Sonora Pass Research Articles

Magmatically folded and faulted schlieren zones formed by magma avalanching in the Sonora Pass Intrusive Suite, Sierra Nevada, California

AbstractThe southwestern margin of the Late Cretaceous Sonora Pass Intrusive Suite, northern Sierra Nevada, California (USA), preserves a densely populated zone of magmatic structures that record dynamic magmatic layer formation and deformation (faulting and folding) within a solidifying upper-crustal magma mush. This zone consists largely of coupled melanocratic (or schlieren) and leucocratic bands hosted within the 95.6 ± 1.5 Ma Kinney Lakes granodiorite (Leopold, 2016), with orientations approximately parallel to the intrusive margin and with inward younging directions. Schlieren consist of a high modal abundance of medium-grained ferromagnesian minerals (hornblende + biotite), zircon, sphene, apatite, opaque minerals, and minor plagioclase and interstitial quartz. Leucocratic bands are dominated by coarse-grained feldspar + quartz with minor ferromagnesian and accessory minerals. Whole-rock geochemical and Sr and Nd isotopic data indicate that the schlieren are derived from the Kinney Lakes granodiorite by effective mechanical separation of mafic minerals and accessory phases.We interpret that the schlieren zone at the margin of the Kinney Lakes granodiorite formed by large-scale collapse of crystal mush by “magma avalanching,” facilitated by gravity, local convection, and possibly by host-rock stoping at the margin. This process eroded a significant portion of the solidifying margin of the chamber and resulted in the formation of magmatically deformed layered structures, which experienced further mingling, re-intrusion, magmatic erosion, and recycling processes. We envisage that magma avalanching of magma mushes in plutons can be achieved by any unstable process (e.g., tectonic, fluid-assisted, stoping, or gravity-driven) in large, long-lived magma-mush chambers.

Initiation of Sierra Nevada range front–Walker Lane faulting ca. 12 Ma in the Ancestral Cascades arc

The eastern escarpment of the Sierra Nevada (USA) forms one of the most prominent topographic and geologic features in the Cordillera, yet the timing and nature of fault displacements along it remain relatively poorly known. The central Sierra Nevada range front is an ideal place to determine the structural evolution of the range front because it has abundant dateable Cenozoic volcanic rocks. The Sonora Pass area of the central Sierra Nevada is particularly good for reconstructing the slip history of range-front faults, because it includes unusually widespread and distinctive high-K volcanic rocks (the ca. 11.5–9 Ma Stanislaus Group) that serve as outstanding strain markers. These include the following, from base to top. (1) The Table Mountain Latite (TML) consists of voluminous trachyandesite, trachybasaltic andesite, and basalt lava flows, erupted from fault-controlled fissures in the Sierra Crest graben-vent system. (2) The Eureka Valley Tuff consists of three trachydacite ignimbrite members erupted from the Little Walker caldera. These ignimbrites are interstratified with lava flows that continued to erupt from the Sierra Crest graben-vent system, and include silicic high-K as well as intermediate to mafic high-K lavas. The graben-vent system consists of a single ∼27-km-long, ∼8–10-km-wide approximately north-south graben that is along the modern Sierran crest between Sonora Pass and Ebbetts Pass, with a series of approximately north-south half-grabens on its western margin, and an ∼24-km-wide northeast transfer zone emanating from the northeast boundary of the graben on the modern range front south of Ebbetts Pass. In this paper we focus on the structural evolution of the Sonora Pass segment of the Sierra Nevada range front, which we do not include in the Sierra Crest graben-vent complex because we have found no vents for high-K lava flows here. However, we show that these faults localized the high-K Little Walker caldera. We demonstrate that the range-front faults at Sonora Pass were active before and during the ca. 11.5–9 Ma high-K volcanism. We show that these faults are dominantly approximately north-south down to the east normal faults, passing northward into a system of approximately northeast-southwest sinistral oblique normal faults that are on the southern end of the ∼24-km-wide northeast transfer zone in the Sierra Crest graben-vent complex. At least half the slip on the north-south normal faults on the Sonora Pass range front occurred before and during eruption of the TML, prior to development of the Little Walker caldera. It has previously been suggested that the range-front faults formed a right-stepping transtensional stepover that controlled the siting of the Little Walker caldera; we support that interpretation by showing that synvolcanic throw on the faults increases southward toward the caldera. The Sonora Pass–Little Walker caldera area is shown here to be very similar in structural style and scale to the transtensional stepover at the Quaternary Long Valley field. Furthermore, the broader structural setting of both volcanic fields is similar, because both are associated with a major approximately northeast-southwest sinistral oblique normal fault zone. This structural style is typical of central Walker Lane belt transtension. Previous models have called for westward encroachment of Basin and Range extension into the Sierra Nevada range front after arc volcanism ceased (ca. 6–3.5 Ma); we show instead that Walker Lane transtension is responsible for the formation of the range front, and that it began by ca. 12 Ma. We conclude that Sierra Nevada range-front faulting at Sonora Pass initiated during high-K arc volcanism, under a Walker Lane transtensional strain regime, and that this controlled the siting of the Little Walker caldera.

