64 C. R. KEYES INTERMONT PLAINS OF THE ARID REGION 



scale. Lately, the long accepted opinions regarding the Basin Eango 

 structure have been brought seriously into question. As a result a new- 

 interest in the subject has been kindled. 



That the tectonics of the regions mentioned have never received the 

 careful consideration that the natural advantages for study would seem to 

 demand is due partly to a singular notion -which has prevailed from the 

 first, that these regions present structures of great simplicity and hence 

 require little detailed examination. 



Physiographically the dominant feature of the Great basin and the 

 Mexican tableland is the plain. The most conspicuous features are the 

 mountains. These, like isolated, lofty piles, rise abruptly above the even, 

 sea-like plains as do volcanic isles out of the ocean. Areally the highlands 

 occupy much the less space. One-fifth of the siirface of the region is 

 mountain; the plains cover four-fifths. 



To the casual observer the smooth surface of the plains appears to have 

 few exposures of bedrock and to be everywhere deeply mantled by loose, 

 and for the most part rather coarse, rock-waste. The mountains, on the 

 other hand, are bare and free of vegetation, permitting their structure to 

 be displayed as nowhere else in the world outside of arid countries. The 

 geologic structure of these regions has been in the past largely inferred 

 from the parts protruding alwve the level of the plains, while those por- 

 tions beneath that level have been generally regarded as inaccessible and 

 have been almost altogether neglected. 



The general tectonics of the two regions under consideration and the 

 Basin Eange structures need not be taken into account now, for the reason 

 that they will receive special attention in another place. In the present 

 connection only the characters of the substructure of the plains is to be 

 considered. 



Foundation of the akid Plains 

 definition of "bolson" 



Although views regarding the geologic structure of the Great Basin 

 region and of the Mexican tableland have differed more or less in details, 

 there has been a consensus of opinion that the mountains are being worn 

 down to build up the intervening plains, and that finally the former will 

 be all carried down into the latter. A common opinion is the one ad- 

 vanced by Le Conte, which regards the mountains as the elevated margins 

 of huge tilted blocks and the plains the depressed and buried portions of 

 tlie same blocks.^ Starting with this conception, it is an easy step to the 



= American Journal of Science (3), vol. sxxviii, 1889, p, 259; also Elements Geology, 

 5th edition, 1904, p. 242. 



