MATURE STAGE OF ARID RELIEJF 557 



ranges is their base, where plain sharply meets mountain without the 

 intervention of foothills. The hard mountain rock is encroached upon 

 at the level of the general plains surface as the sea gnaws away a line of 

 its bordering cliffs, until, in many instances at least, the surface of the 

 intermont plain extends into the mountain blocks distances of several 

 miles. No more astonishing revelation was ever experienced by one who, 

 on first entering the arid region of the West thoroughly believing in the 

 prevailing theory of basin-range structure, was compelled to admit the 

 facts so clearly presented that the sharp, straight line of meeting of 

 mountain and plain was not a faultscarp at all, and that the major line 

 of displacement was usually situated several miles out on the basin plain.®^ 

 If the deflative hypothesis of regional desert-leveling and lowering be 

 accepted we have in the desert ranges a stream type hitherto unrecognized. 

 The streams of this class have no history previous to the youthful stage 

 of the present arid cycle ; they have no prospect of relations with streams 

 of any later cycle. Their birth, their span of life, their extinguishment 

 are definitely circumscribed. They are the only existing streams we 

 know of that do not have some sort of inherited relations with the waters 

 of previous geographic cycles. They are the only streams the complete 

 life histories of which may be distinctly traced at every stage. They are 

 the only streams where origin is clearly fixed in time and sharply limited 

 in space. 



DURATION OF ARID MATURITY 



The period of transition from arid youth to arid old age must be ex- 

 ceedingly brief. Compared with the corresponding stage of the normal 

 nnoist-climate c3Tle it is almost ephemeral. So short is it that it can 

 hardly be recognized as a distinct stage. Strongly supporting this con- 

 clusion are recent observations in New Mexico, Arizona, and Sonora. 



As the broad belts of weak rocks, previously profoundly faulted, undergo 

 through deflative influences the enormous denudation so manifest on every 

 hand, the effect is not only rapidly to wear them down, but the narrower 

 belts of resistant mountain rock are also encroached on as the latter are 

 brought into stronger and stronger relief. In the case of the region just 

 mentioned, where thicknesses of upward a mile have been removed, the 

 hard masses of mountain rock have been eaten into at the base of the 

 ranges for distances of 3 to 5 miles, and even more. There is thus left 

 a lofty central ridge with precipitous slopes rising out of the plains as 

 volcanic isles out of the sea. 



Titles as the Organs, the Needles, the Castle Domes, and the Eagle 

 Tails, locally applied to some of the desert ranges, well express the strik- 



<« Science, n. s., vol. xxxill, 1911, p. 466. 



