372 PACIFIC ISLANDS. 
science. The views of M. C. Prevost, of the Geological Society of 
Paris, have been so nearly followed, both as regards the nature of 
volcanic action and the formation of volcanic mountains,* (though 
unaware of the fact when first written out,) that his name is entitled to 
special mention in this place. The science is greatly indebted to him 
for his lucid exhibition of the principles of volcanic action, in which 
he has led the way against much error. 
I MINERAL CONSTITUTION OF THE BASALTIC 
DS AUN DiS Ob iE Hh EeAlC EMT C: 
The rocks of the mountains in the Pacific have been described as 
varying between basalt and clinkstone or porphyry, the former pass- 
ing into the latter as feldspar becomes the predominant and finally 
the constituting mineral. ‘The basalts are either homogeneous and 
wholly uncrystalline; or they contain crystals of augite or feldspar, or 
grains of chrysolite, or magnetic iron; again, they are compact or 
vesicular; and the latter pass into scoriaceous and obsidian varieties. 
The feldspathic rocks are mostly a clinkstone, or a compact rock, like 
the base of porphyry, and approaching trachyte; and they pass into a 
variety of crystalline texture like syenite. The feldspars as far as 
examined are soda-feldspars. ‘This is the case with the lavas of 
Kilauea, the glassy crystals of Maui, which are near Andesine, and 
other crystals from Samoa. 
The terms dasalt and basaltic lava, used in this volume, are neces- 
sarily indefinite, inasmuch as the rocks themselves vary indefinitely 
in character ; we therefore use them in preference to more specific 
terms. ‘They imply the presence of augite and some feldspathic 
mineral as the essential ingredients. ‘The nature of the rocks can be 
better learned from the particular descriptions, than from any names 
that are applied to them. 
We propose to inquire whether any facts observed will illustrate 
the origin of these different rocks, and the peculiar conditions and 
relations under which they occur. 
The most striking fact connected with these relations has been 
pointed ont as noticed in most volcanic regions, viz., the occurrence 
of feldspathic varieties at the centre of the mountains, while the ex- 
* Bulletin de la Société Geologique de France, especially vols. xi. p. 188, and xiv. 
p. 270. 
