1240 
leucite basalt of East-Borneo, discovered by Prof. MOLENGRAAFF }). 
This last-mentioned biotite is not resorbed, and is often deposited in 
the rock of the Ringgit round the older resorbed crystals, however 
with different optical orientation of the optical groundmass. These little 
brownish-red crystals which also occur dispersed through the rock, 
and as a rule do not show any definite shape, enclose particles of 
the groundmass. The groundmass consists of leucite, augite and 
ore. Sometimes the leucite attains somewhat larger dimensions 
than the majority of the crystals of the groundmass, without forming 
real phenocrysts, the augites are column-shaped and colourless or 
light-green, the ore is plentiful in the rock. 
Macroscopically one sees already locally in little cavities neogenic 
minerals of very sinall dimensions, many of which show the shapes 
of crystals of leucite or sodalite, or also of feldspar. Under the 
microscope one sees in these little cavities isotropic crystals, 
together with neogenic feldspar and sometimes some biotite, whilst 
the dark background against which the prisms of augite set off be- 
tween crossed nicols, is often interrupted by anisotropic portions, 
which for some distance have the same optical orientation, and poi- 
clitically surround the particles of the groundmass. These anisotro- 
pic portions sometimes consist of kalifeldspar, twinned according to 
the Carlsbad-law ; polysynthethic twins have not been observed, but 
the zonal structure which is often distinguishable, points also to the 
„presence of plagioclases. These minerals have been formed after 
the crystallisation of the groundmass of the rock and point to pneu- 
matolytic elements in the magma, which have been set free after the 
final crystallisation; consequently they are no normal constituents 
but products of the autopneumatolysis in the rock. Locally they 
may occur in considerable quantities. 
The enclosures of this leucite are coloured light-red or brownish, 
and contain little phenocrysts of plagioclase where they have not 
been altered into a hypo- or eryptoerystalline groundmass. 
In some of the enclosures examined the plagioclases show micros- 
copically a well developed zonal structure, in others they are 
only slightly so, or sometimes not at all. Major twins according to 
the Carlsbad-law of the polysynthetically twinned feldspars occur. 
The basicity of the feldspars sometimes decreases regularly from the 
centre io the margin, in which case transitions were observed from 
labrador or bytownite to oligoclase or andesine, but alternations of 
!) H. A. Brouwer. On micaleucite basalt from Eastern-Borneo. These Proceedings, 
June 26, 1909 p. 148. 
