79 



CHAPTER YIIL 



THE MEXICAN PLATEAUS, AND THE SLOPES OF SIKKIM. 



Geological Formation of Mexico — The Tierra Calienie — The Tierra Temjplada — 

 The Tierra Fria. 



The Sylvan Wonders of Sikkim — Changes of the. Forest on ascending — The 

 Torrid Zone of Vegetation — The Temperate Zone The Coniferous Belt — • 

 Limits of Arboreal Vegetation — Animal Life. 



THE prodigious height attained in the torrid zone, not only 

 by single mountains, but by vast tracts of land, and the 

 diminution of temperature which is the necessary consequence 

 of their elevation above the level of the sea, enable the in- 

 habitants of many tropical countries, without leaving their 

 native land, to view the vegetable forms of every zone, and to 

 pluck nearly every fruit that is found between the equator and 

 the arctic circle. In Asia, Africa, and Ameri'ca, in the islands 

 of the Indian Archipelago, and in the Hawaian group, where 

 the Manna Loa to>vers to the height of Mont Blanc, and girdles 

 his foot with palms, while snow rests for a great part of the 

 year upon his summit, we find numerous examples of a rapid 

 transition from the torrid to the temperate or frigid zone, often 

 within the range of a single day's journey. 



It would far exceed my limits were I to attempt to follow all 

 these gradations of climate throughout the wide extent of the 

 tropics ; but a short description of the Mexican plateaus, and 

 of the slopes of Sikkim, which I have selected as remarkable 

 instances of the wonderful change of vegetation resulting from 

 the progressive elevation of the land, will, I hope, prove not 

 uninteresting to the reader. 



After traversing South America and the Isthmus of Darien, 



