414 W. T. BRIGHAM ON THE VOLCANIC PHENOMENA 



Erosion. 



The effects of erosion have been considered evidences of the age of the Hawaiian moun- 

 tains, and it would be well perhaps to examine the extent to which rains, rills, and rivers 

 rushing rapidly, rend rocky regions, remove rock remains, leaving lofty ledges, perpendic- 

 ular pahs or precipices, and in a word work wonderful changes in the smooth, swelling 

 summits of these volcanic islands. 



The original form of the mountains was probably quite similar to that of Mauna Loa 

 — smooth rounded domes; but the present aspect is, as we have seen, quite different. 

 Wherever the trade winds have driven the rain clouds against the mountains, streams have 

 formed, and while at first they have been absorbed by the porous lava and carried by 

 subterranean paths to the sea level, the decomposition of the lava produced by constant 

 moisture has at last formed a soil capable of retaining surface streams. The nature of 

 the rock has much to do witli the rapidity of this process, and the shape of the central 

 compact core is not without its influence. The continued rains, finally retained on the 

 surface until they have acquired sufficient volume to form brooks, commence the erosive 

 process. 



Mauna Loa is smooth ; no rivers run over its slopes, and its only ravines are the work of 

 streams of lava or the earthquake. The same is true of Hualalai, and both these moun- 

 tains are on the leeward side of the island of Hawaii. A glance at the map (PI. XIV.) 

 will show their position, and also that of Mauna Kea and Mauna Kohala, whose peaks 

 almost entirely intercept the vaporous clouds which the constant winds bring from the 

 ocean. The windward slopes of these latter mountains are deeply channelled by the per- 

 manent streams and frequent freshets, while on the south and west they are as smooth as 

 the sides of Mauna L5a. The gorges on Mauna Kea from Hilo to Laupahoehoe, are both 

 numerous and deep, and indicate as long continued erosion as those on Kauai. Many 

 extend more than half way up the mountain through the dense forests on the north-east, 

 and while some, as that at Laupahoehoe, have been worn down to the sea level so as to form 

 valleys of considerable extent, the majority meet the ocean in a fall of several hundred 

 feet on the very shore. In all these cases the rock is essentially of the same nature 

 and arrangement; and where the streams are of equal size, the height of the fall is about 

 the same, indicating curiously enough the synchronous origin of all these streams. At 

 Laupahoehoe, the stream is not very large, but there are indications of earthquake rents 

 which assisted the eroding waters. I was unable to examine this valley as I could have 

 wished. 



On M£ui (PI. XIII.) the windward portions of Haleakala, comprised in the district of 

 Hana, are grooved in the same way as we have described them on Kea, but like the mountains 

 of Hawaii the destroying valleys have not yet reached two thirds of the height of the 

 summit. The leeward slopes are not eroded. Eeka is lower, and the torrents have completed 

 their work on the north-east, and have even invaded the other sides, until, like the Tahitian 

 mountains, only a skeleton is left This result is owing to the depth of the craters and 

 their shattered walls, as well as to the streams of water. These cavities were so deep that 

 they penetrated the solid core of the mountain to a considerable depth, and thus retained 

 water to supply streams. 



On Lanai and Kahoolawe (PI. Xin.), the ravines are not the work of surface streams. 