Sierra Crest graben-vent system: A Walker Lane pull apart within the ancestral Cascades arc

We show here that transtensional rifting along the eastern boundary of the Sierra Nevada microplate (Walker Lane rift) began by ca. 12 Ma in the central Sierra Nevada (USA), within the ancestral Cascades arc, triggering voluminous high-K intermediate volcanism (Stanislaus Group). Flood andesite (i.e., unusually large-volume effusive eruptions of intermediate composition) lavas erupted from fault-controlled fissures within a series of grabens that we refer to as the Sierra Crest graben-vent system. This graben-vent system includes the following. 1. The north-northwest–south-southeast Sierra Crest graben proper consists of a single 28-km-long, 8–10-km-wide full graben that is along the modern Sierra Nevada crest between Sonora Pass and Ebbetts Pass (largely in the Carson-Iceberg Wilderness). This contains fissure vents for the high-K intermediate lavas. 2. A series of north-northwest-south-southeast half-grabens on the western margin of the full graben, which progressively disrupted an ancient Nevadaplano paleochannel that contains the type section of Stanislaus Group (Red Peak–Bald Peak area). These Miocene half-grabens are as much as 15 km west of the modern Sierra Nevada crest, and vented high-K lavas from point sources. 3. Series of northeast-southwest grabens define a major transfer zone along the northeast side of the Sierra Crest graben. These extend as much as ∼30 km from the modern range crest down the modern Sierra Nevada range front, in a zone ∼30 km wide, and vented high-K lavas and tuffs of the Stanislaus Group from point sources. Range-front north-south and northeast-southwest faults to the south of that, along the southeast side of the Sierra Crest graben, did not vent volcanic rocks (although they ponded them); those will be described elsewhere. We present evidence for a dextral component of slip on the north-northwest–south-southeast normal faults, and a sinistral component of slip on the northeast-southwest normal faults. The onset of transtension immediately preceded the high-K volcanism (within the analytical error of 40 Ar/ 39 Ar dates), and triggered the deposition of a debris avalanche deposit with a preserved volume of ∼50 km 3 . The grabens are mainly filled with high-K lava flows, ponded to thicknesses of as much as 400 m; this effusive volcanism culminated in the development of the Little Walker caldera over a relatively small part of the field. Trachydacite outflow ignimbrites from the caldera also became ponded in the larger graben-vent complex, where they interfingered with high-K lavas vented there, and escaped the graben-vent complex on its west margin to flow westward down two paleochannels to the western foothills. The Sierra Crest graben-vent system is spectacularly well exposed at the perfect structural level for viewing the controls of synvolcanic faults on the siting and styles of feeders, vents, and graben fills under a transtensional strain regime in an arc volcanic field.

