Hill Complex Research Articles

Petrology and geochemistry of Mesozoic igneous rocks, Bükk Mountains, Hungary

Basic and ultrabasic igneous rocks of the Jurassic Szarvaskő complex (southwestern Bükk Mountains, NE Hungary) have previously been interpreted as dismembered fragments of a Mesozoic ophiolitic sequence comprising mainly gabbroic sills intruded into deep-water terrigenous shales. Abundant extrusive pillow basalts, layered ultramafics and plagiogranite bodies are also present. Gabbros and basalts showing slight LREE depletion ( Ce Yb N =0.84–0.93 ) and very slight negative Eu anomalies (0.92–0.98) are considered to be close to liquid compositions, while some gabbros with the lowest overall REE abundances and positive Eu anomalies ( Eu Eu ∗ = 1.14–1.29 ) are regarded as plagioclase+clinopyroxene cumulates. The REE characteristics of the most primitive rocks are very similar to those of N-type MORB. Differentiated magmas show negative Eu anomalies and increasing REE enrichment, consistent with fractionation of plagioclase and clinopyroxene. In most rocks from the Szarvaskő complex, strong enrichment is seen in Ba, Rb and K, compared with the values for MORB. Age-corrected ϵSr t -values of the Szarvaskő complex range from −0.2 (basic rocks) to + 36.4 (evolved rocks). The ϵNd t -values vary between +6.4 (basic rocks) and +3.3 (evolved rocks). The variation in ϵNd t is probably not due to alteration or metamorphism, since simple REE patterns indicate REE immobility, and is considered to be caused by contamination during magmatic fractionation. The high and variable ϵSr t -values could be due to transfer of radiogenic Sr from the surrounding sediments during very low-grade regional Alpine metamorphism. It is suggested that the basic magmas of the complex were probably originally MORB-like, but during evolution in a shallow magma chamber, they became contaminated by terrigenous sediments. The nearby Triassic Darnó Hill complex is much more strongly altered, does not show the same depletion in LREE, and has lower ϵNd t -values. It is considered to have been formed in a different tectonic setting, possibly a continental margin or oceanic island.

Sodium-calcium metasomatism; chemical, temporal, and spatial relationships at the Yerington, Nevada, porphyry copper deposit

Tilting associated with extensional tectonics has exposed 1,800 m of continuous palcovertical relief through the Jurassic-age Yerington porphyry copper deposit. The rarely observed root zones of a porphyry system are characterized at Yerington by sodium-calcium metasomatism. The principal hydrothermal reactions involve the replacement of primary K-feldspar by oligoclase and of primary biotite by actinolite. Copper has been removed. Sodic-calcic alteration developed at deep intrusive contacts nearly simultaneously with potassic alteration and copper mineralization at shallow intrusive contacts. The reactions accompanying potassic alteration are the reverse of those accompanying sodic-calcic alteration. Plagioclase reacted to form K-feldspar; hornblende reacted to form biotite. The potassic and sodic-calcic alteration couplet principally accompanied each of two separate intrusive events: (1) Luhr Hill granite stock and associated porphyry dikes and (2) Walker River granite porphyry dikes. The younger Walker River intrusion was emplaced at slightly deeper levels than the older Luhr Hill complex. Vertically stacked and partially superimposed alteration and mineralization patterns resulted from differences in depth and in timing of emplacement.At lower temperatures, late sodic alteration was superimposed on early sodic-calcic alteration at deeper levels in the system, whereas late quartz-sericite-pyrite alteration was superimposed on potassic alteration at higher levels. Sodic alteration is characterized by the replacement of K-feldspar by albite and of biotite by chlorite. Copper deposited during potassic alteration was remobilized. The transition to lower temperature alteration is associated only with the Walker River intrusion. The hydrothermal system associated with the emplacement of the Luhr Hill complex had not evolved to lower temperature assemblages prior to emplacement of the Walker River dikes.Phase equilibria and geologic reconstruction of depth of emplacement suggest that temperatures during early sodic-calcic alteration were limited to the range of 360 degrees to 480 degrees C at pressures between 300 and 800 bars. Late sodic alteration took place at similar pressures but at temperatures less than 360 degrees C.Nearly simultaneous development of potassic and sodic-calcic alteration resulted from the flow of fluids away from intrusions at shallow levels (potassic alteration) and from the flow of fluids toward intrusions at deeper levels (sodic-calcic alteration). Because K/Na and Na/Ca activity ratios of a fluid in equilibrium with granitic rocks decrease with decreasing temperature, cooling of high-temperature fluids can generate potassic alteration and heating of fluids to high temperatures can generate sodic or sodic-calcic alteration.

