252 J. E. SPURR — ORIGIN AND STRUCTURE OF THE BASIN RANGES 



tivel}'^ small amount of erosion indicated was capable of excavating deep 

 can3^ons and of giving new relief to the mountains. 



EROSION OF SOUTHEASTERN NEVADA DRY VALLEYS 



Description of valleijs. — The Bonneville and Lahontan Lake basins oc- 

 cupied only a fraction of the province which has hitherto been called 

 the Great basin. But the limits of this region as an orographic depres- 

 sion of necessarily interior drainage will have to be considerably con- 

 stricted on closer stud}', until they will very likely include little more 

 than the Bonneville and Lahontan basins, together with the l)road 

 sunken belt lying east of and parallel to the Sierra Nevada and extend- 

 ing from tlie Lahontan basin nearly to the Mojave desert. A large part 

 of southern Nevada has well defined valle3\s forming a part of the Colo- 

 rado River S3\stem, which a week of rain would supply with streams. 

 Meadow valle}^ which heads near tlie Pioche, is tributar}' to the Virgin 

 river, an affluent of the Colorado. For the greater part of the course it 

 is a magnificent canyon, cut sharply in Tertiary lavas and tuffs to a 

 depth which in places reaches 2,000 feet. Down it small quantities of 

 spring water run, })artly under, pjirtly over, the gravels, and very likely 

 some finds its way to the Colorado. The can3'on is continuous farther 

 north with a typical flat desert valley called Duck valle}^ which extends 

 beyond the 39th parallel. On the south Meadow valley is confluent, not 

 far from its end, with the valley of INLuUh" creek, in which flow waters 

 derived from a spring. Above the source of the spring a drainage channel 

 extends northward nearly to the latitude of Eureka. Along this a little 

 water flows, sometimes above and sometimes below the surface gravels, 

 as in Meadow valle3\ In its upper i)ortions it goes by the name of White 

 river, and it is believed b3' the inhabitants that the White River water 

 finds its wa3^ to the Colorado. 



Farther southwest is the Las Vegas valle3% which is even more plainly 

 tributary to the Colorado than the other two, although it carries no run- 

 ning water. It is transverse to the strike of the stratified rocks, and 

 separates the Spring mountain and Las Vegas ranges. 



Age and origin of valleys. — These three valle3"S, with their l^ranches, 

 constitute a drainage S3'stem embracing nearly the whole of southeastern 

 Nevada. That they are valleys of erosion is almost be3'ond question. 

 This entails as a corollary the former presence of streams capable of 

 carving them, and from this corollary the existence of a bygone episode 

 of moister climate is deduced. 



The valleys possess three relations which furnish clues to their age. 

 The first is their membership in the Colorado River drainage system ; 

 the second is their relation to deposits of various periods, and the third 



