The welded tuff of Devine Canyon, the most widespread ashflow tuff in southeastern Oregon, is centered around the Harney Basin lowland south of Burns; it originally covered about 7,200 square miles and had a volume of over 47 cubic miles. Its age is early Pliocene, about 9 million years. Thickness ranges from a few feet to over 100 feet. Most sections under 35 feet thick are entirely vitric; thicker sections are devitrified but have a vitric base. The distribution of the tuff suggests that the source area lies buried beneath Quaternary sediments in the northwest part of the Harney Basin lowland. Downwarping of this lowland may be related to the expulsion of magma from beneath to form the Devine Canyon and other welded tuff sheets. Phenocrysts are mostly alkali feldspar (Or42Ab5s) and quartz with minor iron-rich pyroxene. Alkali feldspar phenocryst content ranges from 1 to 29 percent and increases markedly upwards in most sections. Analyses of 14 samples :show that the devitrified tuff is uniform in composition. Unlike the welded tuffs of southern Nevada, samples that are rich in phenocrysts are similar in composition to samples that are not; this fact• indicates that phenocrysts crystallized in the magma chamber without significant crystal settling. A water pressure of between 800 and 1,800 bars during phenocryst crystallization is indicated by comparison with the experimental data of Tuttle and Bowen. Resorption of phenocrysts at all levels in the magma indicates reduction in water pressure prior to ash-flow eruption, probably by venting of water vapor plus minor air-fall tuff. Eruption onto a very flat plain produced an exceptionally widespread but relatively thin tuff sheet.

The succession of middle Miocene through Pliocene sedimentary and volcanic rocks that defines the Furnace Creek basin was deposited in a tectonic setting involving large dextral displacement on the northwest-trending FurnaceCreek fault, major crustal extension southwest of the fault, and contemporaneous folding. Most of the succession was deposited contemporaneously with plutonic and volcanic activity in the adjoining area to the south.The basin fill, as much as 4 km thick and exposed over an area 60 km long by 10 km wide, consists of the Artist Drive (ca. 14-6 Ma), Furnace Creek (ca. 6-5 Ma), and Funeral (ca. 5-3 Ma) Formations. The section depositionally overlies severely extended crust marked by the brittle attenuation of Paleozoic formations and apparently the ductile stretching of the underlying Proterozoic crystalline complex. The pre-14-Ma northwest-directed extension is best expressed in the brittle attenuation and associated faulting of Cambrian formations in the vicinity of the Billie borate mine. It is also expressed in the intimate fracturing of Ordovician formations beneath less disturbed Artist Drive strata in the Desolation Canyon area of the Black Mountains, the distribution of clasts in a middle Miocene fanglomerate in the southern Funeral Mountains, and the pre-14-Ma ductile fabrics overprinted on the Badwater crystalline complex. The extension predates, by as much as 3 m.y., the previously assumed advent of major extension in the Death Valley region. The Artist Drive Formation, 1.2-2.5 km thick, consists of a mostly sedimentary, fluvial, and lacustrine northeastern facies and a mostly volcanic southwestern facies deposited on opposite sides of a contemporaneous, northwest-trending anticline. The principal volcanic units thicken southward toward the igneous field, but now terminate with normal fault contact against the uplifted Proterozoic Badwater complex.

The Black Mountains are an east-dipping tilted fault block of the Basin and Range province, located on the east side of Death Valley, southeastern California. The mountains are underlain principally by Late Tertiary volcanic, plutonic, and sedimentary rocks, with locally exposed Proterozoic and Paleozoic basement. They are the location of two important controversial features, the Amargosa chaos and the turtlebacks, and an important key to the possibility of large-magnitude extensional faulting. A new geologic map of the north part of the range utilizes a stratigraphy made up principally of the well-known Artist Drive, Furnace Creek and Greenwater Formations and one new informal unit, sedimentary rocks and tuff of Ryan area. Principal borate deposits in the Death Valley region are in a belt extending from Death Valley up Furnace Creek wash and across the Greenwater Range to Amargosa Valley. The borates occur as both lenticular bedded deposits and as veins in various units of the Furnace Creek Formation. Principal borate minerals are colemanite, ulexite, and proberite. I suggest that hot springs related to the volcanism that produced the silicic volcanic rocks of the Greenwater Formation were the source of boron-rich solutions. The Billie Mine was the only active borate producer in 1995.

Introduction 3 Location 4 Topographic and geologic setting 4 Previous work . 4 Present study 4 Acknowledgments 4 Paleozoic and Mesozo-ic sedimentary rocks 8 Hesozoic plutonic rocks 8 Miocene volcanic rocks* * ; 8 Introduction ---8 Nomenclature of the volcanic rocks 9 Older volcanic rocks in west part of area . 11 Units underlying the basalt and andesite of Orevada View 11 Dacite welded tuff 11 Dacite, quartz latite, andesite, and basalt 13 Basalt and andesite of Orevada View 13 Miocene volcanic rocks Continued Alkali rhyolites outside the caldera ' = 40 Double H Mountain 40 Bilk Creek -Mountains . 43 Trout Creek Mountains 46 Lower Unit 46 Upper Units 47 Peripheral areas 47 Interpretation : 47 Intracaldera lavas Peaks 70 29.-Quantitative spectrographic analyses for B, Ba, Y, and Zr in some samples from the alkali rhyolite* of Jordan Meadow 71 30. Calculated partial groundmass composition of some rocks from the. alkali rhyolite of Jordan Meadow 72 31. K/Ar dates on the alkali rhyolite of Jordan Meadow and the overlying lavas : 76.

The Wadi Shuqub quadrangle (sheet 20/41 A) lies in the high mountainous part of the Hijaz plateau in west-central Saudi Arabia The quadrangle is underlain chiefly by Precambrian layered metasedimentary and metavolcanic rocks that are intruded by plutons ranging in composition from gabbroic to granitic, X The metasedimentary rocks include quartzite, phyllitic quartzite, phyllite, slate, argillite, granulite, schist, and marble. The metavolcanic rocks are mostly greenstone and greenstone tuff but include meta-andesite and metadacite. The rocks are in the greenschist and greenschist-amphibolite transition facies. The major plutonic rocks are tonalite and quartz diorite, which form a large batholith in the western part of the quadrangle. Smaller bodies of diorite and gabbro, granite, and complexes of mixed rocks are also present. Basalt of Quaternary age covers the Precambrian rocks near the eastern margin of the quadrangle, and alluvium underlies major wadis and flats. The main part of the Wadi Bidah mining district, including at least four ancient mining sites, is in the southeast part of the quadrangle. The district has produced gold from goldquartz veins, and has some potential for producing gold, silver, copper, zinc, and lead from massive sulfide deposits.