A large‐scale meandering submarine canyon: outcrop example from the late Proterozoic Adelaide Geosyncline, South Australia

ABSTRACTThe late Proterozoic Adelaide Geosyncline, along with overlying Cambrian strata, comprises a thick sequence of sediments and sparse volcanics which accumulated in a major rift and passive margin setting. During late syn‐rift or early post‐rift phases, large volumes of terrigenous and carbonate sediments of the late Proterozoic Umberatana and Wilpena Groups and Cambrian Hawker Group filled the rift. Submarine canyon development was related to at least four of these depositional cycles, the most notable of which resulted in incision and subsequent filling of the major (several kilometres in width and up to 1.5 km deep) submarine canyons by the Wonoka Formation.The Wonoka Formation canyons are not obviously fault controlled. They are interpreted to have been eroded by turbidity currents during a relative low‐stand of sea‐level. They were subsequently filled by a fining‐upwards suite of sediments which reflects subsequent relative rise of sea‐level and carbonate platform development. Ultimately the canyon complex was buried by north‐westerly progradation of overlying fluvial and slope sequences (Billy Springs Beds and possibly correlative upper Pound Subgroup). It is considered likely that more distal elements of this prograding clastic wedge provided the necessary material for canyon erosion, prior to canyon filling and ultimate burial by what may have been elements of the same depositional cycle.It is considered possible that the series of isolated outcrops of canyon cross‐sections within the Wonoka Formation are sections of a single canyon thalweg developed within a considerably broader zone of slope degradation. If this interpretation is correct, then the gorge‐like Patsy Springs Canyon lies in more proximal regions of the basin‐slope, whereas 40 km to the north‐east the lower slope is cut by the Fortress Hill Canyon Complex. Palaeocurrent analyses of channel‐fill turbidites within the canyons imply that the Fortress Hill Complex is in fact the outcropping western edge of a sinuous, incised canyon thalweg.The Wonoka Formation canyons, containing basal sedimentary breccias but only minor conglomerates, are considered typical of passive margin canyon development. They are contrasted with the generally highly conglomeratic channel‐fills observed in outcropping Tertiary and Cretaceous examples of active margin canyons and upper fan valleys.

Landslide due to mass sinking phenomenon: Sunil-Joshimath of U.P. Himalaya example

Mass sinking associated with soil creep and debris displacement is mainly responsible for landslide in Sunil area of Joshimath hill complex. Seepage plays the important role in soil subsidence. The area representing moderate to high relief denudational hills, shows rarely any insitu rock exposures. Gneissic boulders are embedded in loose sandy soil where due to presence of many springs excess seepage occurs, causing a soil creep and displacement of the bou.ders giving rise to local sinking.

Ordovician cauldron subsidence of the Blue Hills Complex, eastern Massachusetts

The Blue Hills Complex, which lies south of the Boston Basin in eastern Massachusetts, is composed of Cambrian sedimentary rocks and Late Ordovician alkalic plutonic and volcanic rocks. On the north, the Complex is separated by a fault from the Boston Bay Group, which very recently has been shown to be late Precambrian. On the south, the Complex is overlain unconformably by the Pennsyvanian rocks of the Norfolk Basin. The bedding and volcanic flow structures of the Cambrian, Ordovician, and Pennsylvanian rocks are essentially parallel and dip steeply south. I believe that the Blue Hills Complex is a Late Ordovician cauldron subsidence that was tilted during the Alleghenian Orogeny and was thrust northward over the Boston Bay Group.

