120 NORTH AMERICA 



abundantly watered than the western Sierra Mad re,, and 

 possess a still wider range of climate, vegetation, and 

 •agricultural possibilities. Fairly dry in the north, the 

 eastern Sierra gradually enters into the moist tropical 

 region, where the succession of vegetations from top to 

 foot of the mountains is broadly as follows : on the 

 upper slopes, which are largely of volcanic origin, there 

 are imposing forests of conifers : in the temperate belt 

 or ' tierra templada ' which receives the maximum of 

 atmospheric moisture, the magnificent broad-leaf ever- 

 green rain-forests of the valleys alternate with the tall 

 coniferous forests of the ridges : further down, in the 

 lower valleys, the rain-forest possesses all the profusion 

 and features of the equatorial selva. Yet the rainfall has 

 decreased, and outside the alluvial and well- watered 

 portions, conditions are dry enough to favour the growth 

 of those deciduous, low thornwoods and jungles which, 

 in Brazil, are described as 'caatingas': the broad coastal 

 plain or ' tierra caliente ' is divided between the jungle 

 and the grass savanas, on account of its reduced rainfall, 

 but wherever the moisture of the soil compensates for the 

 dryness of the atmosphere, the equatorial forest regains 

 all its power. In Tabasco and Campeachy, mahogany, 

 cedar, dyewoods, Mexican rubber, cacao, and vanilla are 

 among the indigenous forest products. At this southern 

 extremity of the plain, the rainy season, which was so 

 short in the north, has lengthened so as to cover eight 

 to nine months of the year and render the climate 

 regularly hot and moist. This condition is reflected in 

 the vigour of the dark forests which cover the plain 

 and the hills. 



Thanks to the breach in the line of high plateaus 

 which is known as the isthmus of Tehuantepec and to 

 the wide opening of the Chiapas valley, the Atlantic 



