430 SMITH AND EDDINGFIELD. 



IGNEOUS ROCKS. 



The diorite. — Even* gradation between diorite and andesite is found 

 in the Benguet region, the differentiation being largely a matter of depth. 

 The distribution of the diorite is somewhat different from the way it is 

 shown on the 1906 map. We found during our recent examination that 

 tj'pical diorite is much less common than was formerly thought to be 

 the case. In fact, the predominant rock is andesite and not diorite. The 

 principal development of the diorite is in the Antamok Valley just below 

 the Benguet Consolidated Mill. Here the rock can be found in large,, 

 fresh masses in the hill side. 



J. P. Iddings who spent four months in 1909-10 in the Philippines 

 studying the rocks both in field and laboratory, describes this rock as 

 follows : 



At Antamok, Benguet Province, there is a medium-grained quartz-diorite with 

 iuequigranular consertal fabric. It consists of plagioclase and considerable 

 brownish-green hornblende, anhedral with respect to each other, but euhedral 

 toward quartz and orthoelase. There is some altered biotite. In places the ortho- 

 clase is intersertal to poikilitic, with inclusions of plagioclase and hornblende.' 



In the analyses of the diorite on page 221 of Eveland's article a cor- 

 rection should be made in the first three samples. Plainly the alumina 

 is far too low and the ferric oxide much too high. The proportions as 

 given in Xo. II are more nearly correct, namely, Al: re = 2: 1. 



The principal rock in the region is andesite. It has its gi-eatest 

 distribution in the eastern half of the district, where it is found on the 

 highest hills and in the lowest vaUeys. This rock is often mistaken for. 

 diorite, but while it does have dioritic phases it occurs as a porphyritic 

 rock in nearlv all exposures examined. As a typical andesite and a t^'pical 

 diorite can originate from the same parent magma and vary through the 

 difference in rate of cooling, we do not necessarily find a sharp line 

 between the two as has heretofore been considered to be the case in 

 this region. TTe have been unable to draw such a line save in one 

 place, namely in the lower Antamok Valley near the Camote property, 

 and here the diorite is a later intrusion, cutting off the Camote vein 

 sharply. 



The Tertiary lavas. — The Tertiary lavas comprise earlier andesite, 

 later andesite, and intrusives. In the deep valleys we find the finer 

 grained and also more nearly holocrystaUine varieties. It is diflBcult 

 to locate the contact, but the great difference between this and the variety 

 found on the high levels makes it fairly certain that there is a time 

 break. In fact, in Major Creek we have found a series of sediments, 

 not easUy traced it is true, but plainly shown at one or two points, which 



'This Journal, Sec. A (1910), 5, 169. 