San Luis Obispo Transform Fault and Middle Miocene rotation of the Western Transverse Ranges, California

A fault‐disrupted, linear belt of lower middle Miocene pyroclastic and volcanic rocks was erupted 15–17 m.y. B.P. along a trend 140 to 200 km long in the southern Coast Ranges and along the northern flank of the western Transverse Ranges of California. Offset segments of the belt are coincident with the Oceanic‐West Huasna, Santa Maria River‐Little Pine, and Lompoc‐Solvang fault zones. The volcanic rocks thin eastward and also vary in thickness along strike, the latter reflecting the undulation of ‘porpoising’ highs and lows of basins. To account for the linear distribution of over 3000 km2 of bimodal volcanic rock types, it is proposed that they were intruded, extruded, and ejected into a continental submarine margin environment along a feature here named the San Luis Obispo transform (SLOT). The fault was near the mid‐Miocene continental margin and evidently joined the early mid‐Miocene Mendocino triple junction with the crest of the East Pacific Rise. Subduction of the Farallon Plate had stopped along the adjacent continental margin before Miocene magmatism in this region. The inferred position of the East Pacific Rise crest was too far to the south, and the inferred position of the Mendocino triple junction was too far to the north between 15 and 20 m.y. B.P. to have been directly related to the Miocene volcanic event near 35°N latitude at that time. A part of the western Transverse Ranges was transported northwestward immediately following Miocene volcanism. Earlier movement is possible, but evidence is inconclusive. Microplate transport was concurrent with the development of pull‐apart structures and the ≃12‐ to ≃16‐m.y.‐old period of volcanism along the southern margin of the microplate within the Los Angeles basin. Variable paleomagnetic directions that display clockwise deflections in the Oligocene (e.g., Morro Rock‐Islay Hill Complex) and Miocene igneous rocks (e.g., western Transverse Ranges) in the vicinity of SLOT can be accounted for by extension and rotational tectonics during pull‐apart development, rotation of blocks between faults during the northwestward transport of the western Transverse Ranges along SLOT, and by rotation and possible northward translation of a part of the western Transverse Ranges.

Rb-Sr whole-rock isochron evidence for the age of the Malvern Hills complex

The Malvernian igneous complex of the Malvern Hills consists mainlyof plutonic rocks and their locally deformed equivalents. The igneous rocks are calc-alkaline and range from ultra mafic diorite to granite in composition, although diorite and tonalite are the most abundant rock types. A Rb-Sr whole-rock isochron plot for representative samples yields an age of 681 ± 53 Ma with an initial 87 Sr/ 86 Sr ratio of 0.7049±0.0005(2σ errors).This is interpreted as the age of crystallization of the igneous complex, and the relatively low initial 87 Sr/ 86 Sr ratio is considered to preclude a long crustal history for the source region. The Malvernian complex is thus regarded as a late Precambrian addition to the crust. It has been compared on geological grounds with the Johnston Complex and the Stanner-Hanter Complex, and isotopic ages between 640 and 700 Ma for all these complexes indicate that they may represent the earliest phase of igneous activity in the evolution of continental crust below England and Wales.

