Geological Survey. The portion of the map representing the Muddy and Virgin mountains was based on scant information, 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 five months of field work in 1919. This abstract will state only the most essential facts and conclusions regarding the topography, stratigraphy, structure, and physiography of the region mapped.

Acknowledgments.-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. Acknowledgment is also due the U. S. Geological Survey, which furnished surveying instruments and field equipment, although the writer was not a regular employee of that organization. Mr. Harold S. Cave, at that time a graduate 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 advice from Professors Charles Schuchert and H. E. Gregory 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 River, 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 Mountains have a number of sharp peaks, the highest reaching an elevation of 7,700 feet. St. Thomas Gap, a low pass of

structural origin, permits easy passage between the Grand Wash and Virgin valleys. The highest part of the Muddy Range is Callville Mountain, an abrupt mass shaped roughly like a large letter "C" with the opening toward the south. The general elevation of the mountain above sea-level is about 3,000 feet, but many peaks rise to an elevation of 4,000 feet or higher. Muddy Peaks, the highest points, are nearly 5,800 feet above sea-level. Both north and south of Callville Mountain the average elevation is slightly more than 2,000 feet, and abrupt ridges rise several hundred feet higher. Everywhere the surface is nearly destitute of vegetation and has been carved into extremely rugged forms.

The valleys of Grand Wash, Virgin River, and California Wash are each several miles in width, and have a total area approximately equal to that of the intervening ranges. The stream courses occupy comparatively narrow inner valleys, from which there is a gradual rise toward the mountain walls on a series of broad terraces, partly dissected into badlands. Colorado River has cut canyons through the ranges, but its valley is wide and comparatively open in crossing the intermontane troughs. South of the Muddy Range the Colorado is less than 700 feet above sea-level.

ROCK SERIES AND FORMATIONS.

West of the Grand Wash Cliffs sedimentary formations are separated into two especially distinct groups. The first is a thick series resting on pre-Cambrian schists and ending upward with Mesozoic sediments. This group clearly corresponds to the series exposed in the Grand Canyon and on adjacent plateaus, and, as in the case of that series, the limestones, sandstones, and shales belonging to many periods have remarkable conformity among themselves. The second group has a distinctly younger aspect, and is separated from the older series by an unconformity of the first order. Below this break the youngest rocks exposed are not higher in the time scale than middle Mesozoic, whereas the oldest rocks of the younger series are probably later than middle Tertiary. Other unconformities within the younger group indicate continued unrest and an incomplete sedimentary record since the disturbances which first tilted the older rocks.

Exposures of pre-Cambrian rocks occupy large areas both north and south of St. Thomas Gap. The rocks are schists and gneisses, injected with coarse-grained granite. Above these crystalline rocks lie 70 to 100 feet of red arkose, 150 feet of gray quartzite and sandstone, several hundred feet of greenish shale, and limestones. thousands of feet thick. The limestones are in large part Pennsylvanian, Mississippian, and Devonian, but probably the Ordovician and Cambrian periods are also represented. The clastic sediments at the base probably

correspond to the Middle Cambrian Tapeats sandstone and Bright Angel shale of the Grand Canyon district.

Rock formations younger than the Paleozoic are exposed in St. Thomas Gap. They are identical with formations in the Muddy Mountains.

Rocks of the Muddy Mountains.

Paleozoic Formations.

Devonian System.-Limestones of Devonian age are the oldest rocks recognized in the Muddy Mountains. They are in Callville Mountain, a great block which has been thrust over Jurassic sandstone. All of the beds between the overthrust plane and recognized Carboniferous are here referred to the Devonian, but further work may show that some of the lower strata belong to older periods. Devonian fossils were found in a zone about 50 feet thick, more than 300 feet below the base of the Mississippian. Below this fossiliferous horizon there are 900 feet of limestone in which no fossils were found except a few algæ. As the beds are of the same general nature and as no unconformity could be found in the series, the entire thickness of 1,200 feet is tentatively included in one formation, which will here be called the Muddy Peaks limestone because of its prominence in the Muddy Peaks mass.

The limestone is dense and hard, and many beds have a siliceous appearance. Layers are regular and heavy, ranging from 2 to 20 feet in thickness. In color they are either very dark from included carbonaceous matter, or decidedly light, beds or zones of the two alternating. Lenses and thin layers of sandstone occur at intervals, especially near the top of the formation. At the base brecciation and shattering are extreme through a thickness of 100 to 500 feet.

Most of the fossils collected from the formation have been misplaced, and therefore a complete list is not available for publication in this paper. The following partial list has been identified by Dr. E. O. Ulrich: Heliophyllum sp. undet. (calyx less than 1/2 inch in diameter), Zaphrentis sp. undet. (calyx 1 inch or less in diameter), small Stromatoporoid of undetermined genus and species, Atrypa aff. reticularis (differs from the typical form in