GREAT BASIN OVERTHRUSTS 
257 
face of this huge segment is apparently as fine a fault-scarp as can 
be imagined. This mountain face is planed flat. It is steep. 
Horizontal alignment is long and straight. Mountain meets low¬ 
land plain sharply. The transverse salients are characteristically 
truncated. A plane of mass movement is clearly discernable near 
the base, 400 feet above the level of the intermontane plain. For 
the first time since the Basin Range hypothesis was promulgated, 
half a century ago, human eye actually beholds an unmistakable 
fault-line bounding a mountain block exactly where theory de¬ 
mands that it shoud be. 
Ocular and demonstrable evidences of Basin Range structure 
and isostacy are not yet. Face of the Muddy Mountain may be 
expression of fault-scarp; but it is not a normal dislocation which 
theory leads one to expect. The fault observed is an overthrust. 
Tough, massive Paleozoic limestones recline directly upon Meso¬ 
zoic strata — friable Jurassic sandstones in some places, and 
unctuous Triassic shales in other situations. 
A Tectonic setting directly opposite to that always premised is 
disclosed. Since the Muddy Range is manifestly of the same age 
as that of the thousand and one other desert ranges the genetic 
significance of this fact is obvious. In the light of the new situa¬ 
tion Isostasy and Basin Range Structure have certainly to be 
evaluated anew. 
If ever there were typical development of Basin Range structure 
of the kind in which Gilbert, Dutton, and Powell took such keen 
delight to liken to tossed ice-cakes of a northern river jam at time 
of the Spring break up, then the Muddy Mountains of south¬ 
eastern Nevada should present incontestible proofs. These moun¬ 
tains constitute as characteristic a Desert Range as anv" other in 
the entire Great Basin. They form an isolated, north and south 
serrate ridge extending from the Moapa Indian Reservation to the 
Rio Colorado, at the Great Bend, and thence on far southward 
through the Black Range which rises abrutly on the east side of 
the valley. 
The Muddy Range emerges steeply out of a broad intermontane 
plain, known as, but incorrectly so-called, a wash, the California 
Wash of early Mormon travelers. Its west face is so long and 
straight and steep that it would readily pass for a normal fault- 
