396 



BOTANICAL GEOGRAPHY. 



to those tracts corresponding in character to the arid parts of the 

 plains, already pointed out, in which a great similarity of feature 

 exists in some cases, for instance in the almost total absence of 

 forest. Above oOOO ft , and between that and 13000, forests predo- 

 minate. At similar elevnti )ns, and in such a variety of aspects, have 

 I seen the vegetation totally changed by the presence of moisture, 

 that the diflferences of solar influence are not appreciable, I have 

 passeJ over, and over again, fro n humid forests into dry wootls of 

 Pinus longefolia with the usual characteristics, without any thing 

 but the difference of humidity to account for the change. 



These differences of humidity may be supposed to depend upon 

 two causes, general, and local. The general causes are the open 

 aspect of mountains presented to prev;iiling winds impregnated with 

 vapjur derived from an expanse of ocean, or of marshy plains. 

 Thus the aridity of the lower mountain ridges of Bootan, below 

 6000 ft. may be accounted for by the atmosphere to that elevation, 

 being drained of its moisture by the excessive raia thrown down 

 upon the Kliasyah mountains, which intercept the S. W. Monsoon 

 before reaching Bootan. The higher altitudes of the Bootan 

 mountains being so much more lofty than the Khasyah, are not shel- 

 tered by them, and consevjueutiy receive their fuil proportion of 

 rain from the atmosphere. 



The local circumstances affecting the degree of moisture of the 

 soil, depend on geological causes, as the issue of springs, depending 

 on the permeabU or impermeable nature of the strata, of which the 

 rocks are composed. The chemical nature of the rocks ttiemselves 

 I consider to have otherwise a very slight influence under equal cir- 

 cumstances of moisture. The vegetation at similar elevations is much 

 the same, nor do I know a single instance in \Yhich a given plant 

 may fairly be said to be limited to a given soil. 



Of the orders found in Melville island, the following occur on 

 plains within the tropics. 



Cyperacea), Grasses, Juncus* PolygoneJE, Scrophularineae, Campa- 

 nulaceae,* Cichoraeeae,* Corymi)iforie, llanuneul.icege,* Cruciferae,* 

 Caryophylleae,* llosaceae, Legumiaosa). Amentace^. 



But Ericiueae, Pai)aver.icece, and Saxifrage®, are wanting. 



Of the orders that are present, those marked with an asterisk ar© 

 rare, ami when represented by European forms, are of annual dura- 

 tion, and wintry occurrence. 



