THE MEXICAN PLATEAUX. 



33 



scooped out enormous gorges hundreds of yards deep, whicli converge in still larger 

 ravines before reaching the level of the plains. The best known of these barrancas 

 are those of the sierras of Tepic, of the Colima and Orizaba volcanoes and neigh- 

 bouring highlands. Sometimes a whole day is required to reach a village which 

 may be seen perched on a terrace only a few miles distant ; but in the inter- 

 vening space the traveller has perhaps to cross four or five deep troughs, whose 

 crumbling slopes are scored by dangerous zigzag tracks. In some of the older 

 barrancas the slopes are entirely concealed by a dense vegetation. 



But while nature is destroying in one place it is building up in another. 

 The plateaux, the isolated mountains, and even the volcanoes of comparatively 



Fig. 14. — Igneous Regions and Volcanoes of Mexico. 

 Scale 1 : 12,000,000. 



106' 



West or breenwich 



Active Volcanoes. Eruptive Rocks. Sedimentary Eoeks. Extinct Volcanoes. 



The blank spaces have not yet been thoroughly surve; ed. 

 .^^^__^^^___^ 180 Miles. 



recent geological date, as well as the flanks of giants such as Popocatepetl, are 

 found to be covered with an argillaceous or marly layer to an average depth of from 

 15 to 30 feet. These layers are composed entirely of dust brought by the remoUnos 

 de polvo, little whirlwinds ri.'-ing at intervals on the plateaux, " like movable 

 minarets, disappearing and reappearing incessantly." But this dust itself, which 

 now completely clothes the hill- sides, can come only from other formations of 

 recent origin, from the so-called tepetate, a clay detached by the rains from the 

 rocks, and elsewhere deposited in the form of fine alluvial matter. 



South of the uplands, lying between the two border ranges, the surface of the 

 plateaux is occupied by a series of plains, the beds of old lakes, or inland seas. 

 One of these is the Bajio, a long sinuous depression which winds for about 125 

 36 