Stratigraphy of Oceanus Procellarum basalts: Sources and styles of emplacement

The appendix is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D. . 20009. Document J80‐005; $1.00. Payment must accompany order.The basalts within Oceanus Procellarum have been named the Oceanus Procellarum Group and subdivided into four major geologic units on the basis of morphology, spectral reflectance, and other remote sensing information in order to develop a framework of regional stratigraphy for the largest lunar mare. The oldest mare basalt unit identified, the Repsold Formation (blue color, low albedo, high crater density, very weak 3.8‐cm radar backscatter, and a weak 1‐μm reflectance) crops out in NW Procellarum and may be correlative with Apollo 11 and 17 high‐titanium basalts. The next youngest unit, the Telemann Formation (red, higher albedo, high crater density, high DL values, and an average to strong 1‐μm reflectance) extends over wide areas of Procellarum and crops out as extensive surface units and as local patches surrounding earlier, topographically higher units. Numerous large sinuous rules in the Aristarchus Plateau were a major source for some of these deposits. Pyroclastic deposits forming the dark red mantle were also emplaced at the Aristarchus Plateau during these eruptions. The Hermann Formation (intermediate color, low albedo, intermediate crater density, and average to strong 1‐μm reflectance) occurs as extensive plains developed from less voluminous eruptions from sources at the Marius Hills complex, the Rümker Hills, and in Nubium, Cognitum, and near Delisle. The youngest unit, the Sharp Formation (blue color, very low albedo, low crater density, and a strong 1‐μm reflectance) is volumetrically small and is derived primarily from vents near the highland/mare boundary. Many Sharp Formation members have associated sinuous rules. Basalts of the Hermann Formation were sampled by Apollo 12; those of the Telemann Formation may be similar to the Luna 24 VLT basalts, while those of the Repsold Formation may be comparable with the Apollo 11, 17 and Luna 16 samples. The Sharp Formation, the youngest basalts, remains unsampled. The total volume of Procellarum basalts is estimated at 8.7×105 km3 (about 10% of the total mare volume). The relative volumes of the individual formations are not reflected by the surface area covered because the youngest flows tend to be thin and widespread. For example, the Sharp Formation has a surface area of 72,000 km2 but an estimated volume of only 1.8×104 km3, while the Repsold Formation has a present surface area of only 22,000 km2 but an estimated volume of 2.1×105 km3. The vast majority of lavas in Procellarum were extruded early in the phase of lava flooding, prior to about 3.5 b.y.

Paleomagnetism of the Morro Rock‐Islay Hill Complex as evidence for crustal block Rotations in central coastal California

Paleomagnetic directions obtained from seven hypabyssal intrusive bodies of the late Oligocene Morro Rock‐Islay Hill igneous complex of central coastal California display clockwise deflections from the expected mid‐Tertiary direction. Both alternating field and thermal demagnetization experiments show these magnetization directions to be stable and essentially free from secondary overprinting. The mean paleomagnetic direction for all sites is rotated clockwise 49°. Moreover, different bodies show different amounts of clockwise rotation, with declinations ranging from 27° to 76°. These results can best be explained by the clockwise rotation of several local crustal blocks. It is suggested that these rotations are the result of right lateral shearing acting on extensionally loosened crustal fragments within a pull‐apart basin. This combines two previously suggested ideas, the ‘ball bearing’ rotation hypothesis of Beck (1976) and the pull‐apart basin process discussed by Crowell (1974), both of which appear to be important tectonic processes in the extensively sheared continental margin in California.

The age of the Roberts Arm Group, north-central Newfoundland

Felsites from the Roberts Arm Group have yielded a Rb–Sr isochron age of 447 ± 7 Ma, with an initial 87Sr/86Sr of 0.7063, and a genetically related granite pluton has yielded an isochron age of 464 ± 13 Ma, with an initial ratio of 0.7064. The Mansfield Cove oceanic plagiogranite complex, which is in fault contact with the Roberts Arm Group, has yielded a minimum zircon 207Pb/206Pb age of 594 ± 10 Ma. This plagiogranite, and basaltic rocks of the Hall Hill complex which partly surround it, thus provide a basement of suitable age upon which the Roberts Arm Group may have been deposited. The age of this basement corresponds to that of the opening of the lapetus Ocean.The stratigraphic age of the Roberts Arm Group, indicated by these ages, is Middle or possibly Late Ordovician. The volcanics were deposited either before or slightly after the Caradocian black shales of the Notre Dame Bay region. In the former case, Ordovician volcanism around western Notre Dame Bay ceased at about the time of obduction of the allochthons of western Newfoundland. In the latter, volcanism of the Roberts Arm Group marks a north to westward shift of volcanism within the central mobile belt of Newfoundland that occurred in late Middle or early Late Ordovician time. If the Buchans Group is of Silurian age and correlative with the Roberts Arm Group then this lithostratigraphic unit is probably diachronous, younging to the south.

