40 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
of limestone litter its rounded volcanic hill; these, however, represent 
unmistakably disrupted cliff masses, and are the broken fragments 
resulting from the volcanic outbursts. 
THE Vo.tcanic Rocks. 
A good collection of volcanic rocks of the more common types from 
the Fiji area was secured, and a brief summary of their macroscopic 
and microscopic characteristics is given below. 
As regards the general macroscopic features of the Fiji andesites, they 
may be described as compact, reddish, or greenish rocks weathering into 
spheroidal blocks, and showing dark green augite and plagioclase pheno- 
crysts in a fine-grained base. The development of the large augite 
prisms is very pronounced. 
In section, two generations of plagioclase are distinguishable : first, a 
crop of large idiomorphs with corroded edges, and showing well-developed 
concentric zoning. These also show albite twinning, and their general 
outline is tabular, although often they occur in nests or bunches, one 
crystal being partly or wholly enclosed by others. Next, a crop of small 
lath-shaped sections. The “ felt-like aggregate of feldspar microlites ” so 
characteristic of andesites is also well marked. The angites constitute 
an important factor in the mass. They are usually represented as 
monoclinic prisms, yielding octagonal sections, and showing concentric 
zoning, and twinning parallel to the orthopinacoid in a marked degree. 
Rhombic pyroxene polarizing in high colors and almost indistinguishable 
from olivine in thick sections is also common. The augite is often 
replaced by viridite. The following are some detailed descriptions of 
some of the thin sections I prepared of the Fiji volcanic rocks : — 
No. 1. Levuka. Specific gravity, 2.9. A compact augite-andesite. 
The augites are in the form of phenocrysts and yield fine octagonal sec- 
tions. Concentric zonal structure and twinning parallel to the ortho- 
pinacoid is common. ‘Two generations of plagioclase, the older and 
larger showing kaolinization and concentric zoning, the newer crop con- 
sisting of lath-shaped sections. The augite has been replaced by viridite 
in places, and much magnetite is scattered throughout a felt-like base. 
This is a fragment from a lava agglomerate. 
2. Mango. Specific gravity, 2.57. Taken from a lava flow. An 
extremely dense andesite with idiomorphs representing the first crop of 
plagioclase. The “andesite” ground mass is well developed, and large 
particles of magnetite occur in a great development of magnetite specks. 
