222 The Philippine Journal of Science 1914 



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material. The limestone which occurs just below the ore and 

 accompanying skarn lies upon a complex of effusive and intrusive 

 rocks. Dikes and flows are found in the bedded rocks. The out- 

 crop is exposed in the northwestern slope of a spur of Mount 

 Silao, and can be traced over a length of 600 meters and a width 

 of from 20 to 70 meters. It strikes north 20° east, and dips 

 about 45° west-northwest, in strict conformity with the bedded 

 rocks. 



The ore is principally magnetite carrying quartz and pyrite 

 which appear in each case to be present both as primary and 

 secondary minerals. Eddingfield studied microscopic slides of 

 the Camiaching ore in which quartz and pyrite were apparently 

 original constituents with magnetite, while the occurrence of 

 small veins of secondary quartz and pyrite in the ore was com- 

 monly observed in the field. The ore grades into wall rocks, 

 and the gradation stage consists of an increasingly leaner ore 

 of magnetite with altered pyroxenes or amphiboles, pyrite, and 

 quartz ; fibrous aggregates in the green wall rock were identified 

 as probably enstatite by Smith. Samples of the limestone re- 

 placed by red hematite were obtained ; likewise veinlets of mag- 

 netite up to 15 centimeters in width cut the limestone, and 

 "horses" or rounded blocks of limestone are found inclosed in 

 the main body of magnetite (Plate III). 



The precise relation of the intrusive rocks to the ore at Cama- 

 ching was not determined. Many of the indurated fine-grained 

 fragmental rocks are quite similar to the igneous rocks mega- 

 scopically, and the two types could not be distinguished satis- 

 factorily. A rock which is exposed with ore on both sides of it 

 by one of the small creeks flowing across the outcrop and which 

 exhibits the appearance of a dike was examined in thin section 

 by Smith and classed as porphyritic andesite with phenocrysts 

 of green hornblende and decomposed plagioclase feldspars. 

 Another igneous rock from near the ore deposit Smith found to 

 be holocrystalline and to contain principally pj^roxene and plag- 

 ioclase feldspar with some quartz. This rock might be classed 

 as a diorite. 



THE MONTAMORONG ORE DEPOSIT 



The Montamorong outcrop is exposed near the eastern margin 

 of the granite area by a small stream which is tributary to 

 Maasim River. It is about 7 kilometers south-southwest of 

 Camaching, and lies a little to the west of a line from Camaching 

 to Hison. A shallow pit a meter or more in each dimension has 

 been sunk in the outcrop. Intrusive rocks, ophitic in texture 



