ECONOMIC GEOLOGY OF THE BAGUIO DISTRICT, 431 



lie between these two andesites. The andesite in the canons below is 

 much finer grained and has the following characteristics: 



The lower andesite. — Sample from Gold Creek near Kelley's mine. 



Megascopic. — There are two principal facies exposed here, light and dark. 

 The dark facies has the appearance of a typical andesite, bluish gray in color, 

 dense black ground-mass with short stumpy pla^ioclase crystals with glistening 

 surfaces. The rock is quite even in texture. The light phase has a "pepper 

 and salt" appearance, with innumerable green specks, which are hornblendes. 

 Both specimens are fresli and hard, breaking with conchoidal fracture. 



Microscopic. — Both facies are quite porphyritic. The light phase contains 

 plagioclas^e, amphibole. and magnetite in an holocrystalline ground-mass of 

 quartz and felspar. Xo quartz phenocrysts were seen. This is a typical horn- 

 blende andesite. In the dark phase we have plagioclase (labradorite) both 

 augite and hypersthene pyroxenes, all these occurring as large and small pheno- 

 crysts in a cryptocrystalline ground-mass of quartz and felspar with some glass. 

 Magnetite is common. The felspars are marked by the inclusions they contain. 



The upper andesite. — This formation is widely distributed on the 

 summits of the majority- of the ridges in the eastern portion of the dis- 

 trict, but is particularly well shown along the Kias ridge above the 

 Major Mine. This rock has the following characteristics : 



Megascopic. — This rock has not by any means as fresh an appearance as 

 the lower andesites, for the reason that the upper andesite is more exposed. On 

 fresh surface the rock appears gray. It has larger and fewer phenocrysts than 

 those just described. 



Microscopic. — The slide shows plagioclase and smaller augite phenocrysts in a 

 ground-mass which is distinctly glassy. ^Magnetite is abundant. The rock in 

 thin section reveals considerable decomposition of the minerals which can not be 

 seen with the naked eye. 



Andesite breccia.- — ^Eveland has already described this formation under 

 the name of tuff.* 



While there is some tuff included in this formation, it consists mainly 

 of irregular fragments, large and small, of coai-se and fine-grained 

 andesitic material, and would be called more properly an agglomerate, 

 as the constituents are of a heterogeneous nature. Eveland's term "erup- 

 tive conglomerate" was meant to convey the idea of two distinctive 

 characteristics, namely, the volcanic origin and the heterogeneous make-up 

 of the mass. Therefore, we shall refer the reader to his description 

 of it as further attention to it in this paper is unnecessary. 



IXTBUSIVES. 



Eveland has already discussed this class of rocks in the Baguio dis- 

 trict. "We shall add nothing here to his description, but later work has 

 shown the necessity of modifying a statement which he makes on page 

 226 of the discussion already referred to : * * * "Many small dikes 



*This Journal, Sec. A (1907), 2, 231. 



