Heterogeneous Assemblage Research Articles

Proterozoic gneisses of the Malton Complex, near Valemount, British Columbia: U–Pb ages and Nd isotopic signatures

Proterozoic gneisses of the Malton Complex in the vicinity of Valemount, British Columbia, occur in a series of lithologically and structurally complex, fault-bounded slices of crystalline basement and interleaved cover. Gneisses of the Malton Complex span the Southern Rocky Mountain Trench and underlie the western part of the Rocky Mountain fold and thrust belt and the eastern part of the Omineca Belt of the Canadian Cordillera. Structural and stratigraphic relationships indicate that they formed the basement upon which an enigmatic quartzite unit and the Late Proterozoic Windermere Supergroup were deposited.The Yellowjacket and Bulldog gneisses, on the east side of the Rocky Mountain Trench, have yielded four U–Pb zircon crystallization ages of ca. 1870 Ma, with εNd(T) values of −2.6 to −3.4. The Hugh Allan gneiss, also on the east side of the trench but separated from the Yellowjacket gneiss by a major thrust fault, includes leucocratic granite gneiss having a zircon U–Pb age of [Formula: see text], which has intruded an older (undated) lithologically heterogeneous assemblage of gneiss. The basal Windermere succession of the Valemount region is inferred to be younger than ca. 740 Ma, since these intrusions are not found within the Late Proterozoic stratified rocks.Augen granitoid orthogneiss of the Malton Range on the west side of the Rocky Mountain Trench has been dated as [Formula: see text] using zircons. A second sample yields data suggesting an age between 2050 and 2100 Ma, but its interpretation is uncertain because of scatter in analyses and possible zircon inheritance. The latter sample has an εNd(T) at 1990 Ma of −2.6. Nd model ages for the Malton, Yellowjacket, and Bulldog samples range from 2.45 to 2.56 Ga, indicating that the igneous protoliths were derived from a source that probably had some component of Archean crustal material involved.The U–Pb ages and Nd model ages are quite similar to those of rocks underlying portions of Alberta and the western Canadian Shield, specifically the Fort Simpson terrane, the Great Bear magmatic zone, and parts of the Thelon–Taltson arc. This evidence, as well as structural and stratigraphic arguments, links the Malton Complex gneisses with those of the Canadian Shield, precluding their derivation by large-magnitude displacements from the southwestern United States. Structural analysis indicates that they restore to locations 100–200 km southwest of their present exposure.Structural, stratigraphic, and isotopic data indicate that the Southern Rocky Mountain Trench is not a suture.

Modelling of BOD exertion curve

The BOD exertion phenomenon, being a result of complex biochemical reactions between the heterogeneous assemblage of biota and the complex organic substrate, shows a lag phase, followed by a rising curve terminated at a plateau, and again a saturating type increasing curve yielding an ultimate carbonacious BOD. For the substrate rich in nitrogenous compounds, there is a further increase in the BOD curve, saturating at the second stage ultimate BOD value. The BOD kinetics models in vogue describe a particular phase or stage only, and lack in general applicability over the entire exertion process. Presented herein is a generalized BOD exertion equation valid over all the phases and stages of the phenomenon.

