210 0. R. Keyes — Geological Structure of Bolson Plains. 



main line of the Atchison, Topeka and Santa Fe railway the 

 shallow cuts frequently expose the indurated bed-rock. 



In the La Jara valley, 25 miles south of Santa Fe, where for 

 many miles the plain is covered by gravels/where no indurated 

 rocks are exposed, and where the beveled surface of the 

 Cretaceous was thought to lie many feet beneath the present 

 surface of the plain, drill-holes showed the gravels to be in 

 places scarcely a dozen feet in thickness. 



In the broad plain between the Ortiz and Sandia Mountains, 

 east of Albuquerque, similar conditions prevail. At the Una 

 del Gato, the relations of the gravels and the underlying rocks 

 were found as represented in the accompanying cut : 



Figure 3. Beveled edges of Cretaceous sandstones, on which Bolson gravels 

 rest, Una del Gato, New Mexico. 



If any of these gravels attain the thickness ascribed by 

 Shumard and others (500 feet), these measurements have no- 

 where been found even approximated to in the bolsons which 

 have come under personal observation. 



It must be concluded that the bolsons, or at least some of 

 the principal ones, are not simple structural valleys, in the 

 sense that that term is usually applied, though casual observa- 

 tion so indicates them. Neither is the detritus covering the 

 bolsons so enormously thick as has been claimed. 



State School of Mines, Socorro, New Mexico. 



