THE LLANOS OF MEXICO. 31 



tremendous outburst of 1793, when the ejected scoriae were said to be wafted in one 

 direction as far as Yera-Cruz and Perote, in another all the way to Oaxaca. The 

 disturbances have been renewed in recent times. 



According to the unanimous testimony of the natives the two volcanoes of Orizaba 

 and Tuxtla " hold converse together " by means of muffled rumblings like the sound 

 of distant thunder. The headlands of lava projected seawards by Tuxtla form the 

 eastern extremity of the winding volcanic zone, whose central axis, about 730 miles 

 long, coincides very nearly with the 19th parallel of latitude, and is continued far 

 into the Pacific westwards to the Hawaii Archipelago. The uninhabited Revilla- 

 Gigedo islands, which lie on the track of this conjectural volcanic fault, are 

 probably of igneous origin ; Poulett Scrope mentions the fact that vessels navigating 

 those waters frequently find the surface covered with floating pumice. 



The region of the Mexican volcanoes also coincides with the principal zone 

 of earthquakes, whose undulations are usually propagated in the direction from 

 east to west in a line with that of the burning mountains. The province of Jalisco 

 especially is much exposed to these seismic movements. Buildings erected on 

 granite or porphj'ry rocks suffer more than others from such disturbances. 



The Eastern Sierra Madre, whose culminating peaks are the Cofre and Citlal- 

 tepetl, forms, like the western system, a southern continuation of highlands lying 

 within the United States frontier. The parallel ridges of the Apache Mountains, 

 which are disposed in the direction from south-west to north-east, and which are 

 pierced by the gorges of the Rio Bravo, reappear on the right or Mexican side of 

 that river. Here they develop a long line of Jurassic limestone ramparts running 

 south-eastwards and presenting precipitous slopes whose sharp crests are here and 

 there pierced by a few eruptive cones. 



These crests do not exceed an average altitude of about 3,500 feet ; but like 

 the western range they rise gradually southwards, and in the neighbourhood of 

 Saltillo some of the summits already attain an elevation of 6,600 feet. In these 

 regions of north Mexico the two converging eastern and western sierras are not 

 yet connected by any transverse ridges, but are, on the contrary, separated by vast 

 plains and by basins of quaternary alluvial matter which were formerly deposited 

 by extensive inland seas, and which under the action of the winds have since 

 assumed the form of elevated dunes. Here they take the name of Ua)ws, like the 

 grassy savannahs of Venezuela ; but in Mexico these old lacustrine beds have a 

 different vegetation, and they are moreover divided into distinct depressions by 

 small ridges of volcanic or other hills rising above the plains. These ridges are 

 for the most part disposed in the direction from north-west to south-east, parallel 

 with the two great border ranges, and thus form narrow gulches, ravines or canons, 

 which are traversed by rivulets and highways. 



One of these steppes is the Llano de los Cristianos, which occupies some thousand 

 square miles south of the Rio del Norte and its affluent, the Rio Conchos, and which 

 is divided into a multitude of secondary plains by numerous sierras and chains of 

 hills. Farther south the Llano de los Gigantes, so called from the remains of 

 gigantic animals found in the clays and sands formerly supposed to be those of 



