THE PETROGRAPHY OF JAVA AND CELEBES 237 



small phenocrysts of augite, which in thin section are seen to be 

 zonal. The groundmass consists of abundant small leucites with 

 an interstitial matrix composed of augite, magnetite, and minute 

 prismoids of plagioclase. The chemical analysis and norm are 

 very similar to those of No. 5, but the mode differs in having much 

 leucite and no biotite. The rock is a leucite tephrite. 



The rock of analysis No. 8 is gray, dense, aphanitic, and non- 

 porphyritic, from Kali Sekatak. In thin section it is seen to be 

 holocrystalline, and to consist of prisms of augite with some mag- 

 netite, and many leucites with anhedral orthoclase and anhedral 

 alkalic plagioclase, or lime-soda feldspar surrounded by orthoclase. 

 From the chemical analysis and norm it is seen to be the most 

 femic variety from this region that has been analyzed. There 

 are 13 per cent of normative leucite, and nearly 15 of normative 

 nephelite which does not appear as modal nephelite, but must be 

 represented by albite molecules, and by a greater amount of leucite 

 than appears in the norm. The rock may be considered a variety 

 of non-porphyritic vicoite. 



While no crystals of nephelite have been recognized in any of 

 the thin sections of these rocks, some of the specimens collected 

 have numerous cavities with small white hexagonal crystals with 

 basal planes, which appear to be altered nephelite. In other 

 specimens there are cavities containing brilliant square prismatic 

 crystals terminated by pyramidal planes over the edges of the 

 prism, which have the index of refraction and habit of stilbite. 



In a region where there are such highly potassic lavas as those 

 of Mt. Mouriah it is to be expected that the lavas of more recent 

 date should contain notable amounts of potash. Through the 

 kindness of Dr. Verbeek a study was made of the thin sections of 

 rocks collected by him and deposited in the Bureau of Mines, in 

 Batavia, in order to learn whether orthoclase-bearing varieties 

 of the andesitic lavas could be found. The collection contains few 

 holocrystalline rocks which show orthoclase borders around pris- 

 moids of plagioclase, which might be called shoshonites or trachy- 

 andesites. In most cases studied the rocks have a glassy matrix 

 which might contain whatever orthoclase molecules were present 

 in excess of those entering plagioclase crystals. However, the 

 strongly porphyritic glassy lava which occurs in ejected blocks 



