234 RELATION BETWEEN CLIMATE AND VEGETATION 



in tlie bogs and marshes wliicli are left behind, a most luxuriant 

 vegetation is certainly to be found. In America, however, 

 there is no liarniony of masses, nor that equal distribution of 

 the powers whicii produce a lasting fruitfulness ; neither is there 

 the pure ethereal atmosphere whicii we meet with in the moun- 

 tain-forests of Java. 



So remarkable a difference leads naturally to an inquiry into 

 the causes which produce such a universal and almost unex- 

 ampled exuberance of vegetation. I cannot venture myself just 

 now to enter into an explanation of all wliich might throw light 

 upon the subject ; I can only touch upon a few principal points 

 to which my observations and experience clearly point. These 

 show that besides the generally favourable influence of the 

 tropics and mere elevation, there is a marked difference of 

 ground and a peculiar nature of the soil, to which the great 

 force of vegetation must be attributed. It is well known that 

 all these islands have been formed by former volcanic convul- 

 sions ; tliese, in comparison with what have occurred in other 

 parts of the earth, were evidently of a later origin, and the 

 subterraneous fires may still be in action ; at all events, their 

 influence, whether earlier or later, manifests itself almost every- 

 where. This explains why volcanic rocks, particularly of Trachyte 

 and Dolerite, make tlieir appearance everywhere, and form the 

 great bulk of the ground and mountains, with the exception of 

 the limestone, which is always produced in tlie sea. Nowhere 

 other or older rocks appear to be produced by the precipitation 

 of water. It is, therefore, evident that the ground — that soil 

 which excites our admiration by its inexhaustible fertility — can 

 have only been produced by different kinds of volcanic rocks. 

 The composition of these rocks is well known, as also the in- 

 fluence of tlie weather upon them ; but it is perhaps not so well 

 known how generally and constantly and quickly the decomposi- 

 tion of these rocks and their transformation to soil frequently take 

 place, notwithstanding their hardness. The combined influence 

 of atmospheric air and water, and constant changes from heat to 

 cold, from dryness to moisture, upon a rock formed by sedi- 

 mentary deposit, and therefore highly susceptible of all external 

 influences, particularly of those of moisture — their granular 

 junctions, tlie openings and clefts in them, — all these things 

 facilitate the influence of every active agency. Water, sinking 

 down into such rocks, causes frequent slips; heavy rains, 

 swollen torrents, often tear the loose parts asunder, sweep 

 them away in their current, and, by an incessant wearing down, 

 gradually render them smaller and smaller. Volcanic action is 

 not less active in the process of destruction ; powerful liquids, 

 confined and elastic gases, sulphureous vapours, acids, salts, even 