Gravity and magnetic investigation of the Red Hill Complex, Nelson, New Zealand

Abstract Gravity anomalies suggest the Red Hill Complex consists of a keel-shaped core of peridotite 7 km thick surrounded by a sheath of serpentinite 0 · 5km thick. Aeromagnetic anomalies over the Complex are consistent with this shape, but difficulty in obtaining a close fit between calculated and observed anomalies suggests that it is not uniformly magnetised and that the measurements were not made at a sufficient distance above it for the magnetisation to be approximated to a uniform direction. The measurements show that ultramafic rocks do not intrude associated sediments of the Maitai and Pelorus Groups, but it is uncertain whether they intrude associated volcanics and sediments of the Lee River Group. It is unlikely that the Complex was intruded as a dike, as previously supposed. The geophysical measurements are unable however to determine whether the Complex is the remnant of a shallow, basaltic magma chamber, or an enormous block in a tectonic melange.

Application of Sr and O isotope relations to the petrogenesis of the alkaline rocks of the Red Hill complex, New Hampshire, USA

The Red Hill ring complex in central New Hampshire is composed of apparently cogenetic syenites, nepheline-sodalite syenite, and granite. The ages and petrogenetic relations among five of the six recognized units have been investigated by rubidiumstrontium and oxygen isotope analysis of whole rocks and separated minerals. Whole-rock samples from three syenite units are consistent with a single Rb-Sr isochron which gives an age of 198±3 m.y. and an initial (87Sr/86Sr)o ratio of 0.70330±0.00016 (±2 sigma; λ=1.42× 10−11y−1). However, Sr isotope data for two other units, nepheline syenite and granite, are not consistent with this isochron but rather indicate higher initial ratios which range from 0.7033 to about 0.707. Whole-rock O isotope analyses give δ18O values which range from+6.2 to+9.3‰ Sr and O isotope analyses on mineral separates indicate that observed whole-rock variations in (87Sr/86Sr)o are primary and are not due to any secondary process. The fact that the isotope systematics correlate with rock type, suggests that crustal interaction is likely to have played a significant role in the development of this over-and undersaturated association. Such process(es), while still not fully delineated, could be of fundamental importance to the genesis of associations of critically undersaturated and oversaturated intrusives. The data support the idea that interaction between magmas and crustal materials strongly influenced the compositional relations of similar complexes elsewhere including those of the White Mountain magma series.

The origin and emplacement of the Serpentine Hill Complex, Western Tasmania

Abstract The Serpentine Hill Complex is an ophiolite dismembered by tectonic emplacement and subsequent faulting. It consists of ultramafics, gabbros, dolerites, and volcanics (mainly basaltic). The layered ultramafics (dominantly orthopyroxenite and harzburgite) and layered hypersthene gabbro are thought to be cumulates, and have been intruded by pegmatitic gabbro and microgabbros. Three types of serpentinite can be recognized in the western part of the complex, black serpentinites, green serpentinites, and sheared contact serpentinites. The black serpentinites are generally unsheared, consist essentially of lizardite, and have preferentially replaced olivine‐bearing ultramafics. The sheared contact serpentinites consist mainly of chrysotile and antigorite, and may have formed during tectonic emplacement. The formation of the green serpentinites (lizardite‐chrysotile mixtures) appears to have accompanied localized deformation which occurred after tectonic emplacement of the complex. It is postulated that...

