52 J. D. Dana on Denudation in the Pacifie. 
melt away into ridgy hills “se —< and finally into the palm- 
covered plains bordering the 
On our descent, we Folhosed: ‘the western side of the Papaua 
Valley, along a narrow ridge such as we have described, but two 
or three feet wide at top, ‘aud enclosed by precipices of not less 
than a thousand feet. Proceeding thus for two hours, holding to 
the bushes which served as a kind of balustrade, though occasiou- 
ally startled by a slip of the foot one side or the other—our path 
suddeuly narrowed to a mere edge of naked rock, and, more- 
over, the ridge was inclined a little to the east, like a tottering 
wall. Taking the upper side of the sloping wall, and trusting 
our feet to the bushes while clinging to the rocks above, carefully 
dividing our weight lest we should precipitate the rocks and our- 
selves to the depths below, we continued on till we came to an 
abrupt break in the ridge of twenty feet, half of which was 
perpendicular. By means of ropes doubled around the rocks 
above, we in turn let ourselves down, and soon reached again a 
width of three feet, where we could walk in safety. T’wo hours 
more at — brought us to slopes and ridges where we could 
breathe eely. 
The pliant here described characterize all parts of the 
_ island. ‘Towards the high peaks of the interior, the ridges which 
radiate from, or connect with them, become mere nountain walls 
with inaccessible slopes, and the valleys are from one to three 
thousand feet in depth. The central peaks themselves have the 
same wall-like character. It is thus with Orohena and Pitchiti, as 
well as Aorai; and owing to the sharpness of the summit edge, 
rather than the steepness of the assent, Orohena is said to be 
quite inaccessible. Dr. Pickering and Mr. Couthouy, in an ex- 
cursion to a height of five thousand feet on this ridge, met with 
difficulties of the same character we have described. 
Without citing other goon we continue with the author’s 
remarks on the origin of these v 
The causes operating in the Pacific, which may have contrib- 
uted to valley-making, are the following: 
1. Convulsions from internal forces, or ——- action. 
2. Degradation from the action of the 
3. Gradual wear from running water derived from the rains. 
4, Gradual eine, si 5p throug the agency of the elements 
and growing vegeta 
The action of scion forces in the formation of valleys, is 
finely illustrated 1 in the great rupture in the summit of Hale-a-kala 
on Maui. ‘The two valleys formed by the eruption are as exten- 
sive as any in the Hawaiian Group. being two thousand feet deep 
at their highest part, and one to two miles wide. ‘They extend 
the interior outward towards the sea. Above, they open into 
