Contents xix 



PAGE 



Advantage of the study of tropical lands. 



The rounding of pebbles, generally attributed to the me- 

 chanical action of streams, is due to the chemical action of 

 subaerial weathering. When an irregular piece of rock is 

 exposed to subaerial weathering, or any other external chemical 

 decomposing agent, the resultant form of the undecomposed 

 kernel of the fragment is necessarily an ellipsoid . . . 273 



Weathering in Tenerife 274 



Similar case observed by the Prince of Monaco in the 

 Cape Verde Islands 275 



The " Crumble " formation. 



This is the name which I invented for my own use in re- 

 cording the occurrence of a common surface feature of tropical 

 and equatorial regions, when on board the " Challenger." I 

 found in all such countries that the rocks were decomposed 

 to a depth of many metres, the residual material often re- 

 maining in situ, with such a fresh appearance that it was 

 difficult to imagine that it could be anything but unaltered 

 rock. It was only necessary, however, to touch it with a 

 stick or even with the fingers, for it to " crumble " into frag- 

 ments of all sizes down to sand and clay. In almost every 

 place within the tropics visited for the first time the rocks were 

 logged as consisting of "the crumble formation." Outside 

 of the tropics this formation occurs only in a rudimentary 

 form. The crumble formation owes its existence to a high 

 atmospheric temperature and to humidity. It is the typical 

 formation produced by the subaerial weathering of rock in 

 situ. Very slight mechanical disturbance is sufficient to 

 cause degradation under gravity, with the production, first of 

 taluses ; then as the taluses undergo further gravitational 

 degradation, they flatten out, and the final product of the 

 crumble formation and of subaerial weathering is the pampa, 

 prairie, steppe or desert, which are the native names for the 

 same formation in homologous climatic regions of the earth. 

 Demonstration that the generally accepted doctrine that 

 rock-fragments, which are found so frequently covering the tops 

 of mountains, are split off from the parent rock by the energy 

 liberated by water freezing in its interstices, is untenable . 278 



No. 7. BEOBACHTUNGEN UBER DIE EINWIRKUNG DER STRAHL- 

 UNG AUF DAS GLETSCHEREIS. (Extract of paper read before 

 the physical section of the Schweizerische Naturforschende 

 Gesellschaft at its meeting at Basel on 6 September, 1910, 

 and printed in its Verhandlungen, I, p. 330) .... 280 



