ORIGIN OF VALLEYS. 379 
ingredients may exist in places below, where there is not the iron 
and magnesia necessary to augite and hornblende. 
III. ORIGIN OF THE VALLEYS AND RIDGES OF THE 
PACIFIC ISLANDS. 
The general characters of the valleys of the Pacific islands have 
been particularly noticed in our remarks upon the several groups. It 
is, therefore, only necessary in this place to present briefly a system- 
atic view of the facts, preparatory to our consideration of the causes 
from which they have resulted. 
The valleys have usually a course from the interior of the island 
towards the shores ; or when the island consists of two or more distinct 
summits or systems of heights (like Maui) they extend nearly radiately 
from the centre of each division of the island. ‘They are of three 
kinds : 
I. A narrow gorge, with barely a pathway for a frisky streamlet at 
bottom, the enclosing sides diverging upward at an angle of thirty to 
sixty degrees. They have a rapid descent, and are bounded by de- 
clivities from one hundred to two thousand feet or more in elevation, 
which are covered with vegetation though striped nearly horizontally 
by parallel lines of black rock. There are frequent cascades along their 
course; and at head, they often abut against the sides of the central 
inaccessible heights of the island. The streamlet frequently has 
its source in one or more thready cascades, that make an unbroken 
descent of one or two thousand feet down the precipitous yet verdant 
walls of the amphitheatre around. 
II. A narrow gorge, having the walls vertical or nearly so, and a flat 
strip of land at bottom more or less uneven, with a streamlet sporting 
along, first on this side, and then on that, now in rapids, and now with 
smoother and deeper waters. ‘The walls may be from one hundred to 
one thousand feet or more in height; they are richly overgrown, yet 
the rocks are often exposed, though every where more than half con- 
cealed by the green drapery. 
These gorges vary in character according to their position on the 
island. Where they cut through the lower plains, (as the dividing 
plain of Oahu,) they are deep channels with a somewhat even cha- 
racter to the nearly vertical walls, and an open riband of land at 
bottom. The depth is from one toe three hundred feet, and the breadth 
as many yards. Farther towards the interior, where the mountain 
slopes and vegetation have begun, the walls are deeply fluted or fur- 