Discovery of a Remnant Population of Sierra Nevada Red Fox (Vulpes Vulpes Necator)

The Sierra Nevada red fox (Vulpes vulpes necator) occurred historically throughout the high elevations of California's Sierra Nevada and southern Cascade Mountains. Before this study, the only known remaining population in California consisted of ≤ 20 individuals restricted to the Lassen Peak region in the southern Cascades. In August 2010, we photographed a red fox in the Sonora Pass area of the Sierra Nevada, > 100 km from the Lassen Peak region. To determine if multiple individuals were present and were indigenous, we set up additional camera stations, collected genetic samples (saliva, scat, hair, and a carcass), and conducted a comparative genetic analysis between these individuals and historical and modern reference samples. Photo-detections identified at least three individuals based on pelage characteristics. Genetic analyses identified two females and one male, whose microsatellite profiles suggested they were closely related. A genetic assignment analysis indicated that all three individuals clustered most closely (> 95%) with historical samples from the Sierra Nevada, and were distinct from those in the Lassen Peak region. Additionally, mtDNA and microsatellite alleles unique to each population confirmed that the Sonora Pass individuals represent a second remnant California population of Sierra Nevada red fox. Reduced genetic diversity relative to historical levels in both remnant populations was consistent with small populations. Follow-up surveys are needed to determine the abundance and distributional extent of the Sonora Pass population, combined with research on both populations to assess demographic trajectories, determine threats, and to inform conservation efforts.

An in-depth look at distal Sierra Nevada palaeochannel fill: drill cores through the Table Mountain Latite near Knights Ferry

The ∼10.4 Ma Table Mountain Latite (TML) consists of high-K trachyandesite lavas that likely erupted from the Little Walker Center near Sonora Pass and flowed 80 miles (130 km) through the palaeo-Stanislaus river channel to Knights Ferry in the Sierra Nevada foothills. Complete sections through the proximal facies of the flow stratigraphy are common in the high-Sierra and along range-front faults, but in much of its distal facies in the Sierra Nevada foothills, the internal features of the unit are poorly exposed. Condor Earth Technologies offered us the opportunity to study three complete vertical sections through the distal facies of the TML through access to three drill cores. These cores, spaced 1500′ (457 m) apart and oriented oblique to inferred flow direction by 30°, are referred to here as cores A, B, and C, in a down-palaeoflow direction. Previous outcrop studies of the distal TML over the past century have concluded that the distal TML consists of a single flow. This is true of core B, which is dominated by a single 144′ (44 m) thick flow, underlain by a thin (∼5′, 1.5 m) flow with a vesiculated top, inferred to represent a toe of the thick flow, inasmuch as there is no baked zone or weathered contact between them. This contrasts sharply with core C, where the 50′ (15.2 m) thick TML consists of four flows 5–12′ (1.6–3.6 m) thick, each also defined by vesiculated tops, but differing in having weathered tops 1–2′ (0.3–0.6 m) thick, inferred to record an eruptive hiatus between each flow. The third flow in core C appears to be geochemically distinct from both overlying and underlying units, although all of the core samples are petrographically similar. The TML in cores C and B overlies coarse-grained andesitic volcaniclastic debris and fluvial deposits. By contrast, in core A, the TML overlies very fine-grained siltstones interpreted to record deposition in still water, probably a small lake produced by damming of the river by the thick flow in core B. The TML in core A is 20′ (6 m) thick, and is composed of eleven 1–3′ (0.3–1.0 m) thick couplets, each consisting of a clay-altered hyaloclastite breccia passing upwards into a relatively fresh, nonbrecciated, vesiculated top. We interpret these couplets to represent thin toes of a flow that generated steam explosions when they came into contact with a standing body of water. Palaeomagnetic remanence data (inclination only) on lava flows from the cores are consistent with the Classic Table Mountain direction and inconsistent with directional results from any other Stanislaus Group lava flows published to date. Thus, by palaeomagnetic correlation, the lavas studied here were emplaced between 10.36 ± 0.06 and 10.41 ± 0.08 Ma. In addition, remanence results suggest that the three sampled flows from core C were erupted over a time period shorter than the secular variation rate (i.e. less than a few centuries) and the Knights Ferry portion of the TML was emplaced within several centuries of the average Classic TML age range spanning the Sierra Nevada. This indicates that the weathered flow tops at Knights Ferry formed very quickly.

