382 



FALL OF RAIN. ' 



[Ch. XV. 



them 



soon burst their barriers. ' Bay and night/ says Dr. Hooker 

 ' we heard the crashing of falling trees, and the sound of 

 boulders thrown violently against each other in the beds of 

 torrents. By such wear and tear rocky fragments swept 

 down from the hills are in part converted into sand and fine 

 mud ; and the turbid Ganges, during its inundation, derives 

 more of its sediment from this source than from the waste of 

 the fine clay of the alluvial plains below.'* 



In the districts above alluded to, and in other regions on 

 the verge of the tropics, a greater quantity of rain falls annu- 

 ally than at the equator. 



Rainles 



s regions are 



generally situated in the interior of 

 great continents, as in the great Sahara of Africa, and in 

 parts of Arabia and Persia. In these cases, the moisture 

 which the winds derive from the nearest sea is expended on 

 the lands nearer the coasts. If there are exceptions to this 

 rule, or coast regions destitute of rain, as that extending 

 from the north of Chili in lat. 30° south to Peru in lat. 8° 

 south, it is where the prevailing winds are intercepted by a 

 chain like the Andes, and made to part with all their moisture 

 before they reach the lower regions to the leeward. 



Prom such facts the reader will infer that in the course of 

 successive geological periods there will be great variations in 



the quantity of rain falling in one and the same region. 



At 



one period there may be no rain during the year ; at another, 

 a fall of 100 or 500 inches ; and these two last averages may 

 occur on the opposite flanks of a mountain-chain, not more 

 than 20 miles wide. While, therefore, the valleys in one dis- 

 trict are widened and deepened annually, they may remain 

 stationary in another, the superficial soil being protected from 

 waste by a dense covering of vegetation. 



In the course of ages, the height of the land and its position 

 relatively to the ocean will be more or less changed, and we 

 must be careful when speculating on the quantity of pluvial 

 action in past ages, and the rate of the excavation of valleys, 

 to remember, that there may have been periods of drought as 

 well as of flood, the fall being- in defect, as well as in excess 

 of the present annual mean. 



* .[looker's Himalayan Journal, ined. 



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