Lesothosaurus, “Fabrosaurids,” and the early evolution of Ornithischia

ABSTRACT New materials of Lesothosaurus diagnosticus permit a detailed understanding of one of the earliest and most primitive ornithischians. Skull proportions and suturai relations can be discerned from several articulated and disarticulated skulls. The snout is proportionately long with a vascularized, horn-covered tip. The premaxillary palate is broad and vomers are long and fused anteriorly. Unlike many later ornithischians, the postpalatine vacuities are broadly open. The basal tubera are short and gently depressed, and the epiotic contributes to the sidewall of the braincase. The mandibular symphysis is spout-shaped, and the dentition is marked by oblique wear facets, in contrast to earlier reports. The tooth-to-tooth wear facets and form of the predentary-dentary articulation suggest long-axis rotation of the mandibular rami during mastication. The forelimb is proportionately very short, with a partially opposable pollex. The ischium lacks an obturator process. The reduced hallux is held well above the substrate during locomotion. L. diagnosticus is diagnosed below on the basis of apomorphic features. Other “fabrosaurids” constitute a heterogeneous assemblage of poorly known ornithischians and hatchling prosauropods that do not share any apomorphic features with L. diagnosticus. Fabrosaurus australis is a nomen dubium, and, as a consequence, the family Fabrosauridae is invalid. Echinodon becklesii may represent a primitive heterodontosaur, and Tawasaurus, Fulengia, and portions of the holotype of Technosaurus represent hatchling prosauropods. Pisanosaurus mertii may be the most primitive ornithischian, as indicated by the form of the distal crus and astragalus, but a more precise phylogenetic assessment will require additional remains.

Characterization of a succinate-fermenting anaerobic bacterium isolated from a glycolate-degrading mixed culture

A motile curved rod-shaped bacterium was isolated in pure culture from a glycolate-fermenting mixed culture by using citrate as the growth substrate. The purified strain, designated 19gly1, was an obligate anaerobe growing optimally in freshwater medium at neutral pH and 37°C. The organism was gram-negative, lacked cytochromes, and had a DNA mol % G+C ratio of 36. Strain 19gly1 grew on only a limited range of substrates, including citrate and succinate. No growth occurred on glycolate, on carbohydrates or on H2 plus CO2. Metabolism was by fermentation only. The strain was different to previously described species of bacteria and assigned to the heterogeneous assemblage of “Campylobacter-like” strains. Strain 19gly1 has been deposited with the Deutsche Sammlung von Mikroorganismen as DSM 6222.

AQUATIC COLEOPTERA OF SPRINGS IN CANADA

AbstractThe habitat preferences of each of the 663 species of aquatic Coleoptera known from Canada and Alaska were categorized as lentic, lotic, spring-inhabiting, other, or unknown. Most species were assigned to a single habitat type although some occur in more than one habitat. The distribution of species among these habitat types is as follows: lentic, 61%; lotic, 23%; springs, 8%; other, <1%, unknown, 8%. The 63 spring-inhabiting species are distributed among the families Dytiscidae (38 species), Hydrophilidae (nine), Hydraenidae (eight), Chrysomelidae (subfamily Donaciinae) (six), Haliplidae (one), and Dryopidae (one). The diversity of these families in springs only approximately parallels their diversity in the total fauna. Several relatively diverse families (Gyrinidae, Scirtidae, and Curculionidae) are absent from springs as are some predominantly lotic families (Amphizoidae, Elmidae, and Psephenidae). About half the spring species are western (occurring in Manitoba and west), about a quarter are eastern, and a quarter are transcontinental. Most of these spring species are known from the conterminous United States and it is suggested that spring habitats within Canada are being colonized slowly from southern refugia.The habitat affinities of spring-inhabiting Dytiscidae are examined in detail. Of the 260 species occurring in Canada, 38 species occur in springs and represent about 60% of all beetle species in Canadian springs. The spring fauna of dytiscids comprises four elements: nine (24%) inhabit springs only, 11 (29%) are lotic species that also occur in springs, 12 (31%) are species that use a broad range of habitats, and six (16%) are species known otherwise only from lentic habitats. The dytiscid fauna of springs is a heterogeneous assemblage derived from many separate phylogenetic elements.

Polymorphism in pollen ofSalvia leucantha(Lamiaceae)

Abstract LM and SEM study of the pollen from single specimen of Salvia leucantha Cav., gathered from Kumaon mountains in Western Himalaya have shown significant variations in aperture number and type, shape and size of pollen grains, hitherto not reported. Normal pollen is 6-colpate but 4, 5, 7, 8, 9, 10, 11-colpate to spiraperturate also occur to make it a heterogenous assemblage. In shape, pollen varies from oblate, suboblate, oblate-spheroidal, prolatespheroidal to subprolate, whereas, in size it ranges from 15 μm to 40 μm. Furthermore, besides the marked variations in monads, the polymorphism is also expressed in terms of dyads and triads. Exine ornamentation however, does not vary, it being reticulate-retipilate sexine pattern in all the different types under LM and showing double sculpturing under SEM.