New evidence for alternating effusive and explosive eruptions from the type section of the Stanislaus Group in the ‘Cataract’ palaeocanyon, central Sierra Nevada

High-K volcanic rocks of the Stanislaus Group record magmatic events that occurred when the Sierra Nevada microplate began to calve off from the western edge of the Nevadaplano at 10–9 Ma. Despite the fact that these rocks have been studied in the Sierra Nevada for 120 years, their stratigraphy is not well known, because they were largely deposited in a complicated palaeocanyon network that drained the Nevadaplano and are preserved on high peaks in roadless areas. We present new 1:6000 scale mapping and a detailed measured section in the Bald Peak–Red Peak area of the Sonora Pass, in the upper reach of the ‘Cataract’ palaeocanyon, or palaeo-Stanislaus river. The Bald Peak–Red Peak area was proposed as the type section for the Stanislaus Group in 1953 but had not been mapped or measured in sufficient detail to demonstrate stratigraphic relations. Our work demonstrates the following stratigraphy (from base to top): (1) Table Mountain Latite (TML) trachyandesite (latite) and basaltic trachyandesite (shoshonite) lava flows, with phenocrystic clinopyroxene and skeletal plagioclase and groundmass olivine, and minor olivine basalt and basaltic andesite lava flows; (2) Tollhouse Flat Member of the Eureka Valley Tuff (EVT) welded ignimbrite, with abundant phenocrystic biotite; (3) a single trachydacite lava flow with phenocrystic amphibole, referred to here as the Lava Flow Member of the EVT; (4) By-Day Member of the EVT welded ignimbrite, which lacks phenocrystic biotite; (5) Upper Member of the EVT, consisting of unwelded ignimbrite; and (6) a single, very thick (60 m) aphyric basaltic trachyandesite (shoshonite) lava flow that we assign to the Dardanelles Formation. Our work confirms for the first time that the Dardanelles Formation exists as a unit that overlies all members of the EVT, and that its geochemistry is the same as that of the TML in the type section. Our documentation of a trachydacite lava flow within the EVT in the central Sierra Nevada shows that widespread effusive volcanism alternated with the explosive volcanism, rather than merely preceding and following it, and that magmas of trachydacitic composition were erupted as lavas as well as pyroclastic flows. The preservation of all formations and members of the Stanislaus Group in the ‘Cataract’ channel suggests that it was an important palaeogeomorphic feature.

The Mono Arch, eastern Sierra region, California: dynamic topography associated with upper-mantle upwelling?

A broad, topographic flexure localized east of and over the central and southern Sierra Nevada, herein named the Mono Arch, apparently represents crustal response to lithospheric and/or upper-mantle processes, probably dominated by mantle upwelling within the continental interior associated Pacific–North American plate-boundary deformation. This zone of flexure is identified through comparison between the topographic characteristics of the active Cascade volcanic arc and backarc regions with the analogous former arc and backarc in the Sierra Nevada and eastern Sierra Nevada. Serial topographic profiles measured normal to the modern Cascade backarc reveal an accordance of topographic lows defined by valley floors with an average minimum elevation of ∼1400–1500 m for over 175 km to the southeast. Although the accordance drops in elevation slightly to the south, the modern Cascade backarc region is remarkably level, and is characterized by relief up to ∼750 m above this baseline elevation. By contrast, serial topographic profiles over the former arc and backarc transitions of the eastern Sierra region exhibit a regional anticlinal warping defined by accordant valley floors and by a late Miocene–early Pliocene erosion surface and associated deposits. The amplitude of this flexure above regionally flat baseline elevations to the east varies spatially along the length of the former Sierran arc, with a maximum of ∼1000 m centred over the Bridgeport Basin. The total zone of flexure is approximately 350 km long N–S and 100 km wide E–W, and extends from Indian Wells Valley in the south to the Sonora Pass region in the north. Previous geophysical, petrologic, and geodetic studies suggest that the Mono Arch overlies a zone of active mantle upwelling. This region also represents a zone crustal weakness formerly exploited by the middle-to-late Miocene arc and is presently the locus of seismic and volcanic activities. This seismic zone, which lies east of the Sierra Nevada block, includes the northern and central Walker Lane in Nevada and the eastern California shear zone to the south in California. Vertical patterns of surface deformation over this region are characterized by west tilting of the Sierra Nevada block and by block faulting of a late Miocene erosion surface east of the Sierra Nevada.

