THE PHYSIOGRAPHY OF MEXICO 83 



Inasmuch as both eastern and western boundaries of this 

 province might be extended into Central America, it has no south- 

 ern boundary as far as Mexico is concerned. 



It is evident that the coastal plain is divisible into two parts, 

 one submerged and the other a land surface. Of the former there 

 is little to be said, save that its depth below sea-level is in most 

 places very slight; the charts of the United States Coast and 

 Geodetic Survey show but 30 fathoms at a distance of 125 miles 

 out from the mouth of the Rio Grande. 



The landward portion has its greatest width along the Inter- 

 national Boundary, where it attains a width of about 225 miles. 

 South of Brownsville the province narrows suddenly and con- 

 tinues to grow narrower until at Tampico it has practically no 

 development at all. This condition continues with more or less 

 variation as far as Vera Cruz. South of this point there is a gradual 

 widening which increases with distance until, in the peninsula of 

 Yucatan, the plain has a maximum width of over 300 miles. 



Topography. — The most prominent topographic features of this 

 province lie close to the coast line — a multitude of spits, bars, coastal 

 islands, lagoons, and salt marshes. The greater part of the surface 

 above tide is a featureless flat, which terminates rather abruptly at ■ 

 the foothills of the Sierra Madre Oriental. It is not possible to 

 draw a straight line of any considerable length between the coastal- 

 plain type of topography and that of the mountains. Indeed, it 

 is common to find jutting headlands of the Sierras and reduced 

 areas of intrusive rocks almost down to the water's edge/ and on 

 the other hand to find recent marine sediments in the coves 40 or 

 50 miles inland. This condition has already been explained and is 

 mentioned again only to justify the generalization of the boundary 

 line. 



No large or important streams cross the province south of the 

 Rio Grande. The paucity of streams and the number of lakes are 

 noteworthy features of the peninsula of Yucatan. This condition 

 is closely related to the lithology of the bedrock. This consists 

 of thick beds of limestone and shale of Cretaceous age. Where the 

 coastal plain is narrow, as in the vicinity of Tampico, these deposits 



1 V. R. Garfias, Jour. Geol, XX, 666. 



