170 FLORA INDICA. 



between the valley that ascends from one river, to the cor- 

 responding valley that descends to the other river, can only 

 be detected by the observation of the drainage ; whence the 

 two valleys appear to form one. Such is the case with the 

 Dehra Dhiin, which appears to form one continuous trans- 

 verse valley between the Jumna and the Ganges, but which 

 really consists of two valleys ; one descending from the vil- 

 lage of Dehra (which occupies the col) westerly to the Jumna, 

 and the other descending from the same spot easterly to the 

 Ganges. Other Dhuns, again, are simply very broad, open 

 valleys, differing in no physical features from those that occur 

 in other parts of the mountains. In the Panjab- Himalaya, 

 where the tertiary sandstones acquire a great development, 

 two or three such valleys occur in succession before the 

 higher mountains begin. These valleys, or Dhuns, are not, as 

 is very generally supposed, continuous along the whole ex- 

 tent of the Himalaya, and interposed between the tertiary 

 and secondary mountains. They are merely the outer series 

 of lateral valleys, and are always of limited extent. 



In the enormous chain of the Himalaya, which rises nearly 

 from the level of the sea to perpetual snow, we have of course 

 every variation of temperature between tropical or subtropical 

 heat and extreme cold. The diminution of temperature is 1° 

 for 300 feet of elevation in the more humid, and for 400 feet 

 in the drier part of the chain. The elevation of the snow- 

 line, at equal distances from the plains, is nearly uniform 

 throughout the whole extent of the chain, the increase of 

 latitude of the more westerly part being compensated for by 

 the greater distance from the sea, and consequent diminished 

 snow-fall. This level on the outer ranges has been deter- 

 mined to be about 16,000 feet, but it becomes higher on the 

 inner ranges, and in the Tibetan Himalaya is not under 19 

 or 20,000 feet. 



The climate of the Himalaya varies much iu different parts. 

 During the winter season the weather is generally unsettled ; 

 for while the north-east monsoon is blowing over the lower 



