582 PROCEEDINGS OF THE AMERICAN ACADEMY, 



Viti Levu, besides being the largest and most important island of the 

 group, possesses mucli more geological interest than the rest of the islands, 

 since it is essentially different in its rock formations. Wichman has 

 shown that granites, syenites, diorites, and gabbros occur on this island, 

 as well as crystalline schists and limestones ; and he reaches the conclu- 

 sion that the island was an old continental mass composed of these older 

 plutonic rocks which stood above the water during the whole of the 

 Paleozoic and Mesozoic ages. During the Tertiary the island was sub- 

 merged, and received its stratified deposits of limestone, sandstone, and 

 conglomerate, and was also rent by volcanoes. A few specimens from the 

 interior of the island were given to Mr. Agassiz, one of them being a 

 small piece of dioritic granite having the typical hypidiomorphic granular 

 structure, and thus tending to confirm the presence of plutonic rocks on 

 the island. 



The large island of Vanua Levu, which lies northeast of Viti Levu, is 

 said to be similar to Viti Levu in structure, but very little is known 

 about it. Kleinschmidt desci'ibes a visit to the hot springs on it, but 

 apparently made no petrographical collections. The rest of the islands 

 of the group are volcanic or of coral formation. 



The various types of igneous rocks found in the collection might be 

 conveniently included under the following heads: — 



Dioritic granite. Hypersthene andesite. 



Augite andesite. Hornblende andesite. 



Augite-biotite andesite. Basalt. 



Augite-olivine andesite. Olivine basalt. 



Andesites and basalts are the characteristic rocks of the region, and 

 they show the usual variations in structure and mineral components. 

 Augite andesite seems to be the predominating rock of the islands, and it 

 varies from types having a small amount of augite with a large amount 

 of feldspar, with biotite as an accessory, to those in which augite is the 

 dominant constitueut, showing a gradation into a basalt. Hypersthene 

 andesite was shown by one specimen, so it can be considered as of rare 

 occurrence. Hornblende andesite is more common, yet is also very 

 limited in amount compared to the augite andesite. In fact, the more 

 basic type of the andesitic family predominates, augite, olivine, and 

 labradorite forming the most abundant constituents of the specimens. 



In the description which follows each island represented in the collec- 

 tion has its specimen or specimens described in detail. 



