THE PHYSIOGRAPHY OF MEXICO 73 



Lagunas are shallow ephemeral lakes, mostly found in the 

 lowest parts of the bolsons, and fed by streams whose sources are 

 in the neighboring mountains and which flow only during time of 

 storm — the "suicidal" streams previously discussed. 



Medanos are sand hills or dunes, similar to those of our lake 

 shores or the Great Basin. They consist of finely disintegrated 

 particles, derived from the weathering of the plateau, which are 

 constantly moved about, frequently in a general northeast direc- 

 tion, by the high winds. 



In our discussion of the Sierra Madre Occidental province we 

 mentioned a peculiar, stratified deposit of a past but recent geologic 

 age which had been formed in intermontane basins. It must be 

 remembered that the Anahuac Desert Plateau province formerly 

 presented the same topography as is found in the Sierra Madre 

 Occidental province at the present time, the latter by its definition 

 being a survival of former widespread conditions. The dissection 

 and weathering of the type of deposit mentioned results in a char- 

 acteristic feature known in Mexico as "La Brisca." It is a super- 

 ficial volcanic agglomerate consisting of rhyolitic tuff carrying 

 subangular fragments of other igneous rocks and which weathers 

 into striking forms due to inequalities of hardness — pinnacled cliffs, 

 domes, pillars, etc. 1 



The Rio Panuco and the Rio Conchos, a tributary of the Rio 

 Grande, drain the greater part of the province, although a con- 

 siderable area known as the Bolson de Mapimi in Chihuahua and 

 Coahuila is without exterior drainage. This great bolson is not a 

 single basin, for within it are to be found mountains of considerable 

 elevation; but its elevations and depressions are so disposed that 

 none of the water falling in it (which is a negligible quantity) 

 reaches the sea. 2 



The Rio Grande is an interesting river from a physiographic 

 viewpoint. At one time it emptied its waters into a great laguna 

 that lay southwest of the present site of El Paso. It appears 

 then that a headward-working tributary of a stream which prob- 

 ably had the course of the present Pecos-Rio Grande tapped this 



1 Hovey, Bull. Am. Geog. Soc, XXXVII, 539. 



2 Bose, Guides des excursions, iotk Inter. Geol. Cong. Mex., igo6, No. 19. 



