TYPE OF BOLSON PLAINS ^^^R 67 



degrees in the same directions and in places they are even vertical. On 

 the beveled edges of the steeply inclined beds the plains-gravels and clays 

 are laid down, and also broad sheets of basaltic lavas, the latter spreading 

 out from numerous small cones. 



What is most surprising is the fact that the detrital materials covering 

 the plains have such small depth. Over at least two-thirds of the Jornada 

 the rock-floor is frequently exposed, protruding throiigh only a few feet 

 of soil. There do not appear to be the least grounds for assiiming that 

 this plain is deeply filled with debris on account of the Kio Grande having 

 formerly occupied the middle of the basin, as has been recently stated by 

 Lee." None of the well records examined gave any indication of a broad 

 detritus-filled valley; but they do show that the Carbonic limestones and 

 Cretacic sandstones occur only short distances beneath the surface, at least 

 from the west side nearly to the base of the San Andreas range on the 

 east. At the south end of the Jornada there are Tertiary deposits having 

 a thickness of several hundreds of feet. These also appear to be slightly 

 beveled. As these deposits consist of soft, though well stratified, mate- 

 rials, it is manifest that in the various well borings they have not been 

 differentiated from the later surface debris washed down from the 

 peripheral highlands. 



Similar Tertiary deposits of great thickness are known to exist below 

 Albuquerque, above Albuquerque, and at other points on the upper Rio 

 Grande. It is more than likely that in all of these localities there has 

 been a failure to discriminate between the older and younger of the soft 

 deposits, thereby ascribing an unusual thickness to the latter. In the 

 light of recent investigation it is probable that all of the reported data 

 tending to prove the great thickness of the surface deposits of the bolson 

 plains of this regions will have to be examined anew. 



The original types of the bolson plain, as that term has been given 

 technical geographic significance, are thus clearly destructional plains 

 with very thin covering, instead of constructional plains with very thick 

 detrital deposits. If the name is to be retained in geographic literature 

 with a distinct or special significance, it should be only in the widely 

 accepted sense to which the Spanish people have been accustomed. The 

 American adaptation is not only ill chosen, but defined under misappre- 

 liension. The latter usage of the name should be abandoned. 



SUBSTRUCTURE OF TEE E8TANCIA PLAINS 



The Estancia valley is similar to the Jornada del Muerto, adjoins it 

 on the north, and extends 100 miles in that direction to the city of Santa 



' U. S. Geological Survey, Water Supply and Irrigation Paper, no. 188, 1907, p. 21. 



