4:2 C. B. Longwell — Geology of 



Geological Survey. The portion of the map representing 

 the Muddy and Virgin mountains was based on scant in- 

 formation, and the author made no claim of its accuracy. 



Nature of this paper. — The report of which this paper 

 is an abstract will appear as a publication of the U. S. 

 Geological Survey, and will present in detail the results 

 obtained in hve months of field work in 1919. This ab- 

 stract will state only the most essential facts and conclu- 

 sions regarding the topography, stratigraphy, structure, 

 and physiography of the region mapped. 



AcJcnowledgments. — The writer wishes to express his 

 thanks to members of the Geological Department of Yale 

 University, who secured research funds for prosecuting 

 the field work on which this paper is based. Acknowledg- 

 ment is also due the U. S. Geological Survey, which fur- 

 nished surveying instruments and field equipment, 

 although the writer was not a regular employee of that 

 organization. Mr. Harold S. Cave, at that time a grad- 

 uate student in the University of Missouri, spent several 

 weeks in the field and gave valuable assistance. During 

 the preparation of the report continued interest and ad- 

 vice from Professors Charles Schuchert and H. E. Greg- 

 ory were of especial assistance. The most sincere thanks 

 are due them and other members of the faculty who gave 

 helpful suggestions. 



Geography. 



Climate and drainage. — Southeastern Nevada has an 

 average annual rainfall of less than 6 inches, and is part 

 of a large arid region. Due to the proximity of Colorado 

 Eiver, however, the drainage is entirely exterior. Virgin 

 River is a permanent stream, receiving its supply in part 

 from the mountains of southern Utah, and in part from 

 its principal tributary. Muddy Creek, which has its source 

 in a number of large springs. Other stream channels 

 reach every part of the area, but all are dry except at 

 times of infrequent rains. 



Topography. — The general uniformity of surface which 

 characterizes the Colorado Plateaus ends abruptly at the 

 Grand Wash Cliffs. To the west, rugged, barren ranges, 

 trending generally north and south, are separated by wide 

 structural valleys of low elevation. The Virgin Moun- 

 tains have a number of sharp peaks, the highest reaching 

 an elevation of 7,700 feet. St. Thomas Gap, a low pass of 



