valleys, but near the edges of the block they plunge sharply by a succession of falls which total a height of 400 feet or more. The aggradation surface has a general slope conforming to the grades of the larger streams, and also slopes from the mountain walls toward the valley interiors. A considerable part of this highest surface was destroyed during the growth of the next one, which is locally 5 or 6 miles in width and connects with smooth benches cut on solid rock. The inner surfaces or terraces record comparatively short halts of base-level. Within relatively recent time both the Virgin River and Muddy Creek have incised their floodplains, which may properly be considered a third terrace level. OUTLINE OF GEOLOGIC HISTORY. In the Virgin Mountains the geologic record extends backward into pre-Cambrian time, but in the crystalline rocks the record is complicated and obscure. Apparently the clastic Cambrian sediments were deposited on a nearly even surface which cuts across the structure planes of the metamorphic rocks. During much of the Paleozoic era southeastern Nevada was part of the Cordilleran geosyncline, although it was east of the area which received the thickest sediments. Further study will be required to determine the amounts and kinds of sediments deposited previous to the Devonian period. The Upper Devonian sea shallowed to the east and deepened to the west and north. Earlier Devonian invasions may have covered the region, but faunal evidence is not available. Marine invasions occurred in both Lower and Upper Mississippian, in Pennsylvanian, and in Permian times. During the Permian and perhaps during the Upper Pennsylvanian, thick continental and littoral deposits accumulated under conditions of aridity or semiaridity. At the close of the era the region was uplifted without perceptible tilting, and suffered moderate erosion. The Californian Lower Triassic sea deposited limestone, gypsum, and clastic sediments, and a great thickness of continental deposits followed the retreat of the sea. An interval of erosion succeeded, and another epoch of continental sedimentation began with the deposition of the coarse Shinarump conglomerate. A thick series of sand, silt, and clay was furnished by highlands which arose to the south, and the enormously thick and widespread Jurassic sandstones were probably derived from the same source. All of the Mesozoic sediments indicate aridity of climate. In post-Jurassic time the region suffered intense folding, accompanied by overthrusting, probably in connection with the Sierra Nevada disturbance. During the late Mesozoic and the first half of the Tertiary the Muddy Mountain area was probably part of a high region with exterior drainage, and received no deposits which have been preserved. Intense normal faulting occurred, and thick basin deposits were formed, probably in Upper Miocene time. After erosion had subdued the topography there was a recurrence of faulting which formed new basins, in which thick deposits of saline silts and clays accumulated. The present drainage system was established later, probably in Quaternary time, and since its establishment there have been changes of base-level amounting to several hundred feet. Comparatively recent disturbances have deformed late Tertiary and Quaternary deposits, and it is possible that considerable faulting accompanied these latest crustal movements. BIBLIOGRAPHY. Ball, Sydney H.: A geological reconnaissance of southwestern Nevada and eastern California, U. S. Geol. Survey Bull. 308, 1917. Buwalda, J. P.: Tertiary mammal beds of Stewart and Ione valleys in west-central Nevada, Univ. Calif. Publ., Bull. Dept. Geol., vol. 8, No. 19, 335-363, 1914. Carpenter, Everett: Ground water in southeastern Nevada, U. S. Geol. Survey Water Supply Paper 365, 1915. Dake, C. L.: The horizon of the marine Jurassic of Utah, Jour. Geol., 27, 637-646, 1919. The pre-Moenkopi (pre-Permian?) unconformity of the Colorado Darton, N. H.: A reconnaissance of parts of northwestern New Mexico and northern Arizona, U. S. Geol. Survey Bull. 435, 