Geology and Petrology of the Cambria Felsite, a New Oligocene Formation, West-Central California Coast Ranges

Geologic mapping in the Cambria and Black Mountain areas, western San Luis Obispo County, California, has led to recognition of the two stratigraphically and petrologically distinct extrusive felsic igneous units: (1) the Obispo Formation; and (2) a new unit, named here the Cambria Felsite. The Obispo Formation was deposited above the Rincon Shale and below the Monterey Formation during middle Miocene time. The Cambria Felsite rests with angular unconformity on Franciscan rocks of Jurassic and (or) Cretaceous age, and occurs as clasts in the nonmarine Lospe Formation of Oligocene age. Textural and mineralogic intergradations exist between the Cambria Felsite and the nearby upper Oligocene hypabyssal volcanic necks, plugs, lava domes, and dikes of the Morro Rock–Islay Hill complex; this suggests that the Cambria Felsite represents an effusive equivalent of the Morro Rock–Islay Hill. Cambria tuffs are enriched in alkalis plus silica and are depleted in ferromagnesian constituents relative to samples of the Morro Rock type. Chemical contrasts in the two units probably arose either because of a winnowing of airborne crystals from the glassy shards or because of magma fractionation in the conduits followed by pyroclastic eruption of the upper portions of the melt columns. No obvious correlation exists between the two-stage silicic volcanism, separated by a 10-m.y quiescent interval, and middle Tertiary plate tectonics. However, generation of these igneous rocks was probably a thermal response of the western margin of the continental crust-capped Americas plate to underflow of the Farallon plate, resulting in Morro Rock and Cambria units, followed by a complex encounter with the East Pacific Rise, resulting in the Obispo Formation.

MEA Gas Treating Sulphur Production System in the Wildcat Hills Complex

Abstract In the operation of the Wildcat Hills field and plant, modifications have been carried out to increase the maximum throughput, increase the efficiency of the sulphur recovery, decrease the chemical consumption, and decrease downtime and operational and maintenance costs, while maintaining or increasing the safety throughout the operation. Many bottlenecks in plant equipment have been successfully removed with small additional costs, and these, along with operational techniques and methods of control, have increased the capacity and the efficiency of parts of the gas treating system by approximately 30 per cent. Also by proving gas treating and the selection of catalysts, the capacity of the sulphur plant has been increased by approximately 15 per cent. The purpose of this article is to describe the general operation and discuss some of the ways in which many of the problem areas were studied and improved.

Marscoite and related rocks of the Western Red Hills complex, Isle of Skye

The rock from Skye which Harker called marscoite was recognized by him to be hybrid in origin because of the close association in the rock of basic plagioclase, quartz and orthoclase. The position of marscoite in the sequence of rocks forming the Western Red Hills Tertiary complex has now been defined, and evidence for its parentage and the mechanics of its formation obtained. The Western Red Hills intrusive centre, developed after the Cuillin and before the Broadford centres, as suggested by J. E. Richey, consists of five different, high-level granitic intrusions which were followed by a southern and northern series of late intrusions, including marscoites, ferrodiorites, and various additional granitic rocks. The high-level granitic rocks of Skye, which have so far been loosely termed granophyres, cannot all properly be described as such, and the term epigranite is proposed as a general name for them. The earliest rocks belonging to the southern late intrusions are porphyritic epigranites and felsites, having quartz and potash felspar phenocrysts resembling the xenocrysts of these minerals in the marscoite. Then came marscoite, somewhat chilled against the felsite but also back-veined by it. The marscoite in Harker’s Gully, and in other places on Marsco, passes gradually into ferrodiorite which sometimes contains basic andesine phenocrysts similar to the xenocrysts of the marscoite. The ferrodiorite has a composition suggesting that it was derived by extreme fractional crystallization of basic magma. The xenocrysts of the marscoite are highly characteristic and indicate that marscoite was formed by the mixing of a porphyritic acid magma, like that which produced the Southern Porphyritic Felsite, and a porphyritic basic magma, like that which produced the porphyritic ferrodiorite of Marsco. The chemical compositions of the presumed parent materials and marscoite support this view. Because of the even distribution of the xenocrysts in marscoite, the mixing must have been largely effected by the mechanical stirring together of two liquids, in both of which were suspended crystals. Diffusion within the liquid phase must also have contributed in some degree to the ultimate homogeneity of the liquid part of the mixture. The origin of the marscoite of the northern late intrusions is presumably similar to that of the southern, except that the basic parent is believed to be represented by the porphyritic hawaiite blocks in the northern marscoite. The northern marscoite cuts through, and is chilled against, the Glamaig, Eas Mor, and Maol na Gainmhich epigranites. Inwards from the contacts, the marscoite gives place, in a distance of 30 to 50 yd., to a rock here called glamaigite, which consists of rounded, dark patches, usually an inch or so across, and a less dark matrix, in approximately equal amounts. In both dark and light material there are xenocrysts of andesine, potash felspar and quartz, as in the marscoite. The difference in composition of the darker and lighter parts of the rock is not great, but is such as to suggest that the darker would have had a slightly higher temperature range of crystallization. The glamaigite is believed to have originated from a less well-homogenized mixture of basic and acid porphyritic magmas. From the mixture, the slightly more basic parts solidified first, and then flow movements of the magma resulted in rounding of the early semi-solid clots of hybrid material. The central parts of the composite, northern, late intrusions consist of a rock resembling glamaigite but tending to be more uniform and acid in composition. The greater homogeneity of this rock, distinguished as dioritic glamaigite, may be due to its central position within the intrusions, where slower cooling would allow more time for diffusion. It is suggested that the epigranites of the Hebridean igneous province originated by melting of crustal rocks of broadly granitic composition. The heat to produce the melting is believed to have been derived from basic magma intruded into the earth’s crust, the upward transfer of heat being aided by convection in the magma and bottom accumulation of early formed crystals. At some stage, a residual layer of ferrodiorite or hawaiite liquid, produced by fractionation, may have underlain a granitic liquid produced by melting. Two separate systems of convection currents are envisaged in the two liquids, because of the differing densities. At the junction of the two systems of currents, where they would be flowing in opposite directions, there would be an opportunity for mechanical mixing. Ultimately, a mass of hybrid magma may have developed, annular in form and with a forced circulation, which was the source of the marscoite and related rocks.

