DEFINITION OF "bOLSON" 65 



conclusion that the plains are all deeply covered by mountain waste, even 

 to depths of thousands of feet. At first glance on the ground such an 

 explanation of the phenomenon presented seems to leave Mttle room for 

 refutation. 



So firmly has this notion prevailed that Hill, in 1900, in describing the 

 physiographical features of the Greater Texas region (including New 

 Mexico and Colorado east of the Eio Grande), gave it technical fixity by 

 calling the intermont plains in the New Mexican part of the district 

 "bolsons," deriving the name from the Spanish title for these plains, 

 which signifies a purse, or basin without outlet. Hill's definition is essen- 

 tially as follows : 



"These plains, or basins, as tliey are sometimes called, are largely structural 

 in origin. Bolsons are generally floored with loose, unconsolidated sediments 

 derived from the higher peripheral region. Along the margins of these plains 

 are talus hills and fans of boulders and other wash deposits brought down by 

 the mountain freshets. The sediments of some of the bolsons may be of lacus- 

 trine origin. . . . It is essential,- in both the geographic and the geologic 

 discussion, 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 flexed out of the other. The bol- 

 son plains, on the other hand, are newer and later topographic features, con- 

 sisting 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 destructional stratum plains. The bolson plains 

 are constructional detritus plains filling old structural troughs." 



The types which Hill selected to illustrate his definition and which he 

 specifically described were the Hueco, Sandoval (Estaneia plains), and 

 the Jornada del Muerto basins, all of which are in eastern New Mexico. 

 Some minor basins farther to the southward were also briefly mentioned. 



JORNADA DEL MUERTO AS THE TYPE 



About the time of the appearance of the Hill memoir I was engaged in 

 making geologic inquiries into the imderground water si;pplies of the 

 region occupied by the three great bolsons mentioned. These investiga- 

 tions were undertaken for three different railways, and were soon after- 

 ward supplemented by similar inquiries for several mining companies. 

 This work was not hurried geological reconnaissance, but careful investi- 

 gation extending over a period of several years with residence in the 

 region. Its results were measurable quantities, demanding expression in 

 feet and dollars. Some of the more important of these results have been 

 already published in a memoir on the "Underground water conditions of 



