60 J.D. Dana on Denudation in the Pacific. 
thin at summit as they rise towards the crest of the volcanic 
cone, and others have this upper part adjoining the crest want- 
ing, owing to the extent of the degradation, so that two valleys 
have a common head against the vertical bluff. A better model 
of the mountain gorges could hardly be made, and it stands near 
by, convenient for comparison. Diamond Hill, one of these 
cones, is 800 feet high. 
We need add little, in this place, on the 2 a of running 
water, after the statement, based on mathematics, that the trans- 
porting foree varies as the sixth power of the eeanieg if we 
remember that these mountain streams at times increase their 
violence a million fold when the rains swell the waters to a flood, 
all iron on this point must be removed. 
few thousand feet in depth, even in the solid rocks, is no 
great aifair for an agent of such ceaseless activity, during the pe-- 
riods which have elapsed since the lauds became exposed to their 
influence. And when we take into view the lofty heights of the 
Pacific islands, their rapid declivities giving speed to the waters 
aud transported stones and earth, we must admit that of all lauds, 
these are especially fitted for denudation by torrents. 
The nature of the rocks also favors wear and removal. They 
are in sticcessive layers, soft conglomerates or tufas ‘vec ienale 
alternating with the harder basalt or basaltic lava. oreover, 
the rock is commonly much fissured, owing to a tendency toa 
columnar structure ; besides, they are often cellular. The waters 
thus find admission, promoting decomposition and also degrada- 
tion. There are, also, frequent caverns between layers, which 
contribute to the same e 
There is every thing favorable for degradation which can ex- 
ist in a land of perpetual suramer: and there is a full balance 
against the frosts of colder regions in the exuberance of vegeta- 
ble life, since it occasions rapid decomposition of the surface, 
covering even the face of a precipice with a thick layer of altered 
rock, and with spots of soil wherever there is a chink or shelf for 
its lodgment, ‘The traveler on one of these islands ——— a 
valley on a summer day, when the streams are re ted to a 
creeping rill which half the time burrows out of sight, enol 
the rich foliage around, vines and flowers in oratonien covering 
the declivities and festooning the trees, and observing scarcely @ 
bare rock or stone excepting a few it may be along the bottom 
of the gorge, might nesticallaal inquire with some degree of won- 
der, where are the mighty agents which have channeled the lofty 
mountains to their base? But though silent, the agents are still 
on every side at work; decomposition is in slow, but constant 
ene the percolating waters are acting internally, if not at 
reover, at osc season, he would find the scene 
nigel to one of wire rs, careering along over rocks and 
