THE PETROGRAPHY OF JAVA AND CELEBES 245 



In addition to the rocks just described from Java and Celebes 

 there are two from Sumatra that have been analyzed in order to learn 

 whether earlier analyses of rocks from the same localities which 

 showed relatively high alkalies were correct. One is a grayish- 

 black pitchstone with many minute phenocrysts that occurs where 

 the road crosses a stream in the village of Semaboer, on the south 

 slope of Merapi volcano, in Central Sumatra. In thin section it is 

 seen to consist of brown globulitic glass base crowded with micro- 

 lites of prismoid feldspar, pyroxene, and magnetite, with scattered 

 phenocrysts of calcic plagioclase and brown pyroxene. The 

 chemical analysis No. 19 and the norm show that it is dacose, with 

 normal composition for a dacitic andesite-pitchstone without 

 visible modal quartz crystals. 



The second rock from this region is dark greenish gray and 

 aphanitic, with minute phenocrysts of feldspar and mafic mineralsi 

 It occurs in a small stream near the village of Rau Rau, at the 

 east base of Merapi volcano. In thin section it is seen to be almost 

 holocrystalline, with possibly a little pale-brown glass base between 

 prismoids of plagioclase and smaller prismoids and anhedrons of 

 pyroxene, with magnetite. There are abundant small phenocrysts 

 of calcic plagioclase, fewer larger ones of pale augite, and numerous 

 small olivines. The chemical analysis No. 20 and the norm show 

 the rock is an andose with normal amounts of alkalies. It may be 

 called an olivine-bearing pyroxene andesite. 



