全文获取类型
收费全文 | 2310篇 |
免费 | 481篇 |
国内免费 | 1017篇 |
专业分类
测绘学 | 2篇 |
大气科学 | 23篇 |
地球物理 | 285篇 |
地质学 | 3088篇 |
海洋学 | 152篇 |
天文学 | 27篇 |
综合类 | 63篇 |
自然地理 | 168篇 |
出版年
2024年 | 12篇 |
2023年 | 52篇 |
2022年 | 78篇 |
2021年 | 102篇 |
2020年 | 106篇 |
2019年 | 130篇 |
2018年 | 126篇 |
2017年 | 140篇 |
2016年 | 163篇 |
2015年 | 133篇 |
2014年 | 167篇 |
2013年 | 189篇 |
2012年 | 170篇 |
2011年 | 139篇 |
2010年 | 139篇 |
2009年 | 192篇 |
2008年 | 164篇 |
2007年 | 186篇 |
2006年 | 182篇 |
2005年 | 146篇 |
2004年 | 120篇 |
2003年 | 114篇 |
2002年 | 98篇 |
2001年 | 87篇 |
2000年 | 88篇 |
1999年 | 85篇 |
1998年 | 88篇 |
1997年 | 71篇 |
1996年 | 59篇 |
1995年 | 57篇 |
1994年 | 73篇 |
1993年 | 35篇 |
1992年 | 38篇 |
1991年 | 17篇 |
1990年 | 19篇 |
1989年 | 14篇 |
1988年 | 10篇 |
1987年 | 8篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1977年 | 1篇 |
排序方式: 共有3808条查询结果,搜索用时 203 毫秒
891.
892.
Abstract A nearly complete skeleton of a fossil sperm whale from the Middle Miocene age Bessho Formation at Shiga-mura, Nagano Prefecture, the most complete fossil physeterid skeleton found in Japan, is here named Scaldicetus shigensis, new species. Its abruptly tapered rostrum, deep supracranial basin, and high occipital crest are typical of physeterids. Large teeth with crenulate enamel on conical crowns, present in both the palate and mandible, are consistent with the genus Scaldicetus du Bus, 1867, a genus originally based on fossils discovered in the Antwerp Basin, Belgium. Scaldicetus shigensis is relatively primitive, having a relatively long, slender rostrum, large zygomatic arches, large tympanic bullae, prominent occipital condyles, low tooth count and a sloping occipital shield that is deeply emarginated laterally by large temporal fossae. The probable mandibular tooth count is 12, only one more than the primitive eutherian mammalian dentition. Despite its abundant primitive characters, Scaldicetus shigensis has very asymmetrical external nares, comparable to the living sperm whale, Physeter catodon, and is in this way more derived than most of the contemporaneous fossil physeterids. At a time when other sperm whales were more highly evolved, Scaldicetus shigensis retained a primitive occipital shield, large tympanic bullae, and low tooth count, as in primitive Oligocene odontocetes such as Agorophiidae. It was a relict form in Middle Miocene time, and provides an indication of a primitive stage of sperm whale evolution heretofore undocumented. Fossils from elsewhere previously referred to as Scaldicetus, largely on the basis of tooth characters, range in age from Early Miocene to Plio-Pleistocene time. Cranial material that would clarify relationships is lacking for most of these, and this time range seems too long for a cetacean genus. Because of this simplistic approach to identifications, Scaldicetus is probably a grade taxon, not a natural biological grouping. However, the tooth morphology of Scaldicetus shigensis is correct for the genus Scaldicetus, so we provisionally assign it to this genus. Redefinition of the genus Scaldicetus and revision of the Physeteridae are beyond the scope of the present study. 相似文献
893.
新疆罗布泊F4浅坑孢粉组合及意义 总被引:9,自引:1,他引:9
本文对新疆罗布泊F4浅坑沉积物进行孢粉分析,揭示了该区晚更新世晚期-全新世的植物和气候,并结合其它沉积特征研究了该期自然环境的演变。 相似文献
894.
895.
