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导言从司维治(Cvijic)1893年发表《喀斯特现象》一书以来,关于溶蚀斗淋的认识很少有所发展。对于斗淋为什么出现于这些灰岩区而不出现于那些灰岩区还没有令人信服的解释;对斗淋盆状形态形成的过程也还没有定量的说明。 相似文献
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We report the results of SIMS isotopic analyses of carbon, nitrogen, oxygen, and silicon made on 849 small (approximately 1 micrometer) individual silicon carbide grains from the Murchison meteorite. The isotopic compositions of the major elements carbon and silicon of most grains (mainstream) are similar to those observed in larger grain studies suggesting an AGB star origin of these grains. In contrast, the trace element nitrogen shows a clear dependency on grain size. 14N/15N ratios increase with decreasing grain size, suggesting different stellar sources for grains of different size. Typically observed 14N/15N ratios in the small grains of this study are approximately 2700, clearly larger than the values expected from model calculations of AGB stars. In addition to the three dredge-up episodes characteristic for the evolution of AGB stars, extra-mixing of CNO-processed matter in low mass AGB stars appears to be a promising possibility in order to explain the high 14N/15N ratios of the small circumstellar SiC grains. A small fraction of grains shows a silicon isotopic signature not observed in larger circumstellar SiC grains from Murchison. Their stellar origin is still uncertain. The minor type A, B, Y, and X grains were found to be present at a level of a percent, which is similar to their abundance in the larger-grain SiC separates from Murchison. Oxygen isotopic compositions are normal within the experimental uncertainties of several 10%, indicating that oxygen of stellar origin is rare or even absent in the SiC grains. We conclude that most of the oxygen is a contaminant which was introduced into the SiC grains after their formation, e.g., during sample processing in the laboratory. We identified a nitride grain, most likely Si3N4 with little carbon, with highly anomalous isotopic compositions (12C/13C = 157 +/- 33, 14N/15N = 18 +/- 1, delta 29 Si = -43 +/- 56%, delta 30 Si = -271 +/- 50%). The isotopic patterns of carbon, nitrogen, and silicon resemble those of the rare SiC X grains suggesting that these two rare constituents of circumstellar matter formed in the same type of stellar source, namely, Type II supernovae. 相似文献
956.
Nag P 《Population geography : a journal of the Association of Population Geographers of India》1983,5(1-2):60-69
Zambia's urban population increased from 29% to 43% between 1969-80. Female migration, although modest, played an important role in improving the sex ratio in urban areas. In 1980, there were 962 males/1000 females in Zambia as a whole and 1061/1000 in the urban areas. This compares with 960 males/1000 females in Zambia as a whole in 1969 and 1113/1000 in the urban areas. This overall decrease in the number of males/1000 females in urban areas has had implications for Zambian household structure and for the matrilineal social structure. The single male household traditional to urban areas can be expected to be increasingly replaced by complete households of larger size. Family size has tended to be smaller in provinces that have been affected by outmigration. The historically different propensities of males and females to participate in internal migration has meant that the actual number of family units has been smaller than the potential number. However, this pattern can be expected to normalize as female migration continues to increase. In rural areas, this trend should lead to a decline in the number of female household heads and implications for matrilineal land ownership patterns and agricultural output. 相似文献
957.
Sagar P 《Population geography : a journal of the Association of Population Geographers of India》1991,13(1-2):63-84
The author investigates literacy differentials between tribal and non-tribal populations in India, using 1981 census data. "Spatially, the index is high in tradition gripped and economically backward areas of Western Rajasthan, of central highlands and of Western Arunachal Pradesh. It is low in [the] Christian tribal belt of [the] northeast; the early exposed areas of [the] southern peninsula, parts of Gujarat, Maharashtra and Karnataka, and numerically insignificant areas of northern Rajasthan and Western Himalayas." 相似文献
958.
模糊类别制图的空间统计学方法 总被引:4,自引:1,他引:3
类别地图是地理信息系统(GIS)应用中所利用的重要数据类别。这类数据可以从摄影测量和遥感技术得到。用摄影测量方法(影像判读)制作的类别地图常以点、线和多边形的离散目标形式描述,而遥感图像分类方法输出的类别地图以连通光栅块形式表达。不论哪一种情况,在每一个多边形或者光栅块(即制图单元)中仅允许单一类别,边界内部非均匀性和模糊形已经被“过滤”了。这样的类别地图沿用了古曲脆集合论,因为每个制图单元只允许 相似文献
959.
ONTHEPREDICTIONOFTRANSIENTBEDLEVELSDUETOAGGRADATIONANDDEGRADATIONINCLUDINGARMOURINGA.B.PALANIAPPAN1,R.J.GARDE2andP.N.GODBOLE3... 相似文献
960.
1INTRODUCTIONTheaceleratedreleaseofsedimentfromsoilandrocksurfacesanditsmovementto,through,andfromstreamsisthemostpervasivean... 相似文献