J. D. Dana on Denudation in the Pacific. 53 
acommon amphitheatre, the remains of the former crater, the 
walls of which are two thousand feet high. 
As other examples of volcanic action, we may refer to the pit 
eraters of Mount Loa, among which Kilauea stands preéminent. 
This great corral, if we may use a Madeira 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 
valleys; and were the walls on one side removed, it might be- 
come 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 
Nananu, back of Honolulu, (island of Oahu,) which has at its 
head on either side, a peak rising above it toa height of two 
thousand four hundred feet, or four thousand feet above the sea. 
é 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 internal forces. 
he precipice of the eastern mountain of Oahu, is another ex- 
ample of the effect of convulsion in altering the features of islands, 
Catising either a removal or subsidence. 
‘The many fissures which are opened by the action of Kilanea, 
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 
posed to have been exerted during the rise of the 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 vast 
Ocean as any where in 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 degra- 
dation, we should refer to its action on existing shores.* At the 
outset we are surprised at finding little evidence of any such 
action now in progress along lines of coast. The islands, and 
the shores of continents have occasional bays, but none that are 
pening by the action of the sea. The waves tend rather to 
fill up the bays and remove by degradation the prominent capes, 
thus rendering the coast more even, and at the same time, accu- 
Baas te, Tiew here presented is sustained in De la Beche’s Geological Researches, 
