PETROLOGY: IDDINGS AND MORLEY 
107 
No rocks were collected which could be classed as andesites, nothing 
resembling the andesitic lavas so common on Java. 
The rock, whose chemical composition is shown in analysis 1, is dark 
colored with abundant phenocrysts of augite, which are green in thin 
section, and with fewer of olivine. The groundmass is holocrystalKne, 
and consists of leucite and prismoids of plagioclase, partly inclosed in 
poikilitic orthoclase and nephelite. There is some magnetite. The 
mode is richer in leucite than the norm, and poorer in nephelite, owing 
to the presence of albite molecules in the lime-soda-feldspars. There 
is less orthoclase also in the mode than in the norm. 
The rock, from which analysis 2 was made, is from the same locality 
as the first, but differs from it somewhat in habit, being more distinctly 
porphyritic, with phenocrysts of green augite which are markedly 
zonal in thin section. The groundmass consists of augites, small oKvines, 
pronounced leucites, with less conspicuous prismoids of plagioclase, 
and some magnetite. The ill-defined matrix probably contains ortho- 
clase and nephelite. The mode of this rock is much richer in leucite 
than the norm, with other corresponding differences in the felsic con- 
stituents. These two rocks occur at the Fails of Grotjokan, near the 
center of the island. 
The rock of analysis 3 occurs on the southwest coast of the island. 
It is light gray, with small, inconspicuous, phenocrysts of augite and 
scattered oUvines. The holocrystalline groundmass consists of anhedral, 
poikilitic, lime-soda-feldspars, with microcline-Kke polysynthetic twin- 
ning, besides poikilitic orthoclase. Both kinds of feldspar inclose euhe- 
dral microlites of leucite and nephelite. Thic rock is more feldspathic 
than the vicoites just described, and its chemical analysis and norm 
show that it is transitional to the rock of analysis 1, and that it be- 
longs in the magmatic division II. 7. 2. 3 of the Quantitative System 
of Classification. It is proposed to name this division, 11. 7. 2. 3, 
baweanose, for, although the rock from Tandjung Anjer is transitional 
in composition, other varieties of lava will probably be found on Bawean 
which will be still more salic, and will have a composition correspond- 
ing to the symbol 11. 7. 2. 3. 
The phonolitic lavas of the island belong to several varieties. Most 
of them are nephelite rocks with small phenocrysts of orthoclase and 
nephelite in a groundmass of alkaHc feldspars, nephehte and pale green 
augite, with some sodahte; others, with similar phenocrysts, consist 
of minute enhedral nephelites, anhedral, microperthetic, alkalic feldspar, 
and clusters of poikilitic aegirite-augite with small prismoids of what is 
