84 WARREN N. THAYER 



are deeply covered by marine sands supplied by the debris washed 

 down from the eastern edge of the plateau, but on the peninsula, 

 where the plain is very wide and the greater part of it at a con- 

 siderable distance from the mountains, the limestones are close 

 to the surface. These limestones are of a particularly soluble 

 character, and on account of the low, fiat, or undulating surface, 

 the heavy rains of the region soak into the ground and percolate 

 through them, developing great caverns {cenotes). Where the 

 cavern roofs have fallen in the natural channels of circulation are 

 stopped, and water fills the depression; forming shallow ponds 

 (akalcJies). 1 In other places the waters continue to circulate down- 

 ward until they strike a stratum of impervious shale, along which 

 they move in underground channels to the sea, or upon which they 

 form subterranean lakes. These form the only permanent natural 

 sources of water on the peninsula, and as a result they are of con- 

 siderable economic value. 



Physiographic history. — -As in other sections of the Atlantic 

 Coastal Plain, the later history of this province may be summed 

 up as a series of submergences and emergences. At times, perhaps 

 several times, the present land portion has been under water. At 

 other times the present submerged portion has been above the sea, 

 even to the edge of the continental shelf. The tendency at this 

 time is toward a submergence of the present land surface portion. 2 



THE VOLCANIC PROVINCE 



Definition and boundaries. — This province embraces a high 

 mountainous strip of country in which the axes of folding are 

 aligned east-west, and in which the departures from the general 

 level are upward instead of downward, as elsewhere over the plateau. 

 The province extends across Mexico in an east-west direction from 

 Cape Corrientes to Jalapa. Early geographers called it the ''Cor- 

 dillera de Anahuac," 3 but the term has fallen into disuse because it 

 is now recognized that it is as much an integral part of the great 

 Mexican Plateau as the Sierra Madre Occidental. 4 



'Huntington, Bull. Am. Gcog. Soc, XLIV, Soi; V. R. Garfias, Inst. Gcol. Mex., 

 Ill, iS. 



2 C. Sapper, Inst. Geo:. Mex., III. 



• ; Virlet de Aoust, Bull. Soc. Geol. de France, 2d Ser. XXIII, 14-15, 1S66. 



4 Aguilera, Guides des excursions, 10th Inter. Geol. Cong. Mex., 1906, Xo. 7, p. 3. 