Review: From Lead Mines to Gold Fields: Memories of an Incredibly Long Life, by Henry Taylor and Donald L. Parman and Sonora Pass Pioneers: California Bound Emigrants and Explorers, 1841-1864, by David H. Johnson

Book Review| April 01 2007 Review: From Lead Mines to Gold Fields: Memories of an Incredibly Long Life, by Henry Taylor and Donald L. Parman and Sonora Pass Pioneers: California Bound Emigrants and Explorers, 1841-1864, by David H. Johnson From Lead Mines to Gold Fields: Memories of an Incredibly Long LifeHenry TaylorDonald L. ParmanSonora Pass Pioneers: California Bound Emigrants and Explorers, 1841-1864David H. Johnson Ronald H. Limbaugh Ronald H. Limbaugh Search for other works by this author on: This Site PubMed Google Scholar California History (2007) 84 (3): 75–76. https://doi.org/10.2307/25161906 Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Cite Icon Cite Search Site Citation Ronald H. Limbaugh; Review: From Lead Mines to Gold Fields: Memories of an Incredibly Long Life, by Henry Taylor and Donald L. Parman and Sonora Pass Pioneers: California Bound Emigrants and Explorers, 1841-1864, by David H. Johnson. California History 1 April 2007; 84 (3): 75–76. doi: https://doi.org/10.2307/25161906 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentCalifornia History Search This content is only available via PDF. Copyright 2007 CHS Article PDF first page preview Close Modal You do not currently have access to this content.

A Novel Antipredator Mechanism in Salamanders: Rolling Escape in Hydromantes platycephalus

Antipredator mechanisms in salamanders involve behavioral responses that are usually combined with aposematic coloration and skin toxicity (Brodie, 1970, 1977, 1983; Dodd and Brodie, 1976). Behavioral responses range from immobility to active escape, including the adoption of stereotyped postures (Brodie, 1983). Escape mechanisms involve active responses like rapid coiling and uncoiling movements which flip or propel the animal, tail lashing, lateral writhing, and locomotion (Duellman and Trueb, 1986). Although escape responses are usually followed by immobility, no escape mechanism involving passive transport has been reported. Based on field observations, we document a novel antipredator mechanism in the salamander Hydromantes platycephalus that consists of body and tail coiling and limb tucking followed by passive rolling escape. Although coiling is widespread among salamanders, H. platycephalus performs it in the novel context of the steep slopes of the northern Sierra Nevada of California. This results in the peculiar antipredator mechanism of rolling escape. We observed Hydromantes platycephalus at two localities in California, in the vicinity of Sonora Pass: (1) the slopes of Blue Canyon in Stanislaus National Forest, Tuolumne County, and (2) near the Pacific Crest Trail in Toyabee National Forest, Mono County. We found 19 individuals at the first locality on 21 August and 11 September, 1993 and two at the second locality on 11 September, 1993. We observed defensive behavior of 10 individuals from the first locality and two from the second.

The late Cenozoic evolution of the Tuolumne River, central Sierra Nevada, California

