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RICHARD J. ALDRIDGE DEREK E. G. BRIGGS IVAN J. SANSOM M. PAUL SMITH 《Geology Today》1994,10(4):141-145
The teeth, or elements, ofconodonts are among the most important Palaeozoic microfossils. Despite this, the biological relationships of conodonts have long remained enigmatic. Recent evidence of their soft anatomy, and of the ultrastructure of the elements, has finally revealed that they are vertebrates. These discoveries push vertebrate origins firmly back into the Cambrian and add a large cohort of specialists to the ranks of the vertebrate palaeontologists. 相似文献
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Natural Partial Melting of Spinel Lherzolite 总被引：3，自引：1，他引：3
Spinel lherzolite nodules from Dreiser Weiher, Germany, containsmall glass bearing microcrystalline blebs that were formedfrom a partial melt. The partial melting occurred during contactanatexis of lherzolite surrounding a deep seated magma chamberin the mantle. The blebs formed from melt drops that were isolatedfrom each other and not from an intergranular film. The resultssuggest that primary magma originates as small melt drops ina lherzolitic matrix. This evidence indicates a limited mobilityof traces of partial melt in the mantle, and suggests that theaccumulation of primary magmas formed at very small degreesof partial melting is unlikely. 相似文献
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Abstract One of the most talked about issues with respect to development, transportation, and conversion of the energy resources of the Western United States is the water requirement and its consequent impacts upon the ambience of that part of the country. In conjunction with other studies conducted by the US Geological Survey of water use in the conversion and transportation of the West's coal, an analysis of water use and environmental implications of coal slurry pipeline transport is presented. Simulations of a hypothetical slurry pipeline of 1000 mile length transporting 12.5 million tons per year indicate that pipeline costs and energy requirements are quite sensitive to the coal-to-water ratio. For realistic water prices, the optimal ratio will not vary far from the 50/50 ratio by weight. In comparison to other methods of energy conversion and transport, coal slurry pipelines utilize about a third of the amount of water required for coal gasification, and about a fifth of the amount required for onsite electrical generation. An analysis of net energy output from operating alternative energy transportation systems for the assumed conditions indicates that both slurry pipeline and rail shipment require approximately 4.5 per cent of the potential electrical energy output of the coal transported and high-voltage, direct-current transportation requires approximately 6.5 per cent. The environmental impacts of the different transport options are so substantially different that a common basis for comparison does not exist. 相似文献
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Vertical variability and effect of stability on turbulence characteristics down to the floor of a pine forest 总被引：3，自引：0，他引：3
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ABSTRACT. This study examines the residential patterns of immigrant newcomers within a relatively new immigrant destination, Washington, D.C. Particular attention is given to how these patterns are shaped by the newcomers' race and ethnicity. Our analysis is based primarily upon data on immigrant flows from the U.S. Immigration and Naturalization Service for the 1990–1998 period. Evidence from our study suggests a racial and ethnic hierarchy to the residential location of Africans, Asians, Europeans, and Latin Americans, providing some support for the pattern expected in the place‐stratification model. When their residential patterns are compared with those of non‐Hispanic blacks, we find that African newcomers are more likely to settle among blacks than are other regional‐origin groups. However, Africans are also found to reside in multiethnic zip codes where large numbers of newcomers are concentrated. The findings suggest that race is an important but not exclusive factor in explaining the residential choices of recent immigrants. 相似文献
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