272 Mr. J. Y. Buchanan [May 8, 



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 m situ, and its 

 final state is Sipampa. 



Advantafie of the Study of Tropical Lands. — The study of all 

 matters relating to surface geology in temperate latitudes is made diffi- 

 cult by the variability of the meteorological conditions. Had the 

 study of physical geography and of meteorology taken its rise in 

 countries within the tropics, advance would have iDeen much quicker. 

 The surface geology of a place depends mainly on the meteorological 

 conditions. If these are simple the dependence on them of the con- 

 ditions 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-important, 

 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 be- 

 tween 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 edges and corners, 

 by the chemical action of weathering. When gravity dislodges and 

 brings them down into the valley, they only require to be hurried 

 along by one flood to come out clean and round. In these places no 

 one can fail to recognise that the function of the river or stream is 

 mainly distributive. Now, whether the river flows only during one 

 or two days in the year, or is perennial with occasional floods, the 

 nature of the action is the same, and there is no difference in the 

 roundness of the stones. 



I had the good fortune once to witness an example of this. It 

 was in the island of Tenerife, in October 1883. It was said that 

 there had been no rain for six months, and certainly all^the beds of 

 the streams were perfectly dry. During the forenoon clouds collected, 

 and about ! mid-day there was a thunderstorm with a i violent and 

 prolonged deluge of rain. After the rain had been ^pouring for 



