196 Dk. Thomson on the Climate and Vegetation of the Himalaya. 



regular. The effects of the south-west or rainy monsoon diminish step 

 by step, as we advance westward, till on arriving at the valley of the 

 Indus at the western extremity of the Himalaya, it ceases to be observed 

 at all. In these most western portions of the chain, very little rain falls 

 at any season of the year, and the little which does occur, falls in the 

 spring months, and is therefore quite independent of the regular monsoon. 



It is also worthy of note, that in the more western parts of the chain, 

 the climate is extremely dry at all periods of the year, except during 

 the monsoon or rainy season, as it is called in India, while to the east- 

 ward the climate of the mountains shares to a considerable extent the 

 more equable and always moist climate of Bengal. 



The most important point of all, however, regarding the climate in re- 

 spect of its effects on vegetation which requires to be borne in mind, is 

 that a very great portion of the rain which falls is deposited on the first 

 range of mountains upon which the rain wind strikes. I have already 

 pointed out that this is the case with the Khasya range, and it is 

 there highly strikingly illustrated by the fact, that it is only on the very 

 south side of the hills that the rain fall is so enormous, the fall twenty 

 miles north of Churra being probably less than half what it is there. 



This tendency of the rain fall to exhaust itself very considerably on 

 the first range of mountains to which it has access, is peculiarly important 

 in a mountain chain 150 miles in width, its effect being that the upper 

 part of all the large valleys, and especially the interior valleys and their 

 ramifications, are much more dry than those adjacent to the plains of 

 India. Even in the most humid part of the Himalaya, in Sikkim, this dif- 

 ference is extremely marked, and in the more dry parts to the west, 

 (the extreme east interior is not known,) the inner valleys are so dry 

 that rain is scarcely ever known to fall. 



In close connection with the increase of aridity, as we advance from 

 the plains of India to the interior of the mountains, I may mention the 

 increased elevation of the line of perpetual congelation, which has evi- 

 dently the same cause. In the outer lofty ranges of the Himalaya, the 

 snow line is met with at about 16,000 feet, while in the Tibetan part of the 

 chain, many ridges of 20,000 feet of elevation are almost entirely bare of 

 snow. 



Having thus alluded in very brief and general terms to the most pro- 

 minent physical features of the mountain chain of Himalaya, I shall pro- 

 ceed to describe, as rapidly as is consistent with clearness, the general 

 character of the vegetation which is to be observed in its different parts at 

 all elevations, from the plains of India to the uppermost limit of vegetable 

 life. This would be an easy task if the vegetation were uniform through- 

 out the whole chain, but owing to the great variations of climate to which 

 I have just adverted, there is a very great difference in this respect, few 

 indeed of the plants of the eastern extremity of the Himalaya being 

 identical with those which occur in the far west. In general terms, it 



