全文获取类型
收费全文 | 6212篇 |
免费 | 32篇 |
国内免费 | 23篇 |
专业分类
测绘学 | 24篇 |
大气科学 | 1126篇 |
地球物理 | 1402篇 |
地质学 | 3387篇 |
海洋学 | 53篇 |
天文学 | 209篇 |
综合类 | 7篇 |
自然地理 | 59篇 |
出版年
2013年 | 96篇 |
2012年 | 53篇 |
2011年 | 47篇 |
2009年 | 51篇 |
2008年 | 51篇 |
2006年 | 54篇 |
2005年 | 53篇 |
2004年 | 44篇 |
1998年 | 41篇 |
1996年 | 85篇 |
1995年 | 54篇 |
1994年 | 52篇 |
1993年 | 68篇 |
1992年 | 154篇 |
1991年 | 165篇 |
1990年 | 148篇 |
1989年 | 138篇 |
1988年 | 147篇 |
1987年 | 179篇 |
1986年 | 135篇 |
1985年 | 122篇 |
1984年 | 161篇 |
1983年 | 178篇 |
1982年 | 187篇 |
1981年 | 199篇 |
1980年 | 160篇 |
1979年 | 191篇 |
1978年 | 168篇 |
1977年 | 157篇 |
1976年 | 124篇 |
1975年 | 145篇 |
1974年 | 131篇 |
1973年 | 148篇 |
1972年 | 135篇 |
1971年 | 141篇 |
1970年 | 146篇 |
1969年 | 101篇 |
1968年 | 132篇 |
1967年 | 125篇 |
1966年 | 85篇 |
1965年 | 96篇 |
1964年 | 101篇 |
1963年 | 44篇 |
1962年 | 92篇 |
1960年 | 107篇 |
1957年 | 42篇 |
1954年 | 68篇 |
1952年 | 51篇 |
1950年 | 61篇 |
1949年 | 44篇 |
排序方式: 共有6267条查询结果,搜索用时 15 毫秒
181.
Much wildlife habitat is being destroyed by extractive resource industries in mountain environments. This article illustrates how mountain wildlife habitat was restored in a devastated area. A strip mine for coal on the east slopes of the Alberta Rockies, occupied during its operations by Rocky Mountain bighorn sheep (Ovis canadensis canadensis, Shaw 1803), was reclaimed as bighorn habitat. By considering the behaviour and habitat requirements of bighorns in conjunction with an operating coal mine and reclamation process, a large coal mining company has been able to reconstruct the mine site restoration to benefit mountain sheep. Although bighorn habitat requirements are well known, it was not until the animal and its requirements were studied against the backdrop of an operating coal mine that obvious recommendations regarding the maintenance and placement of high steep excavated rock faces were made and accepted by the government regulatory agencies. An open pit mine is a devastated landscape; trees, soil and overburden are removed to retrieve coal that may be up to 215 m underground. Overburden and soil are returned, but the disturbance compares to the barren landscapes left behind by glaciers. Contouring of the land, grading of overburden and soil, seeding with grass and legume mixtures and extensive fertilization are designed to speed plant colonization and soil development. By examining the bighorn's biological needs with respect to specific mining operations, a unique opportunity arose to employ McHarg's design with nature concept. This approach incorporates environmental and societal values into every aspect of development, and promotes the evaluation of the constraints and opportunities arising. The sheep numbered about 200. Their seasonal movements were similar to those found on native ranges. They used the reclaimed areas as winter range and for the mineral licks exposed during mining; in summer, the sheep moved to nearby alpine areas. Two thirds of all sightings were confined to 1.3 km2 of reclaimed grassland; its average productivity (4190 kg/ha) exceeded native ranges (1700 kg/ha). The body mass of female sheep in autumn equalled that of the largest in Alberta, but the skull dimensions were not larger than those of adjacent ranges. Infestation with lungworms was moderate. Lamb production and survival were high. Design criteria should be: feeding areas should be dry and lie within 300 m of escape terrain, which should have a slope of 40% and contain at least three benches. Rock piles should be placed on grazing areas. Mineral licks, a vital welfare factor, already existed within the high walls created by strip mining. 相似文献
182.
Prof. Dr. Thomas K. Rudel 《GeoJournal》1989,19(4):437-446
This paper argues that the process of establishing claims and control over resources, referred to as resource partitioning, structures frontier societies in ways which impede subsequent efforts at rural development. Conflicts over claims to resources create divisions among local elites which prevent the formation of coalitions to promote development. Struggles for control over the flow of resources out of a region cause regional development agencies to pursue transportation policies which retard development by isolating the region from all but one commercial center. A case study of resource partitioning and rural development efforts in the Ecuadorian Amazon illustrates the argument. 相似文献
183.
