. No- 

 thing definite could be learned in regard to their field relations. Xo con- 

 tacts were found between the quartz diorite and the apparently overlying 

 voleanics of the Panique group, but from the position of the latter it is 



" Metamorphism is no longer considered an indication of age ; Tertian* rocks 

 have been subjected to profound d_\Tiamic stresses and are metamorphosed. 

 Without fossils the question can not be satisfactorily settled. W. D. S. 



"T/iis Journal, Sec. A (1910), 5, 155. 