新疆西准噶尔沙尔布尔提山晚志留世、早泥盆世孢子和疑源类包括孢子21属31种和变种,其中新种1个(Amicosporites xinjiangensis);疑源类11属14种。此外还有几丁虫3属3种。根据孢子在地层中分布规律和演化特征,自下而上可划出4个孢子带:1.Synorisporites dittonensis—Api-culiretusispora spicula(DS)带;2.Emphanisporites micrornatus—Streelispora newportensis(MN)带;3.Chelinospora cassicula—Perotriletes subitus(CS)带;4.Grandispora sp.—Apiculatasporites micro-conus(CM)带;上述孢子组合基本上可与中国华南、西南、西秦岭及有关国家、地区相同时代孢子带对比。根据孢子和疑源类材料,对西准噶尔志留—泥盆系界线、古地理和古生态进行了讨论。 相似文献
896.
冀东青龙一带双山子与朱丈子群的时代归属问题 总被引:4,自引:0,他引:4
本文谁了各类变质岩的全岩Rb-Sr等时线所反映的岩石初始或叠加变质变形年龄。角闪岩相的锆石U-Pb一致线和Sm-Nd等时线则因原岩(火成或沉积成因)的差别分别反映生成年龄或变形年龄,实际是受原生水控制。Sm-Nd等时线年龄偏低,还与低温动力变质中水的加入有关。由于本区2450Ma的数据出现频率很高,反映了太古宙与元古宙间的主要地质事件,结合双山子群、朱丈子群中发育可供洲际对比的绿岩带,故二者统归太 相似文献
897.
滇中地区晚三叠世的重力流沉积及其地质意义 总被引:4,自引:0,他引:4
上三叠统云南驿组一段下部是钙质浊积岩,上部为陆源碎屑浊积岩及少量火山碎屑浊积岩;二段的角砾状灰岩有坍塌和重力滑动两种成因,夹钙质浊积岩。罗家大山组一段由火山碎屑质的碎屑流、浊流和颗粒流沉积组成,夹基性熔岩。它们是滇中裂谷的记录,物源区在西边,构成中国西南部三叠纪古特提斯的一部分。 相似文献
898.
采自辽宁省凌源县大新房子村西部晚侏罗世地层中的一种鞘翅目化石,标本保存完好,其分类位置属于花蚤总科,接近于花蚤科和先花蚤科。标本保存了一些原始特征,为此,建立一新科——辽西花蚤科。迄今为止,晚侏罗世和早白垩世的花蚤类化石极为鲜见,文内描述的Liaoximordella hongi及相同层位的化石组合,对辽西地区中生代地层的划分和晚侏罗世花蚤类的演化均具有重要的意义。 相似文献
899.
H.R. Thierstein 《Cretaceous Research》1980,1(2):165-176
The effects of carbonate dissolution on the taxonomic composition of ancient nannofossil assemblages were investigated by deploying them on a deep mooring at depths of 1039 m–5500 m for 117 days in the Sargasso Sea. The relative proportions of only a few taxa change significantly with increasing dissolution: Cruciplacolithus primus is by far the most susceptible and Micula staurophora by far the most resistant species. The average dissolution indices calculated for the deployed assemblages closely follow the gradients of other dissolution indicators in the North Atlantic. 相似文献
900.
ZHANG Jin YUN Long ZHANG Beihang QU Junfeng ZHAO Heng HUI Jie WANG Yannan ZHANG Yiping 《《地质学报》英文版》2020,94(4):988-1006
How the Altyn Tagh fault(ATF) extends eastwards is one of the key questions in the study of the growth of the Qinghai–Tibetan Plateau. Detailed fieldwork at the easternmost part of the ATF shows that the ATF extends eastward and bypasses the Kuantan Mountain; it does not stop at the Kuantan Mountain, but connects with the northern Heishan fault in the east. The ATF does not enter the Alxa Block but extends eastward along the southern Alxa Block to the Jintanan Mountain. The Heishan fault is not a thrust fault but a sinistral strike-slip fault with a component of thrusting and is a part of the ATF. Further to the east, the Heishan fault may connect with the Jintananshan fault. A typical strike-slip duplex develops in the easternmost part of the ATF. The cut and deformed Quaternary sediments and displaced present gullies along the easternmost ATF indicate that it is an active fault. The local highest Mountain(i.e., the Kuantan Mountain) in the region forms in a restraining bend of the ATF due to the thrusting and uplifting. The northward growth of the Qinghai–Tibetan Plateau and the active deformation in South Mongolia are realized by sinistral strike-slipping on a series of NE–SW-trending faults and thrusting in restraining bends along the strike-slip faults with the northeastward motion of blocks between these faults. 相似文献