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 Volcanic 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 augites 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 The augite has been replaced by viridite 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 gravit} T , 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. 



