232 JOSEPH P. IDDINGS AND EDWARD W. MORLEY 



Bawean, between Java and Borneo. Verbeek states that Mt. 

 Mouriah is in large part tuffs and breccia with some flows of mas- 

 sive lava. He describes the rocks as chiefly leucite tephrites and 

 leucitites with small amounts of feldspar in the groundmass ; there 

 being all gradations between these varieties. Olivine-bearing 

 varieties are much fewer. In some of the rocks there are pheno- 

 crysts of leucite 5-10 mm. in diameter, and in exceptional instances 

 15 mm. The groundmass of these rocks is in most instances dark 

 gray to black, dense and aphanitic, less often porous. 



Two of the localities on Mt. Mouriah mentioned in Verbeek's 

 description were visited in 1910: one was the stream Kali Gillinan, 

 near the village Masin, on the south slope of the mountain above 

 Bareng; the other locality was the stream Kali Sekatak, below the 

 village Ragou, above Petjangaan, at the west base of the mountain. 

 The first stream has a narrow channel at the place visited, and 

 washes great bowlders of lava, and has short beaches of smaller 

 bowlders and gravel. The second stream is much larger, flows in 

 more open country below Ragou, and has long reaches of gravel and 

 bowlders. In both places the bowlders represented great varieties 

 of leucitic rocks which were mostly dense and compact and 

 extremely fresh and unaltered by weathering, leucite crystals at 

 the surface of the bowlders being transparent, or only slightly 

 whitened in some instances, but completely altered in others. 

 However, when the rocks are broken the interior portions usually 

 appear to be very fresh, which proves to be the case when their 

 sections are studied microscopically. These leucitic rocks, although 

 considered by Verbeek to be possibly of late Tertiary age, are as 

 fresh and as well preserved as modern lavas, or as many Tertiary 

 lavas in the arid regions of Western America. In places where they 

 have been covered by soil and vegetation for long periods of time 

 they are completely decomposed for a short distance from the rock 

 surface, as Mohr has shown to be the case with andesitic rocks in 

 other parts of Java. Of 37 specimens collected from various 

 bowlders at the two localities named, 8 varieties have been analyzed 

 chemically, the analyses and norms being given in Table I, together 

 with analyses of lavas from two active craters in other parts of 

 Java; a short petrographic description of each follows. 