1910. Dutton, C. E.: Tertiary history of the Grand Canyon district, U. S. Geol. Survey Mon. 2, 1882. Also 2nd Ann. Rpt., p. 126, 1882. Gilbert, G. K.: The basin range system, U. S. Geog. and Geol. Surveys W. 100th Mer., vol. 3, 1875. Girty, G. H.: New species of fossils from the Thaynes limestone of Utah, Annals N. Y. Acad. Sci., 20, 239, 1910. Gregory, H. E.: Geology of the Navajo country, U. S. Geol. Survey Prof. Paper 93, 1917. Hill, J. M.: The Grand Gulch mining region, Mohave County, Arizona, U. S. Geol. Survey Bull. 580, 39-58, 1916. The Yellow Pine mining district, Clark County, Nevada, U. S. Geol. Ives, J. C.: Report upon the Colorado River of the West, Ex. Doc., 36th Lawson, A. C.: The petrographic designation of alluvial fan formations, Univ. Calif. Publ., Bull. Dept. Geol., vol. 7, 325-334, 1913. Lee, Willis T.: A geological reconnaissance of a part of western Arizona, U. S. Geol. Survey Bull. 352, 1908. Marvin, A. R.: Geology of route from St. George, Utah, to the Gila River, Arizona, U. S. Geog. and Geol. Surveys W. 100th Mer., vol. 3, 193-225, 1875. Noble, L. F.: The Shinumo quadrangle, Grand Canyon district, Arizona, U. S. Geol. Survey Bull. 549, 1914. Robinson, H. H.: The San Franciscan volcanic field, Arizona, U. S. Geol. Survey Prof. Paper 76, 1913. Schuchert, Charles: Paleogeography of North America, Bull. Geol. Soc. America, vol. 20, 427-606, 1910. On the Carboniferous of the Grand Canyon of Arizona, this Journal, 45, 347-369, 1918. Spurr, J. E.: Descriptive geology of Nevada south of the fortieth parallel and adjacent portions of California, U. S. Geol. Survey Bull. 208, 1903. Turner, H. W.: The Esmeralda formation, Am. Geologist, 25, 168, 1900. Walcott, C. D.: Permian of the Kanab valley, Arizona, this Journal, 20, 1880. Wheeler, G. M.: U. S. Geog. and Geol. Surveys W. 100th Mer., vols. 1 and 3, 1875. ART. V.-The Stanley Shale of Oklahoma; by C. W. HONESS. [By permission of C. W. Shannon, Director, Oklahoma Geological Survey, Norman, Oklahoma.] INTRODUCTION. Although the Stanley shale was not defined and named until 19021 the strata composing this formation have been known since the fore part of the 19th century, when the early explorers and travelers entered the regions of the outcroppings and wrote brief descriptions of the topography and described some of the exposures.2 3 In January, 1857, the Arkansas Geological Survey was organized and D. D. Owen made State Geologist. His explorations and studies led him into all parts of the State and by 1860 his second report appeared, which had to do with the middle and southern counties of Arkansas, including the area of outcrop of the Stanley in that State. His work was of the nature of a reconnaissance, and his untimely death prevented his writing all of the report; nevertheless he succeeded in determining, at least to his own satisfaction, the general position in the geological column of the Jackfork and Stanley, both of which he assigned to the "sub-Carboniferous," i. e. "at least not lower than the base of the 'sub-Carboniferous'. . . . . and the highest and newest of this great series of sandstones, slates and shales not younger than the base of the true coal measures. 994 Since his time several geologists have studied the Stanley both in Arkansas and in Oklahoma, notably J. A. Taff, George H. Girty, and H. D. Miser. 1 Taff, J. A.: Atoka Folio of the U. S. Geol. Survey, No. 79, page 4 ("from the village of Stanley in the Kiamitia Valley, where it is extensively exposed''). Nuttall, Thos.: A Journal of Travels into Arkansas Territory during the year 1819,"' etc., pp. 296, Philadelphia, 1821. Ward, J. A.: "A Geological Reconnaissance of the Arkansas River," Cincinnati, 1853. Owen, D. D.: 2nd Rept. of a Geological Reconnaissance of the middle and southern counties of Arkansas for 1859-60, pp. 13-153. Philadelphia, 1860. Taff, J. A.: Atoka Folio, No. 79, U. S. Geol. Survey, Atlas. 0 377. Girty, Geo. H.: Fauna of the Caney Shale, U. S. Geol. Survey, Bull. 'Miser, H. D.: Manganese Deposits of Caddo Gap and DeQueen Quadrangle. U. S. Geol. Survey Bull. 660-C. |