Dr. H. H. Arnold-Bemrose—Olivine Nodules in Basalt. 3 
inclusions which give it a very rough appearance under the micro- 
scope. Some of the olivine grains are twinned many times. One twin 
in thin slice 1479 consists of twelve portions, another consists of five 
parts. It is almost impossible to distinguish the boundaries of the 
twins in ordinary light, but in polarized light the whole or part of 
a grain is divided into broad plates which differ ttle from each other 
in colour, the sides of which, corresponding to the trace of the 
twinning planes are often, but not always, parallel to one another. 
The angles of extinction of adjacent portions differ only by a few 
degrees and extinction in all cases is either parallel or nearly parallel 
to the trace of the twinning planes. 
The pyroxene in the nodules is monoclinic and can only be referred 
to augite. In the absence of crystallographic outlines the only method 
of distinguishing between the orthorhombic and monoclinic pyroxenes 
is to measure the angles of extinction with regard to the cleavages 
and to observe the optic figures in convergent light. In the thin 
slices examined there is no certain trace of enstatite or orthorhombic 
pyroxene. On the other hand, the whole of the evidence points to 
the presence of monoclinic pyroxene or augite. Two types of cases 
occur in which the crystalline grain is traversed by parallel cleavage 
lines. In the one extinction takes place parallel to the traces of 
cleavage and in convergent light shows an optic axis just outside the 
field of view. These are augite grains parallel to (100). In the 
other the angle of extinction varies from a few degrees up to an angle 
of 45° and denotes augite from the prism zone. Few cases are present 
of pyroxene showing an almost rectangular cleavage, but im all 
these there is an optic axis nearly in the centre of the field, which 
proves that the mineral is augite and not enstatite. 
Picotite is present in the nodules only to a slight extent. It is of 
a yellowish-brown colour in ordinary light and becomes extinct under 
crossed nicols. 
The presence of olivine and of augite in small groups from almost 
microscopic dimensions up to a length of more than two inches in the 
Calton Hill basalt is in favour of the nodules being segregations from 
the magma and not enclosures of older rock. 
EXPLANATION OF PLATES I AND II. 
[The figures were photographed by the author from the microscope, under polarized 
light with crossed nicols. ] 
Prate [. 
Fic. 1. Thin slice (1464), magnified twenty diameters, showing olivine grains in 
anodule. The olivine is traversed by cracks containing serpentine and minute 
pieces of black iron ore. 
Fic. 2. Thin slice (1464), magnified twenty diameters, showing olivine grains in 
a nodule. The grain of olivine near the centre of the figure is twinned ; one 
portion of the twin extinguishes parallel to the trace of the twinning plane and 
the other at an angle of 4° with it. 
Fic. 3. Thin slice (1479), magnified twenty diameters, showing part of a large 
grain of olivine in a nodule. The olivine is traversed by cracks and shows 
multiple twinning. The twinning planes are not always parallel, and the 
extinction angle of each portion of the twin is either parallel or nearly parallel to 
the trace of the twinning plane. 