Dr. T. C. Whitmore 《GeoJournal》1989,19(4):347-360
The main fields of research in tropical rain forest ecology today are briefly described and the development of the subject over the past forty years is outlined. 相似文献
184.
185.
186.
Dr. J. P. Lorand 《Mineralogy and Petrology》1985,34(3-4):183-209
Summary A volumetrically minor Cu–Fe–Ni–S component derived from the uppermost mantle is found within the Beni Bousera (northern Morocco) and Ronda (southern Spain) Alpine-type peridotites; it occurs today as inclusions within primary silicates or as assemblages disseminated in the intergranular sites of the host rocks. Detailed microtextural and microprobe data indicate that inclusions and intergranular assemblages behaved as two contrasting systems during the low temperature, incipient serpentinization of the host rocks. The former were equilibrated in closed systems whereas the latter behaved as open systems with respect to hydrothermal fluids; as a result, intergranular assemblages were controlled by redox conditions generated by serpentinization. An early stage of alteration is characterized by a slight decrease of the sulfur content and would be due to the first influx of water inside peridotites; a further transformation was produced by anomalous highly reducing conditions responsible for the crystallization of native iron-bearing alloy assemblages as well as for a preferential partioning of Fe from silicate into pentlandite. The production of anomalous, reducing conditions during incipient serpentinization is ascribed to a low permeability of the host rocks with respect to the diffusion of hydrogen out of the sites of serpentinization. Because of its low temperature behaviour, the intergranular sulfide component would not be of use reconstructing the initial composition of the upper mantle sulfide component; thus, it is concluded that only the sulfide inclusions would provide such informations.
With 9 Figures 相似文献
Le comportement du composant sulfuré du manteau supérieur pendant les premiers stades de serpentinisation des péridotites de type alpin; une discussion a partir de l'exemple des massifs ultramafiques de Beni Bousera (Maroc) et de Ronda (Espagne)
Résumé Des traces d'un composant sulfuré du système Cu–Fe–Ni–S d'origine mantellique sont présentes dans les péridotites alpines de Beni Bousera (Maroc) et Ronda (Espagne); il forme actuellement des inclusions dans les silicates primaires ou des plages intergranulaires disséminées dans les interstices des roches. Une analyse minéralogique et chimique détaillée démontre qu'inclusions et plages intergranulaires sulfurées ont eu un comportement opposé dès les premiers stades de serpentinisation des roches encaissantes. Les premières ont été reéquilibrées en système chimique ouvert vis-à-vis des fluides de serpentinisation; en conséquence les paragenèses intergranulaires résultent d'une suite de transformations du composant sulfuré mantellique, controlée par le degré d'oxydo réduction du fluide de serpentinisation. Dans un premier stade, l'entrée de l'eau dans les péridotites a provoqué une légère baisse de la fugacité du soufre et la cristallisation d'assemblages sulfurés riches en mackinawite. L'augmentation du degré de serpentinisation a ensuite engendré des conditions inhabituellement réductrices responsables d'un important fractionnement du fer libéré par la serpentinisation de l'olivine dans la pentlandite ainsi que de la cristallisation d'une paragenèse complexe de phases métalliques incluant localement le fer natif; ces conditions anormalement reductrices sont imputées à une faible perméabilité des roches encaissantes vi-à-vis de la diffusion de l'hydrogène en dehors dessites de serpentinisation. En raison de leur comportement à basse température, les plages sulfurées intergranulaires sont inutilisables pour reconstituer le composant sulfuré du manteau supérieur; sur ce plan, on conclut que seules les inclusions blindées dans les silicates peuvent apporter des informations.
With 9 Figures 相似文献
187.
188.
D. Bronger Dr. 《GeoJournal》1984,8(2):137-146
Not merely the rapid growth of the metropolitan cities in the Third World but the overconcentration of the political and administrative functions together with the economic, social and cultural activities within the metropolitan regions — in short: the primacy —should be considered as the essential component of the term metropolization.An accurate computation of the metropolitan population growth in China is difficult because of the frequent change of city area delimitations after 1953 in connection with the missing distinction between shiqu-, jiaoqu- and hsien-population in the statistics. Therefore a solid assessment of the Chinese policy to restrict the growth of the metropolitan cities seems to be impossible.Recent data reveal that there is still a strong primacy for the metropolitan cities together with a heavy concentration of this primacy in the historical core regions. The evaluation indicates that the efforts towards balanced regional development and reduction of the regional disparities could not be considered as successful so far. 相似文献
189.
190.