270 TIME CONSIDEEATIONS 



however, apparently safe in assuming that in warm and moist 

 climates decomposition follows so closely upon disintegration 

 as to form the more conspicuous feature of the phenomenon, 

 while in dry regions, or those subject to energetic frost action, 

 mechanical processes prevail and disintegration exceeds de- 

 composition. 



Dr. Hugh Worth has noted^ that the product of the weather- 

 ing of dolerite, in England, was not attended with any excessive 

 loss of silica and that the ultimate product was a ferruginous 

 clay, rather than a beauxite, or hydrargillite, as in India. The 

 same features are brought out in our own analyses. How uni- 

 versal or how dependent this difference may be on climate it is 

 yet too early to say. 



Accepting the facts thus far given, there is at once suggested 

 the idea that the lithological nature of sedimentary rocks, as 

 well as their fossil contents, may be regarded as indicative of 

 prevalent climatic conditions. 



The possibility of estimating these conditions by the char- 

 acter of the debris resulting from the degeneration of feld- 

 spathic rocks was first suggested by the geologists of the Indian 

 Survey,^ the undecomposed feldspars in the Panchet (Mesozoic) 

 sandstones being regarded as indicating a recurrence of a cold 

 period during which mechanical forces preponderated over those 

 purely chemical. The same idea was subsequently put forth, 

 quite independently, by the present writer. ^ That rocks in arid 

 regions do actually undergo less decomposition during the 

 weathering processes is shown not only by the fresh character 

 of the residuary material. Judd has shown* that rivers like 

 the Nile, draining regions of great aridity, though in a con- 

 dition of high concentration from prolonged evaporation, carry, 

 in solution, smaller proportional amounts of derived salts than 

 do those of humid regions. 



Russell has noted that in the Yukon River region of Alaska 

 disintegration so far exceeds decomposition that the talus from 

 the mountains composed of loose, angular masses of rock quite 

 free from vegetation, forms what he calls debris streams, which 



1 Geol. Mag., Jan., 1904. 



2 Geol. of India, 2a ed., Vol. I, p. 201. 



3 Bull. Geol. Soc. of America, Yol. "VII, p. 362. 



* Report on Deposits of tlie Nile Delta, Proc. Royal Society of London, 



vol. ^Jv]a.li^, looO. 