Erosional remnants of volcanic rock deposited in a 10-m.y.-old channel of the Tuolumne River permit its partial reconstruction. Projection of the reconstructed channel west to the Central Valley and east to the range crest, together with several assumptions about the position of the hinge line and changes in channel gradient, allows estimates of the amount of uplift at the range crest during the past 10 m.y. At Tioga Pass, this amounts to as much as 1,830 m, as compared to the 2,150 m estimated in an earlier study for Deadman Pass at the San Joaquin River 30 km to the south. Comparison of the geometry of these river systems leads to the conclusion that 10 m.y. ago an ancestral range of hills occupied the present site of the Sierran crest, and, although of relatively moderate relief, it was a barrier to westward drainage even before late Cenozoic uplift. At that time, the San Joaquin River was apparently the only river flowing westward across the range from well south of Mount Whitney north to Sonora Pass. The Tuolumne River evidently never extended east of this range. Comparison of the ancient channel with the modern channel of the Tuolumne River permits analysis of the later evolution of the river system and the development of Hetch Hetchy Valley and the Grand Canyon of the Tuolumne. At Rancheria Mountain, where the volcanic in the ancient channel was highest, the river was forced to shift laterally southward around the dam and adjacent to the volcanic infilling, and start its new channel in granitic bedrock. Near Rancheria Mountain, as much as 1,525 m of new channel incision has taken place in the past 10 m.y., and the modern channel is about 915 m lower than the abandoned channel. An undetermined amount of this downcutting was from glacial erosion. The Tuolumne river system provides no direct evidence for timing the onset of uplift, but the shape of the 10-m.y. old channel at Rancheria Mountain suggests that uplift had been underway for some time before the volcanic infilling. This timing is compatible with evidence from the upper San Joaquin River. Hetch Hetchy Valley on the Tuolumne is a much fresher glaciated valley than is Yosemite Valley. Hetch Hetchy was filled to the brim with glacial ice as recently as 15,000-20,000 yr ago (Tioga glaciation), whereas Yosemite Valley probably has not been filled for 750,000 yr or more (Sherwin glaciation). Thus the upper reaches of Yosemite Valley cliffs have been shaped by spalling rather than by glacial scour and are much more irregular than those in Hetch Hetchy.

Variation among conspecific insect populations in the mechanistic basis of diet breadth

Post-alighting oviposition behaviour of adult females was studied in two oligophagous populations of the butterfly, Euphydryas editha. Comparison of these two populations shows that similar patterns of host use can result from dissimilar patterns of plant-insect interaction. In one population, Schneider's Meadow, the variation of host use among individual insects resulted directly from heritable behavioural variation of preference rank. In the other population, Sonora Pass, this behavioural variation was absent. Variation of host use arose more from plant distribution and abundance than from variation among insects. At Schneider's Meadow, any of the three host species present could be ranked highest by a particular butterfly, and we found all possible permutations of preference rank order in pairwise comparisons of these hosts. Because host specificity was high, and because all three host species were abundant, butterflies usually found their preferred host species and expressed their preferences by laying eggs on the host species they preferred. At Sonora Pass, all butterflies that expressed preferences ranked host species in the same order, but many egg batches were laid on the second-ranked host. Host specificity of those insects that discriminated was significantly lower than at Schneider's Meadow. Because of this low specificity and because the top-ranked host was rare and clumped, many females failed to find this species before the second-ranked host also became acceptable.

Uranium in Tuffaceous Fluvial Sandstone, Sonora Pass, California: ABSTRACT

Uranium in Tuffaceous Fluvial Sandstone, Sonora Pass, California: ABSTRACT

POTASSIUM-ARGON DATES OF THREE PLEISTOCENE INTERGLACIAL BASALT FLOWS FROM THE SIERRA NEVADA, CALIFORNIA

Potassium-argon dates of 90,000 ± 90,000 years and 60,000 ± 50,000 years on the basalt in Sawmill Canyon of the Sierra Nevada, California, that underlies a moraine correlated with the Tahoe Glaciation and overlies pre-Tahoe till show that the Tahoe is probably less than 100,000 years old. Pre-Tahoe basalt near Sonora Pass gives a date of 150,000 ± 30,000 years. These data support the correlation of the Tahoe with early Wisconsin. Tahoe Glaciation in the southernmost Sierra Nevada seems to have been no less extensive than pre-Tahoe glaciation. The basalt that forms the Devils Postpile, believed by Matthes (1960) to have been extruded between his El Portal (Sherwin?) and Wisconsin glaciations, gives a date of 940,000 ± 160,000 years.