The rapid analysis of single marine cells by flow cytometry

Analytical flow cytometry (AFC) is a novel technique for the rapid (more than 103 s-1) analysis and sorting of single cells based upon simultaneous, multiple measurements of laser-induced particle fluorescence, light scatter and impedance. Originally developed for biomedical use, AFC is now being adapted to analyse single-celled organisms such as phytoplankton and bacteria which are present as trace but functionally important components in seawater. Marine AFC has been used to analytically differentiate and sort these organisms from the heterogeneous assemblage of particles present in seawater. Chlorophyll autofluorescence is an unique biomarker for photosynthetic organisms and has been used to analyse phytoplankton cytometrically both in the laboratory and at sea. A theoretical and practical framework for the cytometric quantitation of cellular chlorophyll in phytoplankton based on autofluorescence is presented. Other subcellular constituents such as enzymes, lipids, nucleic acids and toxins in phytoplankton have recently been analysed by AFC using immuno-, induced or applied fluorescent labelling techniques. Examples are presented together with novel developments in fringe areas of cytometry that are likely to influence AFC of single marine cells in the near future.

RbcL sequence divergence and phylogenetic relationships in Saxifragaceae sensu lato.

Phylogenetic relationships are often poorly understood at higher taxonomic levels (family and above) despite intensive morphological analysis. An excellent example is Saxifragaceae sensu lato, which represents one of the major phylogenetic problems in angiosperms at higher taxonomic levels. As originally defined, the family is a heterogeneous assemblage of herbaceous and woody taxa comprising 15 subfamilies. Although more recent classifications fundamentally modified this scheme, little agreement exists regarding the circumscription, taxonomic rank, or relationships of these subfamilies. The recurrent discrepancies in taxonomic treatments of the Saxifragaceae prompted an investigation of the power of chloroplast gene sequences to resolve phylogenetic relationships within this family and between the Saxifragaceae and other major plant lineages. Sequence data from the gene rbcL (ribulose-1,5-bisphosphate carboxylase, large subunit) reveal that (i) Saxifragaceae sensu lato is at least paraphyletic, and probably polyphyletic, (ii) the genera Parnassia and Brexia are only distantly related to other members of Saxifragaceae, and (iii) representatives of the Solanaceae (subclass Asteridae) appear more closely related to Saxifragaceae (subclass Rosidase) than traditionally maintained. These data illustrate the value of chloroplast gene sequence data in resolving genetic, and hence phylogenetic, relationships among members of the most taxonomically complex groups.

The Solund–Stavfjord Ophiolite Complex and associated rocks, west Norwegian Caledonides: geology, geochemistry and tectonic environment

AbstractMetabasalts of the Upper Ordovician Solund-Stavfjord Ophiolite Complex of the westernmost Norwegian Caledonides, show N-to E-MORB affinity, with high Th/Ta (or Nb) ratios giving evidence of subduction influence. The Solund–Stavfjord Ophiolite Complex is overlain by a heterogeneous assemblage of sedimentary and volcanic rocks, the Stavenes Group, of which the Heggøy Formation of metasandstones and phyllites conformably overlies the metabasalts of the Solund–Stavfjord Ophiolite Complex. The Heggøy Formation contains, in places, abundant metabasalt pillow lavas and minor intrusions, geochemically similar to those of the Solund–Stavfjord Ophiolite Complex, and basic metavolcaniclastites of island arc tholeiite (IAT) composition. This indicates that the Solund–Stavfjord Ophiolite Complex and Heggøy Formation developed in a marginal basin between a continental margin and an active subduction system, for which the present-day Andaman Sea may provide a realistic model. The other magmatic rocks of the Stavenes Group, showing both calc-alkaline and alkaline affinities, are less well time-constrained, but they are thought to represent an advanced stage of the island arc development, and ocean island build-up, respectively.

