ORIGIN OF VALLEYS. 381 
towards the sea. Above they open into a common amphitheatre, 
the remains of the former crater, the walls of which are two thousand 
feet high. 
Other examples of volcanic action are seen in the pit craters of 
Mount Loa, among which Kilauea stands pre-eminent. ‘This great 
corral, if we may use a foreign word, is a thousand feet deep, one to 
two miles wide, and over three long, so that it forms a cavity which 
may compare advantageously with many large valleys; and were 
the walls on one side to be removed, it would become the head of a 
valley like that of Hale-a-kala on Maui. 
As an example of this kind of valley upon islands which have lost 
their original volcanic form, we venture to refer to the wide Nuuanu, 
back of Honolulu, (island of Oahu,) which has at its head on either 
side a peak rising above it to a height of two thousand four hundred 
feet, or four thousand feet above the sea. 
The immense amphitheatre to the west of the lofty Orohena and 
Aorai, on the island of Tahiti, is remarkable for its great breadth, and 
the towering summits which overhang it; and if not a parallel case to 
that of Maui, that is, if the head was not originally the great crater, 
there must have been a subsidence or removal of a large tract by in- 
ternal causes. 
The precipice of the eastern mountain of Oahu, is another ex- 
ample of the effect of convulsion in altering the features of islands, 
causing either a removal or subsidence. 
The many fissures which are opened by the action of Kilauea, 
might be looked upon as valleys on a smaller scale, and the germs of 
more extensive ones. But with few exceptions, these fissures as 
soon as made are closed by the ejected lava, and the mountain is here 
no weaker than before. Those which remain open, may be the means 
of determining the direction of valleys afterwards formed. 
Action of the Sea.—The action of the sea in valley-making is often 
supposed to have been exerted during the rising of land; and as such 
changes of level have taken place in the Pacific, this cause, it would 
seem, must have had as extensive operation in this ocean as any 
where over the world, especially as the lands are small and encircled 
by the sea, and there is, therefore, a large amount of coast exposed, 
in proportion to the whole area. 
But in order to apprehend the full effect of this mode of degrada- 
tion, we should refer to its action on existing shores.* At the outset 
* See also De La Beche’s Geological Researches, page 192. 
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