GEOLOGICAL STEUCTURE. 
287 
islands and the tranquil sea, combine to form one of 
the finest panoramas. 
Mdndoleh possesses a great advantage over the 
other islands in several springs of water, which though 
scanty, would — from being perennial — afford an abun- 
dant supply, if economized by tanks. The temper- 
ature of these, 77° of Fahrenheit, while the air was at 
83", shows that their sources are in the loftier regions 
of the mountain''^ 
* This island— Mondoleh— is principally basalt, a rock composed 
of felspar, hornblende, and magnetic iron ore : with porpliyritic 
crystals of basaltic hornblende embedded in it, of an amygdaloid tex- 
ture ; the pores elongated horizontally,— filled witli inesotype, either 
in crystals, or in compact masses of crystalline character, or with a 
green earthy matter, or with some other spathic mineral; the last 
mentioned exhibiting crystalline fomis of the hexagonal system, as, 
rhomboedrons with elongated hexagonal axes, combined with others 
of turned,— or of shortened axes. This combination may be possibly 
new. 
Sometimes the crystals are arranged in kidney-shaped groups, 
indicated by the snlphuret of iron, but in such a manner, that each 
specimen can be distinguished and examined in respect of its 
crystallization. 
In the higher parts of the island the ainygdaloidal texture of the 
basalt ceases, and it seems to be of a more solid character with 
shivery cleavage; crystals of basaltic homblende more distinct are 
embedded in it and few crystals of olivine. These formations are 
intersected by a second kind of basalt in vertical veins from two to 
four feet in breadth, running east and west. 
The rock is quite solid, of a rather deep blue colour, with embedded 
crystal of basaltic hornblende and olivine. The separation assumes 
a columnar character in a horizonal position and hexagonal form. 
A minute examination of the basalt discloses evidences of different 
periods of cooling ; as, on the sides of the vein it is quite compact 
