Ice and its Natural History 273 



and decay has removed support, gravity claims the fragment 

 as its own, and the result is a talus. 



Whether the precipice sheds a fragment in a year, or a 

 thousand of them in a day, the primary shape of the talus 

 is the same. If there is no lower level for it to descend to, it 

 weathers in situ, and its final state is zpampa. 



Advantage of the Study of Tropical Lands. The study of 

 all matters relating to surface geology in temperate latitudes 

 is made difficult by the variability of the meteorological con- 

 ditions. Had the study of physical geography and of meteor- 

 ology taken its rise in countries within the tropics, advance 

 would have been much quicker. The surface geology of a 

 place depends mainly on the meteorological conditions. If 

 these are simple, the dependence on them of the conditions of 

 the surface geology is easily traced. The determining primary 

 features of tropical meteorology are heat and moisture, and 

 these together produce a secondary agency which is all-im- 

 portant, namely intensity of chemical action. This secondary 

 agency is by far the most effective in altering the relief of the 

 surface of the land. Its importance cannot even be guessed 

 by those who have not visited tropical or equatorial regions, 

 and studied the soil and the rocks from which it has been 

 formed. An important difference between the climate of 

 tropical and of temperate regions is that, in the former, the 

 wet weather is concentrated into a few months of rainy 

 season, and the dry weather into a few months of dry season, 

 while in the latter there is no such separation ; wet and dry 

 weather are distributed indiscriminately throughout the year. 

 One secondary effect of the temperate climate is that the local 

 streams have some water in them all the year round, which 

 may be swelled to floods at any time of the year. In tropical, 

 or rather sub-tropical regions, the stream beds are dry during 

 the greater part of the year ; and, in some cases, for whole 

 years together. During the long, hot summer the rocks on 

 the mountain sides, which always, even in the driest season, 

 retain moisture below the surface, are eaten into along every 

 discontinuity and loosened up into fragments, which, so long 

 as they remain in situ, tend more and more to lose their 

 B. 18 



