106 
PETROLOGY: IDDINGS AND MORLEY 
and sharp peaks with steep spurs and deep valleys, covered with tropi- 
cal vegetation, constitute the dominant features of this picturesque and 
thickly populated island. 
Verbeek, who with Fennema described Bawean in their report on Java 
and Madura, noted the presence of leucitic and nephelitic lavas, to- 
gether with trachytes, andesites and intermediate varieties. During 
a visit of four weeks in 1914 one of us collected rocks from numerous 
localities and studied the relations of the lavas to one another, so far 
as the covering of soil and vegetation would permit. Some of the 
specimens collected have been analyzed by Dr. Morley, and are the 
first rocks from Bawean to be analyzed, so far as known to the writers. 
The bulk of the lavas appear to be dark basaltic rocks with small 
phenocrysts of augite. They vary somewhat in habit, owing to the 
size and prominence of the phenocrysts and the denseness of the ground- 
mass. They occur mostly as breccias, or tuffs containing blocks of 
various sizes, and to a less extent they are massive flows and dikes. 
They appear to constitute the body of the central mountains, and to be 
the older, or main mass of erupted material. With these basaltic rocks 
are associated lighter colored massive lavas, phonoHtes and trachytes, 
which form hills, spurs and ridges in all parts of the island. They ap- 
pear to have been erupted from fissures, possibly from parasitic cones, 
on the flanks of the basaltic volcanoes, and probably were the latest 
eruptions in the district. However, no definite relations, or contacts, 
of the various bodies of lava were observed, the relative ages of the 
basaltic and phonolitic lavas being indicated by their modes of occur- 
rence, which are like those of similar lavas in other volcanic districts 
in this part of the world. 
Upon microscopical study it is found that the basaltic lavas are almost 
wholly leucite-bearing olivine-augite rocks with subordinate amounts 
of feldspar, both calcic plagioclase and alkahc feldspar, in part antho- 
clase, besides nephehte. These minerals vary in relative amounts in 
different lavas, and are variously developed according to the degree 
to which the rocks have been cystallized. In some instances the 
constituents of the groundmass are so minute as to be obscure and identi- 
fiable with difficulty. In such cases it is the alkalic feldspar and nephelite 
or leucite that is ill-defined. Judging by the better crystallized rocks, 
three of which have been analyzed chemically, analyses 1, 2, 3, the 
basaltic rocks are mostly vicoites, that is, leucite-nephelite-basalts with 
subordinate amounts of alkalicalcic feldspar and orthoclase. Some 
have more plagioclase and approach leucite-basanites or leucite- tephrites. 
