MOUNTAIN CHAINS AND RIVERS. 395 



" The land may increase or diminish in area ; river systems 

 may be flexed ; but the strong ridge drawn across a continent 

 by a mountain range, will remain through all these revolutions 

 of time a great divide that the rivers cannot pass. Rivers 

 frequently run parallel with, or between mountain ridges 

 and ranges for long distances ; and then break through them, 

 but rarely or never bisect them." * 



Notable instances of such rivers are to be found in 

 the Himalayan system, as a glance at the map at once 

 shows, and it may be that though these great moun- 

 tains may have been upraised in late tertiary times, 

 still it seems probable hundreds of thousands, it may even 

 be millions of years have come and gone, since these 

 mighty ranges of snow-clad peaks assumed their 

 present aspect in all its leading features. 



Leaving now the question of the origin of mountain 

 ranges, let us briefly consider them in their aspect 

 as factors in the regulation of climates. When we 

 consider how small a thing they are in comparison 

 with the earth itself, their influence in this respect is 

 very much more considerable than might otherwise have 

 been expected. 



It is generally supposed that an increase in eleva- 

 tion of about 300 feet is sufficient to produce a fall in 

 the temperature of about one degree Fahrenheit one 

 thousand feet would therefore represent something more 

 than 3 degrees. Viewed in this light, it must be obvious 

 that a lofty mountain range in the equatorial regions 

 will represent every variety of temperature existing 

 from the equator to the pole. The line of perpetual 

 snow may be fixed in tropical regions at an altitude 



* The Origin of Mountain Ranges, by T. Mellard Reade, C.E., 1886, 

 pp. 3067. 



