86 MEXICO, CENTRAL AMERICA, WEST INDIES. 



village, is only 1,000 feet high. Most of the high grounds skirting the plains of 

 the isthmus affect the form of " tables " ; seen from the surrounding mountains, 

 they merge almost entirely with the lowlands. 



According to Spear, a geologist attached to one of the numerous expeditions 

 that have studied the isthmus of Tehuantepec, the terraced formations consist 

 partly of cretaceous rocks deposited at a time when the Atlantic and Pacific 

 Oceans were here connected by a broad channel. After their upheaval the 

 flanks of these chalk cliffs became overlaid on both sides by more recent tertiary 

 and quaternary formations. The land still continues to encroach insensibly on 

 the ocean ; the Pacific Coast, formed of late alluvial matter, is continually 

 advancing seawards, while the lagoons along the shore are gradually drying up. 

 In the isthmus of Tehuantepec low-lying tracts occupy a larger space relatively to 

 the whole region than in any other part of Mexico. 



The two oceans were also at one time connected farther north by another 

 marine passage, and the so-called " Valley " of Mexico in the very centre of the 

 Anahuac tableland is a remnant of this old branch of the sea. Towards the close 

 of Mesozoic times the marine waters winded over these lands which at present 

 stand over 6,500 feet above sea-level, and the volcanoes now surmounting them 

 had not yet discharged their lava streams. At this epoch the contour line of the 

 Gulf of Mexico also lay far more to the west than in our da3'S. The rich silver 

 mines are nearly all situated in the two Sierra Madrés north of the " Valley," 

 and are disposed along certain definite lines. Thus their main axis appears to run 

 due north-west and south-east between Batopilas and Guanajuato, and the famous 

 argentiferous lodes of Zacatecas, Fresnillo, Sombrerete, and Durango all lie on or 

 near this axis ; the lories themselves are disposed in the same direction. 



Rivers and Lakes. 



The form of the Mexican plateau with its narrow escarpments, and its border 

 ranges disposed parallel with the seaboards, combined with the dry climate of the 

 northern and central regions, has prevented the development of any large fluvial 

 systems with extensive ramifying arteries. Of all Mexican rivers the most impor- 

 tant, if not for its volume at least for its length and for the part that it plaj's as 

 the political frontier-line between the Anglo-Saxon and Hispano- American repub- 

 lics for over 720 miles of its course, is the Rio Bravo, or Rio Grande del Norte. The 

 Mexican part of its basin comprises about 94,000 square miles, or one-third of the 

 whole area of its drainage ; but it receives scarcely any copious or perennial streams. 

 Most of their beds are dry except during the rainy season, and their waters, ren - 

 dered saline by lodging in shallow basins, give a brackish taste to the Bravo itself. 



The largest affluent on the Mexican side is the Rio Conchos, whose headstreams 

 are fed for a distance of over 200 miles north and south by the eastern slopes of 

 the great Sierra Madre between the States of Sonora and Chihuahua. From the 

 Eastern Sierra Madre flows the Rio Salado, or " Salt River," whose very name 

 indicates a prolonged period of drought. In the same range rises the Rio San 



