I have before explained that the conditions are such that the cold periods of the northern and southern hemispheres were concurrent3. Through this cause, while the glacial epoch2 was being perfected, the ice followed down the mountain ranges of both hemispheres; and, while gathering4 on the lands of the temperate5 latitudes6, it also spread over a portion of the tropical zone. It is reported that traces of ancient glaciers are found in India, and also in Central America and in tropical South America. In fact, the denudation8 caused by ancient glaciers on the elevated lands of the tropics are too well defined to be attributed to any process of weathering, while Alpine9 plants of the same species are found near the summits of mountains in the tropics as well as in the high latitudes of both hemispheres.
This fact goes to show that a portion of the lowlands of the tropical zone have experienced a temperature favorable for the55 growth of Alpine plants. And, judging from the tropical islands I have visited, situated10 in the cold currents which flow down the eastern sides of the oceans from the high latitudes, I think they show strong traces of having during some remote period been subject to the action of glaciers. The island of St. Helena, situated in the southern tropical Atlantic, has the appearance of having been heavily iced during a frigid11 age. Its steep ravines, which deepen as they approach the sea, recall to the southern voyager the ice-worn islands of the high latitudes. It seems improbable that these deep ravines which penetrate12 the hard volcanic13 rock, on their short course to the sea, could have been caused by their scanty14 brooklets.
The bowlders scattered15 over the island are not in harmony with the weathering process, while the obliteration16 of its craters17 seems to point to a more rapid process of erosion than could be attributed to weathering.
Professor Agassiz, in his “General Sketch19 of the Expedition of the ‘Albatross,’” states that the Galapagos Islands are of volcanic origin, and that their age does not reach beyond the earliest Tertiary period; and his report seems to favor the impression of their having undergone denudation sufficient to slough20 off large portions of the rims21 of the older craters, and also the eastern face of Wenman Island. On Hood’s Island, at the time of my visit, its crater18 had entirely22 disappeared.
The highest portion of the island, which was the probable site of its ancient crater, showed no trace of its former existence; yet at the foot of this low mountain, on its southern side, I saw a large collection of loose bowlders, composed of hard volcanic rock, which were mostly free from soil and other débris, and easily moved from their places, while the spaces afforded by the loose piles of dark basaltic rocks afforded a secure retreat for numerous owls23 and lizards24. Beyond the rocky piles to the southward a horizontal area of land was strewn with bowlders to the sea, which was some two miles distant from the higher land. The bowlders which covered the plain were somewhat56 smaller than those at the foot of the mountain, as none of the former were more than three or four feet in their longest measurement.
They seem to have been formed from thin strata25 of lava26, which were broken in pieces from pressure, such as the action of ice could perform. In fact, the crowded and angular and somewhat worn blocks of lava presented a different appearance from stones thrown from the crater of a volcano, while no such bowlders are found among the recent volcanic eruptions27 on the islands.
The plain so thickly strewn with bowlders, and partly shaded by a tall growth of shrubs28, fell off abruptly29 at the seaside, forming a steep cliff some two hundred feet in height.
The rocky floor at the foot of the cliff received such débris as fell from the sea-washed land; yet it contained few bowlders, they having been washed away by the waves soon after falling.
At one place a steep, dry ravine penetrated30 the land from the seashore, which was dangerous to cross on account of the loose stones resting on its sides. Two or three miles further west, on the level land bordering the sea, a large rookery of albatross were brooding their eggs and chicklings. The land on the south side of Albemarle, near the sea, consists of débris from the eroded31 high lands; and, judging from the crumbling32 cliffs by the sea, it seems that the land at one time extended further seaward.
Besides the excessive denudation which appears to have taken place on portions of these bowlder-strewn lands, we have other unmistakable testimony33 of their having formerly34 possessed35 a frigid temperature. The characteristic Alpine flora36 of these islands points to a time when they were exposed to a cold climate. Furthermore, rookeries of seal and albatross, which naturally belong to shores situated in cold latitudes, still exist on these equatorial islands; and, when we consider the favorable position of the Galapagos for the reception of cold during a frigid period, we can well account for the lingering signs which point to their former cold climate.
