0. P. Keyes — Geological Structure of Bolson Plains. 207 



Art. XX. — Geological Structure of New Mexican Bolson 

 Plains ; * by Charles E. Keyes. 



Among the most novel physical features which strike the 

 eye of the traveler as he crosses New Mexico are the broad 

 plains out of which the mountain ridges rise abruptly as volcanic 

 islands out of the sea. These plains are twenty to thirty miles 

 wide, often a hundred miles or more long, and at lirst glance 

 appear nearly level. Closer aquaintance shows that they are 

 basins, inclined towards the center and without marked drainage 

 ways or drainage outlets. 



With their usual keen appreciative distinctions of geographic 

 features, the Spanish aptly call these inclosed plains " bolsons," 

 meaning a purse. Of these plains, the writer who has brought 

 their Spanish name into geographical usage says : 



"These plains, or 'basins,' as they are sometimes called, are 

 largely structural in origin. Bolsons are generally floored with 

 loose, unconsolidated sediments derived from the higher peri- 

 pheral region. Along the margins of these plains are talus 

 hills and fans of bowlders, and other wash-deposits brought 

 down by mountain freshets. The sediments of some of the 

 bolsons may be of lacustral origin. 



"It is essential, in both the geographic and the geologic dis- 

 cussion, to bear in mind the distinction between bolson plains 

 and plateau plains. The plateau plains and the mountains are 

 genetically related, the strata composing the one being bent 

 onto or flexing out into the other. The bolson plains, on the 

 other hand, are newer and later topographic features, consist- 

 ing of structural valleys between mountains or plateau plains, 

 which have been partially filled with debris derived from the 

 adjacent eminences. The plateau plains are usually destruc- 

 tional stratum plains. The bolson plains are constructional 

 detritus plains filling old structural troughs."! 



A distinction between the plateau plains and the bolson 

 plains is as important as it is real. But the statement that bol- 

 son plains are constructional detritus plains in structural 

 valleys does not convey a correct idea of the phenomenon, and, 

 as generally understood, the exact signification of the term struc- 

 tural as applied to the character of these valleys is very apt to 

 be misinterpreted. In a carefully qualified sense the valleys 

 occupied by the bolson plains might perhaps be considered 

 structural valleys, but their history is very much more complex 

 and very different from what might be suspected from casual 

 observation. 



* Read before the New Mexico Academy of Sciences, December 22, 1902. 

 f R. T. Hill : Topographic Atlas of United States, Folio 3, p. 8, 1900. 