AN EXPANDED SURVEY OF THE ULTRASTRUCTURE OF RED ALGAL PIT PLUGS1

ABSTRACTThe fine structure of pit plugs in 90 species of red algae was examined, bringing the total number of species in the continuing survey to 153. The organization of plug caps was confirmed to be a stable, predictable trait within thalli, between generations in heteromorphic life histories, and within the presently recognized orders, with one exception—the Acrochaetiales. Two forms of the outer cap were found in this group, a thin plate, as in the Nemaliales and Palmariales, and a dome, as in Batrachospermales and Corallinales. Variation of pit plug structure indicates that the Acrochaetiales are a heterogeneous assemblage and that pit plugs will be useful in reappraising their systematics. The systematic affinities of several species of uncertain affinities are clarified. Schmitziella endophloea Bornet et Batters is excluded from both orders, Corallinales and Acrochaetiales, with which it previously was allied. Although other ordinal attributions are not precluded by pit plug structure alone, pit plug structure is consistent with placement of Apophlaea sinclairii Harvey and Hildenbrandia rivularis (Liebman) J. Agardh in the Hildenbrandiales, Plagiospora gracilis Kuckuck, Schmitziella endophloea, and Wurdemannia miniata (Duby) J. Feldmann et Hamel in the Gigartinales, and Pseudorhododiscus nipponicus Masuda in the Palmariales.

Dinoflagellate species and organic facies evidence of marine transgression and regression in the atlantic coastal plain

Palynological evidence is used to date and interpret depositional environments of sediments of Campanian, Maestrichtian and early Danian ages cored in three wells from South Carolina and Georgia. The evidence is usefil for distinguishing environments which lithofacies evidence indicates a range from nonmarine to coastal to inner neritic shallow shelf. Numerous dinoflagellate species and an organic facies defined abundant amoprphous debris (amorphous debris facies) distinguish shallow shelf sediments deposited during marine transgression. The nearshore amorphous debris facies of late Campanian age consists of heterogenous assemblages dominated by Palaeohystrichophora infusorioides Deflandre or Hystrichosphaerina varians (May). The farther offshore amorphous debris facies of late early Maestrichtian to late Maestrichtian age consists of heterogenous assemblages dominated by Glaphyrocysta retiintexta (Cookson) and/or Areoligera medusettiformis (Wetzel). The larger number of dinoflagellate species in the offshore facies represents the maximum transgression detected in the investigated interval. A multiple occurrence datum defined by the combination of first appearance, klast appearances and sole occurrence of dinoflagellate species at the base of each interval distinguished by the amorphous debris facies provides the first evidence of marine transgression. Relatively small organic residues consisting of intertinite and few or no palynomorphs define the inertinite facies in nonmarine deltaic and in coastal (lagoonal, tidal flat, interdistributary bary) sediments. Dinocyt☆s are absent in the nonmarine sediments and are represented by few species and few specimens in the coastal inertinite faceis. A third organic facies (vascular tissue facies) is defined by the abundance of land plant tissue. Sporomorph species, including those of the Normapolles pollen group and of pteridophyte spores, comprise a large proportion of the total palynomorph flora in the inertinite and vascular tissue facies. The vascular tissue facies occurs in the proximal prodelta and nearshore shallow shelf lithofacies of early Maestrichtian age. Baed in the sequence of organuic facies, dinoflagellate species abundance, and lithofacies in the investigated wells, the nonmarine and coastal inertinite facies was first deposited and was followed during the late Campanian by a marine trangression when the nearshore amorphous debris facies was deposited. This was followed in the early Maestrichtian by the influx of terrigenous organic matter (vascular tissue facies) in response to deltaic progradation on the shallow shelf. A marine regression occured towards the close of the early Maestrichtian, emplacing the coastal inertinite facies. The major marine transgression occured near the end of the early Maestrichtian, developing a farther offshore amorphous debris facies on an expanded continental shelf which persisted through the late Maestrichtian. The inertinite facies returned during marine regression in the approximate position of the Maestrichtian/Danian boudnary.

