EESULTS DUB TO POSITION 239 



tion, had, however, been sufficient to hold the mass intact until 

 exposed briefly to atmospheric influences. 



The protective action of water, as sometimes shown in the 

 beds of streams and in deep ravines, may be only apparent, and 

 due to the fact that erosion exceeds decomposition, the stream 

 having cut its way down to fresh bed-rock. Professor Dana, 

 to be sure, writing more than half a century ago,^ described the 

 basaltic rocks of Kiama, Australia, as in a condition of advanced 

 decomposition except where protected by sea-water. *Mt is a 

 general and important fact that a rock which alters rapidly when 

 exposed to the united action of air and water, is wholly un- 

 changed when ipimersed in water, or exposed to a constant wet- 

 ting by the surf." "While no exception can be taken to the 

 conclusion regarding those rocks wholly immersed, the question 

 naturally arises in one's mind, if the absence of decomposition 

 products in those rocks constantly wetted by the surf and in 

 many stream beds may not be due, in part at least, to erosion, as 

 noted above. That rocks so situated are in a condition far from 

 fresh, is well known to any petrologist who has attempted to 

 gather specimens. 



In the case of strata lying nearly horizontal, it rarely happens 

 that all possess the same power of resistance, the more friable 

 weathering away with the greatest rapidity, leaving the harder 

 layers for a time projecting in rib-like masses, to ultimately break 

 down in large angular blocks as the support below is gradually 

 removed. Friable beds of sedimentary rock are thus not infre- 

 quently protected by a capping of impervious lava. Continual 

 percolation of water through existing joints and fractures in 

 time, however, erode away, in part, the underlying material, 

 causing the landscape to assume the Table Mountain appear- 

 ance, where each flat-topped hill represents residual masses of 

 a once continuous plateau, now isolated in the manner described. 



It is obvious that where a large series of sedimentary rocks 

 composed, it may be, of interbedded limestones, sandstones, and 

 argillites are turned up on edge and exposed alike to atmos- 

 pheric agencies, they will become eroded very unequally. If 

 chemical agencies alone prevail, the limestone will dwindle 

 away and perhaps give rise to long valleys or depressions 

 walled in by the more enduring sands and shales, and carry- 

 ing upon its bottom a fertile clayey soil representing not 



^Eeports of Wilkes's Exploring Expedition, Geology, p. 514. 



