EDITORIAL 631 



The possibility of faulting in the Serra do Mar is a question 

 of importance in connection with the subject of rock- decomposi- 

 tion in that region, but too little is known of the facts to admit 

 of anything more than s|)eculation on this subject. The region 

 is covered by a dense tangle of forest and undergrowth that ren- 

 der detailed examination extremely difficult, and this difficulty is 

 greatly increased by the decomposition of the rocks and by the 

 lack of artificial exposure that would help to an understanding of 

 the structure. 



Derby thinks the whole question is covered by the proposi- 

 tion of Pumpelly that, other things being equal, the depth of rock 

 decay is due to time and to the permeability and solubility of 

 the constituents. Such a statement admits of no question. But 

 the difficulty with such a way of putting it is that it is simply 

 another way of saying that time, permeability, and solubility are 

 elements of rock decay, for, as to other things being equal, they 

 seldom or never are equal, even in the same region, or in the same 

 rocks. It might be as truly said that other things being equal 

 the rate of decay is determined by the color of the rocks, by 

 their dip or by the dip of their surfaces, or even by the direc- 

 tion of the prevailing winds with reference to their dip. The 

 agencies and processes of rock decomposition are complex. 



We are of the opinion that a comprehensive knowledge of 

 this subject is to be had in a study, not of road and railway cuts, 

 but of the mineralogical changes to be found in deep mines and 

 tunnels. The susceptibility to alterations of copper and iron sul- 

 -phides to sulphates, carbonates, oxides, silicates, etc., offer a deli- 

 cate test of the penetration of the agencies of decomposition. 

 Penrose cites cases of the alteration of such ores to the depths 

 of 600, 1000 and even 1500 feet. (Jour. Geol. II, 1894, p. 295.) 



J. C. B. 