Structural controls on metamorphism and syn-tectonic magmatism: the Portuguese Hercynian collision belt

Upper Palaeozoic collisional movements along the Ibero-Armorican arc were accommodated on oblique strike-slip ductile shear zone systems. In north-central Portugal, these shear zones are coincident with long linear belts of high temperature, low pressure metamorphism and S-type granitic plutons. A model is presented in which crustal thickening subsequent to collision was followed by anatexis at depths of 12–15 km; this was achieved by a combination of (i) increased heat flow, (ii) lowering of melting temperatures caused by fluxing of hydrous fluids, and (iii) a heterogeneous assemblage of greywackes and pelites which encouraged local high ductility contrasts. Shear zones are proposed as the most important influence in controlling the site of development of such belts; anatexis of metasedimentary crust and emplacement of the resulting S-type plutons within these zones took place without an appeal to crustal rifting or an asthenospheric heat source.

A chemotaxonomic evaluation of the genus Beauveria

Intra- and interspecific variation of thirty-two isolates assignable to the genus Beauveria was evaluated using 64 morphological and biochemical characters. Two isolates of Tolypocladium cylindrosporum were included to test generic concepts. Seventeen cluster groups were obtained following a numerical taxonomic analysis, each group being separated by at least one character. Cultural characters were highly variable and could not be used reliably for species determination. Spore form was the most useful criterion to distinguish between species. Biochemical data generally supported species concepts based purely on morphology, with the exception of B. bassiana which comprised a heterogeneous assemblage of strains. There is evidence from API ZYM and esterase patterns that this variability is determined by host (substrate) and geographical origins. B. alba, although morphologically close to B. bassiana, could be separated readily on biochemical characters using principal component analysis. The following species of Beauveria are recognized: B. alba, B. amorpha, B. bassiana, B. brongniartii, B. velata, B. vermiconia and an undescribed taxon close to B. amorpha but morphologically and biochemically distinct. Both isolates of T. cylindrosporum cluster in the same group and, on the basis of present evidence, particularly conidiogenesis, the synonymy of this genus with Beauveria is questioned.

Meristic Composition of Sand Lance (Ammodytes spp.) in Newfoundland Waters with a Review of Species Designations in the Northwest Atlantic

Examination of new meristic data on sand lance (Ammodytes spp.) populations in Newfoundland coastal waters using a combination of discriminant function and modal analyses demonstrates for the first time that co-occurrence is a common feature of the distribution of the two species (A. dubius and A. americanus) provisionally considered to exist in the Northwest Atlantic. In addition, the consistency in the meristic counts of A. dubius between the offshore and inshore samples and the postulated spawning of A. dubius in Newfoundland coastal waters provides good evidence that A. dubius is reproductively isolated from A. americanus. Reanalysis of published data on the vertebral composition of sand lance in the Northwest Atlantic confirms the co-occurrence of A. americanus and A. dubius in inshore areas from West Greenland southwards to the Gulf of Maine. These new analyses taken together with a review of the major distinguishing characteristics of sand lance species in the North Atlantic waters lead us to conclude that the heterogeneous assemblage of sand lance found inshore from West Greenland southwards and offshore from Georges Bank southwards and which has traditionally been classified as A. americanus belongs to a single trans-Atlantic species, A. marinus.

