Constitution' of the batho1ith-Continued Introduction ______________________________________ _ Genernl geologic relations _______________________ _ Previous geologic work _________________________ _ Acknowledgments _____________________________ _ Wu.llrocks u.nd roof rocks ___________________________ _ Pu.leozoic rocks ________________________________ _ Mesozoic rocks ________________________________ _ Structure _____________________________________ _ Evolution of the gross structure _________________ _ Metamorphism ________________________________ _ Age of the batholith ________________________________ _ Older plutonic rocks of the west walL ________________ _ Constitution of the batholith ________________-____ _ Lithologic groupings of the plutonic rocks _________ _ Compositionn.l classifica.tion ______________________ • Mafic plutonic rocks

Cenozoic volcanic rocks of the Devils Postpile quadrangle are of late Pliocene to Recent age and are divided into 11 map units. The suite is alkalic-calcic and ranges in composition from basalt to rhyolite. It includes a rhyolitic welded ash-flow tuff which is probably correlative with the Bishop Tuff, although the two units are geographically isolated by the Sierra Nevada drainage divide. The Devils Postpile itself is a classic example of columnar jointing in the lower part of a lava flow.

The l\iount Morrison quadrangle includes part of the east front of the Sierra Nevada in Mono and Fresno Counties, Calif., ancl lies midway between Mono Lake on the north and Bishop on the south. The Sierra Nevada, characterized by alpine topography, occupies the southern half of the quadrangle and is separated from an area of relatively gentle topography to -the north by a steep ea~Stward-trending escarpment 2,500 to more than 8,000 feet high. Altitudes in the quadrangle range from less than 6,800 to more than 18,100 feet. The geologic units. mapped can be grouped into metasedimentary rod:s of Paleozoic age, metavolcanic rocks of Mesozoic nge, and intrusive rocks of Cretaceous age, which occur chietly in the Sierra Nevada, and extrusive and sedimentary rocks of Cenozoic age, which occupy the area north of the Sierra.The two groups of metamorphic rocks compose the Mount Morrison roof pendant, which underlies 42 square miles in the Sierrn Nevada and consists of a grossly homoclinal sequence of beds a little less than 50,000 feet thick. .The beds strike northwestward and dip steeply, with tops to the west. The common mineral assemblages of the rocks are typical of• the hornblende hornfels metamorphic facies of Turner and consist of quartz, mica, and feldspar in the more aluminous rocks and diopside, tremolite or actinolite, grossularite, and locally, wollastonite In the more calcareous varieties.The metasedimentary rocks of Paleozoic age compOEle the eastern three-fourths of the pendant. They are divided into 27 cartographic units, totaling 82,400 feet in maximum stratigraphic thickness, that range in age from Early Ordovician to Permian ( ?) . The rocks are divided into three blocks by two major fnults which strike almost parallel to the beds. The easternmost block contains six formations, with an approximate total thickness of 8,200 feet. The common rocks in this block are thin-bedded siliceous and pelitic hornfels, marble, slate, metachert, and a lesser amount of thin-bedded siliceous calc-hornfels and• thick-bedded calcareous quartz sandstone. Graptolites of Early Ordovician (Arenig) and Middle Ordovic\an (Caradoc) ages were found at several localities. The middle block comprises 10 formations with a total thiclmess of about 16,900 feet. The lower half of this block consists. chietly of thin-bedded 1 siliceous •and pelitic hornfels, slate, siliceous calc-hornfels, and marble: the upper half con-•Sis.ts of alternate thin-bedded siliceous hornfels and thickbedded calcareous quartz sandstone. Graptolites of Middle Ordovician (Caradoc) age were collected from the lower half, and one poorly preserved graptolite that may be as young as Silurian('!) was collected •near the top The westernmost block comprises five formations totaling 7,300 feet in thickness. Th~ lowest third of the block consists largely of thin-bedded siliceous hornfels and metachert but includes one distinctive PETROGRAPHY About 90 thin sections of the metasedimentary. rocks were examined with a petrographic microscope, and :~ ' .•. ', . .

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Qa Alluvium (Holocene) Silt, sand, and granule-to boulder-sized material having minor amounts of organic debris, gray, yellowish-gray to brown, well -sorted and -stratified. Alluvium shows fining-upward cycles. Underlies active stream beds or floodplain, locally frozen in silty channels; includes placer-mine tailings on sections of Quail, Troublesome, Ophir, and Nome Creeks, streams in Livengood village area, and southeast of Chatanika River in Fairbanks mining district; 0.3 m to approximately 15 m thick.Qg Reworked creek gravels in placer mining areas (Holocene) Placer-mine tailings derived from buried stream gravels worked for gold by methods of pick and shovel, mechanized surface, underground drifting, or dredging methods; shown, as map scale permits, on Fairbanks and Livengood Creeks, elsewhere included in Qa. Commonly less than 9 m thick.Qab Abandoned or inactive flood plain deposits (Holocene) Silt, sand, granule-to pebblesize gray gravel, and organic material. Abandoned stream channels 0.6 to 4.5 m topographically higher than active channels of Qa, elsewhere flat to hummocky and many bogs. Includes small alluvial fans, deposited by minor side streams, and intermixed alluvial sediments and very silty natural stream levees from cyclical flooding of Minto Flats.Commonly frozen. As thick as 30 m. QdSand dune deposits (Holocene) Sand, moderate yellowish-brown, well-sorted, eolian. Grains 65 to 85 percent quartz, yellowish-white, clear to opaque, angular to round; dark gray to black rock fragments, chert, mica, traces of feldspar and light-colored rock fragments. Isolated dunes mostly covered by eolian silt and stabilized by vegetation. As thick as 9 m. QsSwamp deposits (Holocene) Humus, peat, and silt in poorly-drained areas having abundant stagnant water; generally frozen below a depth of about a meter; some small swampy areas included in units Qa, Qab, and Qsu. As thick as tens of meters. QafAlluvial fan deposits (Holocene) Sand, gravel, and boulders, gray to brown, poorly-to wellsorted and stratified, coarse-grained, rounded to angular; clasts locally-derived and deposited under high-energy conditions below major decrease in slope. Locally covered by reworked silt and small amount of vegetation. Thickness varies greatly; only largest alluvial fans shown on map. Qsu Silt, undifferentiated, contains reworked loess, swamp and organic-rich deposits (Holocene) Silt, pale yellowish brown, largely eolian, and in part, locally retransported to lower slopes and valley bottoms by alluvial and solifluctional processes. Local brown to grayish black organic rich layers, masses, and disseminated debris. Poorly drained and frozen as abundant horizontal and vertical sheets, wedges, and irregular masses of ground ice. Thickness ranges from 1 to 61 m. QIC Loess and colluvium Including minor upland alluvium (Holocene) Unsorted mixture of local bedrock fragments and loess, light-gray to brown, angular or subrounded; partially frozen, fairly well-drained, locally occurs as mixture of reworked loess and colluvium in drainage chan...