98 MICHIGAN ACADEMY OF SCIENCE. 



been exposed loiifj to tbe action of -water, the latter had long; ago for- 

 gotten what it was to be dry. .Moisture, then, is the all important 

 factor in denudation where volcanic rocks' are concerned. 



Our hypothetical island stands in the path of the trade wind which 

 condenses its moisture into clouds that hang perpetuall}' about its 

 summit or on its tianks, maintaining an altitude between 1,500 and 

 5,000 feet, keeping the rocks almost perpetually in a damp condition 

 and drenching them at frequent internals with tropical showers. One 

 consequence is a luxuriant vegetation which flourishc^s in s|»ite of the 

 fact that the rain washes away soil as fast as it is formed. 



,Vt first, indeed, the vesicular lava sucks in the rain like a sponge, 

 but it is not long before incipient decomposition of the rock supplies 

 ciay for ]tuddling the surface and soon, geologically speaking, streamlets 

 will begin to seek out the lines of least resistance on the surface and 

 will then set themselves to remove the obstacles they find in their path. 

 So stream erosion is begun. It proceeds slowly at first, for it has to 

 act on a very tough material, but that which mechanical violence fails 

 to accomplish, the magic of chemical affinity eftects, and as soon as 

 any channel at all is formed, this quasi solvent action of water is 

 localized exactly where it will be most effective, in the rocks immediately 

 underlying the bed of the stream. 



Meanwhile vegetation is playing its important part, mosses and ferns 

 wrapping the rocks to keep them wet, the roots of shrubs and trees 

 penetrating every crevice with their j»owerful wedges, vegetable acids 

 and products of vegetable decay making havoc of the molecular structure 

 of the rock. .Vll this is a very familiar story to you, no doubt. So too 

 is the process next to follow, of scoring the slopes of the cone with 

 ravines radiating from the center — begun at the base w^here the volume 

 of water is greatest, passing through the various phases which result 

 in the complex of knife-edge ridges separating deep and steep- walled 

 valleys that characterizes the later stages in the life history- of a 

 mountain. That this succession of changes will proceed with extra- 

 ordinary rapidity in material such as I have described, if conditions are 

 favorable, it is almost superfluous to say. There are however a few 

 points in connection with the progress of this denudation that are 

 especially worthy of remark. Since the chemical effects associated with 

 the presence of moisture are of vastly greater conse(]uence than the 

 mechanical action of the water, the valleys formenl will retain in their 

 lo^er stretches the character of canyons, narrow with precipitous sides 

 and insignificant lateral valleys. The upper j)ortion will assume the 

 V-shape, with numerous V-shaped tributary valleys, or in the later 

 period will take the amphitheater form. The portions of the lower 

 slopes included between the canyons will therefore finally remain as 

 isolated peaks — outliers — connected only by vestiges of the original 

 ridges with the central mass. With this rationale in mind it is generally 

 not difficult to reconstruct in imagination from the remaining frag- 

 ments, even when these constitute an insignificant part of the whole^ 

 the original mountain. 



The elevation of the mountain, especially in a region where the trade 

 wind ]»revails. will have mu<-h to do with the dissecting of its nutss by 

 rain. If the mountain is more than about 7,000 feet high the clouds 