Heterogeneity of a human T-lymphoblastoid cell line

A human T-lymphoblastoid cell line (Jurkat) was cloned, and four resulting sublines were characterized in a variety of ways with the objective of gaining information on heterogeneity in cell lines. Within a few weeks of cloning, distinct cellular morphologies and growth patterns became apparent in the four sublines. Growth rate measurements made over 3 months did not show any significant differences between the sublines. Surface protein profiles obtained by radioimmunoprecipitation using antisera in conjunction with extracts from [ 35S]Met and 125I-labeled cells revealed differences between the sublines. Analysis of total cell DNA showed that one of the sublines possessed only half the chromosome complement of the other sublines and the parental line. Karyotyping confirmed this result and, in addition, demonstrated that chromosome numbers fluctuated around a mean value for each subline. Karyotypic variability became apparent within 2 months of cloning and tended to increase with time in culture. G-banding analysis showed that the analyzed cell populations contained distinctive cytogenetic aberrations. Properties of the cloned sublines were monitored over a 9-month period. One of the sublines that had shown heterogeneous morphology even after 6 weeks maintained the heterogeneity throughout this time. Another subline underwent a marked change in morphology (round to irregular) and growth habit (single cells to large clumps) with increasing time in culture. Interestingly, several alterations to surface proteins accompanied these growth changes. A third subline had relatively stable morphology and chromosome number throughout the 9-month period. The modal chromosome number was hypotetraploid for three sublines and the parent line, but was diploid for another subline. However, it was interesting that progression toward tetraploidy in this subline was apparent after almost 2 years of culturing. The results showed that the original cell line consisted of a heterogeneous assemblage of cell types, some of which were quite unstable. Some implications for research using cultured cell lines are discussed.

Proposal for an Amended Classification of Anaerobic Mollicutes

The taxonomic status of the obligately anaerobic mycoplasmas has remained controversial since their discovery in 1973. For this reason, Anaeroplasma, the single recognized genus, has not been assigned to a higher taxon in the class Mollicutes. Nutritional, biochemical, serological, and genomic data indicate that the anaerobic mollicutes compose a heterogeneous assemblage of organisms, all of which are distinct from facultatively anaerobic mollicutes. A revised and extended classification of these organisms is proposed.

Foliar flavonoid exudates and sectional taxonomy of Hemizonia

The foliar exudates of 18 taxa that represent the four recognized sections of Hemizonia were shown to consist of at least 15 flavonoids. The compounds included six flavonols, six flavones, and three flavonones. No consistent differences among sections were found with regard to these compounds. The chemistry was distinctive for all taxa, except between H. australis and H. pungens subsp, pungens. All species shared four compounds (patuletin, quercetin, 3-methylquercetin, luteolin), and all but two species contained 3-methylpatuletin. The resultant two-dimensional ordination and linkage dendrograms indicated that three principal assemblages could be discerned. The flavonoid data are not completely in accord with morphological, cytological, and eco-distributional analyses of species relationships. The data suggest, however, that flavonoids generally are useful in the differentiation among sections Hemizonia, Centromadia, and Zonamra. The data further indicate that section Madiomeris consists of a heterogeneous assemblage of species which is concordant with other non-chemical data. Flavonoid complements show that the perennials of section Zonamra are closely allied with section Madiomeris and supports the contention that section Zonamra is highly artificial and may require realignment.

Felsic plutonic rocks of the Midyan region, Kingdom of Saudi Arabia—I. Distribution, classification and resource potential