An Oblique Parallel Flaked Point from Missouri

A point with oblique parallel flaking was found in an eroded field on high ground, 2½ miles north of the Missouri River near Kansas City, Mo. Chert flakes scattered over the site indicate occupation over several acres but no other artifacts from the site are now available for study. The location and the eroded condition of this site are similar to those of the Nebo Hill Complex (J. M. Shippee, Nebo Hill, A Lithic Complex In Western Missouri, American Antiquity, Vol. 14, No. 1, pp. 29-32, 1948), but nothing else links them except the presence of the light colored chert flakes.

Note on a xenolith in riebeckite-trachyte, mid Eildon, Roxburghshire

The earliest notice of the occurrence of riebeckite in the rocks of the Eildon Hills was made by Mr T. Barron in 1896.1 He described its characters and mode of occurrence in rocks of felsitic type, which he regarded as lavas. In the 1908 edition of Harker’s Petrology for Students, p. 114, there is a figure of riebeckite-orthophyre from Mid Eildon. In 1914 Lady M‘Robert2 published a paper on ‶Acid and other Intrusions near Melrose.″ Although her paper deals principally with the petrography of the igneous rocks of the district, Lady M‘Robert regarded the Eildon Hills complex as being, probably, the denuded eastern remnant of a composite laccolite, and considered that the present western boundary of the hills represented the central part of the laccolite. The Mid Eildon Hill, with which the present paper is concerned, has been shown by Lady M‘Robert to consist of sanidine-trachyte at the base followed upwards by riebeckite-felsite, which in turn is overlain by augite-olivine-trachyte and riebeckite-trachyte, the latter forming the rock at the hill summit. She considered that these rocks were intruded as sheets during a late period in the history of the ‶Plateau Eruptions″ of Calciferous Sandstone times, and that they corresponded with the trachytic lavas and intrusions of the Garleton Hills and Campsie and Renfrewshire Hills. It is in the riebeckite-trachyte at the summit of the hill that the cognate xenolith was found. The xenolith formed a ball-shaped mass about two inches across. In parts it broke up readily on being hammered,