Potassium-Argon Dates of Some Cenozoic Volcanic Rocks of the Sierra Nevada, California

A date of 19.9 m. y. (early Hemingfordian) from a welded rhyolite tuff at the top of the type section of the Valley Springs Formation, and one of 25.7 m. y. (early Arikareean) from the basal welded rhyolite tuff in the Sonora Pass area, indicate that the Valley Springs Formation was deposited before late Miocene, and that rhyolitic volcanism in the Sierra Nevada began by earliest Miocene time. Three dates of 8.8–9.0 m. y. (early Hemphillian) on biotite-augite latite, which stratigraphically overlies the Table Mountain latite in the Stanislaus drainage, show that the Table Mountain latite is a member of the Mehrten Formation, which contains the late Hemphillian Oakdale fauna and the early Clarendonian Two Mile Bar fauna. A 3.5-million year date on basalt filling a canyon cut in the Chagoopa surface at Little Kern River, a 9.5-million year date on basalt from Jose Basin in Fresno County, and dates of 3.5–3.6 m. y. from basalts in the Upper San Joaquin drainage indicate that the Chagoopa surface and the Mountain Valley stage are at least as old as early Pliocene, and that the Sierra Nevada had a minimum relief of 4000 feet at that time. Basalt beneath McGee Till on McGee Mountain gives a date of 2.6 m. y., and basalt on Coyote Flat gives a date of 9.6 m. y. Basalt beneath Sherwin Till in Owens Gorge has an age of 3.2 m. y. A rhyolite tuff and an overlying basalt in Deep Springs Valley, White Mountains, give ages of 10.9 and 10.8 m. y., respectively.

Ecology Color Variation in a Butterfly and the Problem of "Protective Coloration"

The distribution area of Oeneis chryxus Dbld. & Hew. (Lepidoptera: Satyridae) extends from the arctic regions of North America, south through the high ranges of the Rocky Mountains, the Sierra Nevada and the Great Basin as far south as New Mexico. The butterfly is not uniformly distributed over this area but is found only in the Arcticalpine life-zone and in the unforested ridges of the upper Hudsonian; hence, in the southern parts of its range, it is restricted to isolated islands on the higher peaks. The species is variable with respect to the predominant color on the upper surfaces of the wings, some individuals being a rather dark yellow-brown, others a medium yellow-brown and in extreme instances, yellow-white. The colors appear to be entirely quantitative in nature; individuals showing all different intermediate shades of color can be arranged from darkest to lightest. Within a restricted population, however, the individuals are remarkably uniform and only very occasionally are atypical examples found. Most populations consist of individuals of a rather dark yellowbrown color; occasional ones consist of extremely light or extremely dark forms. In California and western Nevada, the species is represented by two races, a very pale, yellow-white race (ivallda Mead, fig. 5B) and a very dark, yellow-brown race (stanislaus Hov., fig. 5D). The distribution area of each of these races is shown on the map (fig. 1); the localities where the white race has been found are shown by circles and those of the brown race by triangles. The contour line shown on the map approximately encloses the habitable territory of the species, this being the Arctic-alpine and upper Hudsonian life-zones. The color shade of individuals inhabiting the various localities is shown on the diagram (fig. 2). The horizontal scale indicates the geographical position of the locality while the vertical scale indicates the shade of color, darkest at the top and lightest at the bottom; the line connecting the points gives an index of the steepness of the variation gradient (cline as used by Huxley, '39) between the localities. At Sonora Pass, in the center of the range of the brown race, white or light-colored individuals are taken very rarely. Between Ebett and Echo Passes the populations all lighter in color, but entirely white individuals are as yet unknown. South of Sonora Pass to a point north of Tioga Pass where the brown and white races appear to interbreed, the population of the brown race is nevertheless dark, showing no variation gradient. In the very narrow transition zone, however, the population is mixed, showing the entire range of variation from the darkest to the lightest. These facts would seem to indicate that the differences between the brown and white races are not due to a single gene, but rather to a combination of several genetic factors, and that these factors or genes are widely distributed throughout the entire range of the species though with differential concentrations. As far as one can judge without genetical experimentation, the white races from the northern and southern sections are entirely identical even though the intervening area inhabited by the dark race is at least eighty miles in extent. The shortest means of communication between the two regions is through that occupied by the brown one (fig. 1). Furthermore, as the surrounding territory is of lower elevation and is in a different life-zone, it is ecologically unsuited for the existence of this particular species; it could hardly be a region through which migration could 371