About 80 large felsic plutons were emplaced in the Midyan region of the northwestern Arabian Shield between 725 and 570 Ma ago. Polymetallic mineralization, including Mo, Nb, REE, Sn, Ta, U, W and Zr, is associated with particular varieties. Five intrusive suites of regional extent and five other plutons have been identified. The oldest extensive unit is the Muwaylih suite, consisting of trondhjemite, tonalite, diorite and gabbro, with generally trondhjemitic compositional affinities and no known mineral potential. The most voluminous is the Ifal suite, a heterogeneous assemblage of biotite—hornblende monzogranite, granodiorite and quartz monzodiorite in large, oval to irregular plutons characterized by numerous intraplutonic dikes, but with no known mineral potential. The Ifal suite is intruded by plutons of the Atiyah monzogranite, with a variable number of dikes. The Muwaylih, Ifal and Atiyah suites are chemically metaluminous, but the Midyan suite consists of alkali granite to alkali-feldspar granite which is metaluminous to peralkaline. Large, irregular plutons of the Haql suite are composed of perthite leucogranite which is metaluminous to marginally peraluminous or peralkaline. Plutonic rock units of local significance include the Lawz complex, consisting of commonly granophyric syenogranite to monzogranite, the Mowasse quartz syenite and the Sawda (nepheline syenite) complex. Polymetallic Nb Zr deposits, such as those at Jabal Tawlah and Ghurayyah, are the most important known mineral resources and are related to alkali granite of the Midyan suite. Plumasitic specialized plutons, such as the Ratamah granite, have weak Sn W Ta Nb mineralization, and are related to the Haql suite. Apical and contact zones of plutons of these suites are favorable exploration target areas.

Degradation of Quaternary cinder cones in the Cima volcanic field, Mojave Desert, California

Basaltic cinder cones in the Cima volcanic field record a detailed history of progressive erosion in the arid environment of the eastern Mojave Desert. These cones range in age from ∼0.015 m.y. to 1.09±0.08 m.y., as dated by radiocarbon or K-Ar analyses of the youngest lava flows from each cone. Cone heights range from 50 to 155 m, and basal widths range from 400 to 915 m; on younger cones, height/width ratios average 0.17, and crater-width/cone-width ratios average 0.42. The degradational morphology of these cones displays several trends that are closely related to cone age. (1) Crater-width/cone-width ratios decrease from 0.48 on the youngest cone to 0.21 on the oldest cone with a preserved crater. (2) Mean maximum side slopes (Tan Sc) decrease from 0.575 on the youngest cone to an average of 0.41 on the oldest cones studied. (3) Debris-apron–height/cone-height ratios increase from <0.10 on the youngest cone to an average of 0.34 on the oldest cones. (4) Cone drainage evolves from irregularly spaced rills and gullies on the youngest cone to regularly spaced gullies on 0.20- to 0.35-m.y.-old cones to valleys as much as 110 m wide and 10 m deep on cones 0.59 m.y. old and older. These trends form the basis of an empirical model of cinder-cone degradation in arid environments. This model documents (1) an erosional loss of ∼15% of cone volume during the first million years; (2) progressive decline of cone slope (at an average rate of 0.006°/103 yr) and cone height (at an average rate of 2.25 cm/103 yr); (3) initial rapid stripping of the loose cinder mantle from upper-cone slopes accompanied by rapid debris-apron formation; and (4) a gradual transition, between 0.25 and 0.6 m.y., from relatively uniform stripping of upper slopes to localized fluvial dissection of both cone slopes and debris apron. This transition is apparently controlled by a concomitant change from diffuse subsurface drainage within the pervious cinder mantle to concentrated surface flow across the heterogeneous assemblage of agglutinate layers, dikes, and ponded flows of the cone interior.

Prokaryotic and eukaryotic microfossils from a Proterozoic/Phanerozoic transition in China

Cherts of the Doushantuo Formation in the Yangtze Gorges, South China, contain a superbly preserved heterogeneous assemblage of bacteria, cyanobacteria, planktonic algae, submillimetre-sized burrows and problematica. The microbiota has a bearing on defining the Precambrian–Cambrian boundary. Baltisphaeridium and associated spinose microfossils in the chert suggest an early Cambrian age whereas other acritarchs, vendotaenids and pretrilobite small shelly fossils from the conformably overlying the Dengying Formation, indicate a late Proterozoic age for the Doushantuo. This mixed benthic–planktonic assemblage preserved in silicified, laminated, microbial mat-like fabrics, provides a unique sample of diverse microbiotic elements existing at the dawn of the Phanerozoic and records the earliest evidence of meiofauna.