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71.
72.
African education controlled by the state's Department of Education and Training has collapsed. Insufficient spending, low standards and persistent student boycotts have made township schools undesirable places of learning. African students with any financial means now seek secondary education at multi-racial private schools in affluent white suburbs or at street-academies in the centre of Johannesburg. The establishment of schooling services for African pupils in the traditionally white central business district contributes to the wider process of desegregation of South Africa's major urban area, Johannesburg. 相似文献
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Urban systems are increasingly dominated by large cities that house a disproportionate share of telecommunications infrastructure. This paper investigates whether telecommunications is defining a technological divide between large and smaller cities. The paper provides a conceptual basis for large-city dominance and presents the results of surveys that investigate the use of telecommunications in medium and small Indiana cities. The survey data indicate that telecommunications is used by the set of Indiana cities, but is less prevalent among the set of smallest cities. Chi-square tests, however, suggest that city size does not adequately explain these variations in use. 相似文献
76.
Ten sites near the Snake River Plain have consistent differences in their climatic histories. Sites at low elevation reflect the “early Holocene xerothermic” of the Pacific Northwest, whereas most climatic chronologies at high elevation indicate maximum warmth or aridity somewhat later, ca. 6000 yr ago. This elevational contrast in climatic histories is duplicated at three sites from the central Snake River Plain. For sites in such close proximity, the different chronologies cannot be explained by changes in atmospheric circulation during the late Quaternary. Rather, the differences are best explained by the autecology of the plants involved and the changing seasonal climate. The seasonal climatic sequence predicted by multiple thermal maxima explains the high- and low-elevation chronologies. During the early Holocene, maximum insolation and intensified summer drought in July forced low-elevation vegetation upward. However, moisture was not a limiting factor at high elevation, where vegetation moved upward in response to increased length of growing season coincident with maximum September insolation 6000 yr ago. 相似文献
77.
A comparative study of blue and green beryl crystals (from the region of Governador Valadares, Minas Gerais, Brazil) using electron paramagnetic resonance (EPR) and optical absorption (OA) spectroscopy is reported. The EPR spectra show that Fe3+ in blue beryl occupies a substitutional Al3+ site and in green beryl is localized in the structural channels between two O6 planes. On the other hand the infrared spectra show that the alkali content in the blue beryl is mostly at substitutional and/or interstitial sites and in green beryl is mostly in the structural channels. The OA spectra show two types of Fe2+. Thermal treatments above 200° C in green beryl cause the reduction of Fe3+ into Fe2+ accompanied by a change of color to blue. The blue beryl color does not change on heating. The kinetics of the thermal conversion of Fe3+ into Fe2+ is composed of two first order processes; the first one has an activation energy ΔE 1=0.30 eV and the second one has an activation energy ΔE 2=0.46 eV. 相似文献
78.
Benthic fluxes of C, N, P and Si have been measured at two sites in Great Bay Estuary, New Hampshire. Higher fluxes of reactive phosphate, nitrate and reactive silicate were observed at the site where bioturbation is known to occur and the fluxes of NH4 +, PO4 ?3 and reactive silicate at this location were from 3 to 6 times higher than that calculated by simple pore water diffusion models. *** DIRECT SUPPORT *** A01BY019 00010 相似文献
79.
Susan E. Hogg 《Earth》1982,18(1):59-76
Following McGee's (1897) use of the term sheetflood, there developed a variety of conflicting opinions and numerous terminologies relating to this process, many of which are ill-considered and poorly defined. A classification system is devised on both a hydrologic and geomorphic basis which dispenses with many superfluous terms that have been used with respect to sheetfloods. Magnitude of a storm is expressed in terms of the amount of surface runoff generated from a given storm due to the surface conditions.A sheetflood is defined as a sheet of unconfined flood water moving down a slope. The frequency of a sheetflood is relatively low while its magnitude is relatively great. Sheetflow is defined as relatively high-frequency, low-magnitude overland flow occurring in a continuous sheet and is restricted to laminar flow conditions. Sheetwash, a term of geomorphic origin, is considered to be redundant and is superseded by the more meaningful term rainwash defined as the washing action of rain on slopes. 相似文献
80.
Steven Emerson Susan Kalhorn Lucinda Jacobs Bradley M Tebo Kenneth H Nealson Reinhardt A Rosson 《Geochimica et cosmochimica acta》1982,46(6):1073-1079
A simple mass balance for dissolved manganese(II) in waters containing low levels of oxygen in Saanich Inlet indicates that the residence time for Mn(II) removal to the solid phase is on the order of a few days. The average oxidation state of Mn in particulate material sampled from the region of Mn removal was 2.3–2.7, and electron micrographs revealed structures characteristic of bacterially formed Mn precipitates. Radiotracer experiments utilizing 54Mn(II) indicated that removal of Mn from solution in the region of active uptake was substantially blocked by a poison mixture, demonstrating that Mn(II) binding to particulates is catalyzed by bacteria in this environment. 相似文献