AS Eel IT. 295 
place, these varieties are all of common occurrence on the island. 
They pass into vesicular varieties, which sometimes appear almost 
scoriaceous. ‘The rocks also contain magnetic iron, which, in the form 
of sand, may be found in many places along the seashore. The com- 
pass is often rendered useless on the island by the local attraction of 
the rocks. Bearings taken on the mountains were found to vary two 
or three points on changing the position of the instrument. The sye- 
nitic rock referred to has the appearance of common syenite, yet con- 
tains no distinct grains of quartz. It consists of a crystalline feldspar, 
(near albite,) with a few acicular crystals of hornblende. It appears 
to be only a feldspathic variety of the same igneous rocks that consti- 
tute the island. ‘The trachytic basalt differs from the other varieties 
in containing more feldspar, and a consequent lighter colour and less 
specific gravity. It is intermediate between the common basalt and 
the syenite just mentioned. 
The conglomerates are of every degree of coarseness, from a rock 
with rounded stones six inches through, to a fine yellowish-brown 
earthy tufa, containing only disseminated crystals of chrysolite or 
augite. Some of the conglomerates present a singular mottled appear- 
ance, from the many colours of the fragments composing them : stones 
and pebbles of ash-gray, brown, red, and grayish-blue colours being 
imbedded together in a brownish-yellow or brick-red base. ‘The frag- 
ments, though angular in many instances, yet in the cliffs toward the 
shores are very commonly rounded. 
Entering the valleys from the seashore, the first thing in the struc- 
ture of the hills which strikes the attention, is the regular stratification 
of the rocks, and a dip or inclination toward the sea. The dip varies 
from three to ten degrees, or, more rarely, fifteen. Wherever exa- 
mined, it is uniformly from the centre of the island outward. In 
many instances the slopes of the summit of a ridge correspond with 
the dip of the subjacent layers; but generally the denudation which 
has taken place has increased somewhat the rapidity of these slopes. 
Although these declivities are generally overgrown, yet here and there 
dark lines of rocks appear through, giving a riband character to the 
surface, and exhibiting the usual seaward dip. 
The rocks constituting these layers are mostly vesicular varieties 
of the light and dark gray basalt, sometimes becoming red or brown. 
Between these layers, are others of the conglomerate and tufa 
described. I observed no exposed section where the exact character 
of the several alternations could be ascertained. Indeed, through the 