57 During the perfection of an ice period the western shore of South America was covered with an ice-sheet from the summits of its mountain range to the sea, extending northward37 as far as the latitude7 of 38° south.
This vast ice-sheet, situated in a region of great snow-fall, was constantly sending icebergs38 into the sea, where they were borne northward by the cold Humboldt current directly toward the Galapagos Islands; while, on the other hand, in the northern latitudes, in regions of great snow-fall, such as Alaska and British America, numerous icebergs were launched into the ocean, to be currented southward to the Galapagos seas. Thus during the frigid epoch the equatorial waters surrounding the Galapagos group was one of the greatest gathering places for floating ice to be found on the globe.
And here the frigidity39 stored up in the glaciers of the higher latitudes was set free, thus chilling the waters as well as the atmosphere of that region. The Alpine flora of the American coast mountains was probably carried by floating ice to the Galapagos, while its rookeries of albatross and seal date back to a cold period. And it seems that these cold-weather animals, with the assistance of the cool Humboldt current, may be able to preserve their rookeries at the equator until the advent40 of another ice period. In connection with the evidences of a cold climate having possessed the Galapagos, there are ample traces of ice-sheets having flowed over a large portion of the high lands of tropical America, and in some places the ice may have flowed down to the sea, especially where the large rivers now empty; and it is said that masses of clay, mixed with sub-angular stones, have been found in Brazil, which goes to prove the glaciation of portions of that tropical land during a remote age. Professor Louis J. R. Agassiz, during his research in the Amazon valley, found bowlders resting near the summits of the low hills of that region, which he attributed to the action of ice. The spread of glaciers on southern continents and islands is shown on map No. 1.
58 In Science, Nov. 17, 1893, Mr. J. Crawford published a summary of his discoveries in Nicaragua, during ten months of nearly continuous exploration since August, 1892.
The author of this report says: “The numerous eroded mountain ridges41 and lateral42 terminal moraines of that tropical region give unquestionable evidences of the former existence of a glacial epoch, which covered an area of several thousand square miles in Nicaragua with glacial ice. The ice-sheet covered a large part of the existing narrow divide of land (containing about 48,000 square miles) between the Pacific and Caribbean Sea.” And it is likely that other large areas of tropical America were glaciated at the same time, especially in regions of great precipitation.
The island of Cuba, during a portion of the ice age, probably supported heavy glaciers, and obtained an average temperature as low as South-western New Zealand at this age. According to the description given by J. W. Spencer, of the Cuban land, great valleys have been excavated43, the lower portion of which are now fiords, reaching in one case at least to seven thousand feet in depth before gaining the sea beyond. Thus, while keeping in view the glacial condition of Central America during the frigid period, it seems that the great Cuban excavations44 were partly the work of glaciers of the same cold epoch.* Judging from such reliable statements, it is probable that the climate of tropical America during the frigid age was somewhat colder than obtained in the tropical regions of the eastern continent, owing to the wide connection of the Atlantic with the Arctic Ocean as well as with the antarctic seas, and because of its shores possessing59 a larger area of glaciated lands in proportion to its size than the Pacific and Indian Oceans, and also owing to the tropical Atlantic containing so small a portion of the world’s waters which lie within the torrid zone, and its equatorial current being separated by continental45 lands from the great equatorial stream of the Pacific and Indian Oceans.
*The meeting of the British Association for the Advancement46 of Science, September, 1895, was reported in Science of October 18, where mention is made of an interesting paper by Mr. R. B. White, on “The Glacial Age of Tropical America,” in which he described a number of apparently47 glacial deposits in the Republic of Colombia, almost under the equator. He spoke48 of moraines forming veritable mountains, immense thicknesses of bowlder clay, breccias, cement beds, sand, gravels49, and clays, beds of loess, valleys scooped50, grooved51, and terraced, monstrous52 erratics, and traces of great avalanches53.
Therefore, the tropical Atlantic waters must have been reduced to a lower temperature during a frigid age than the tropical waters of the Indian Ocean or the western part of the tropical Pacific, as a large portion of the great equatorial current of the latter oceans, during its western movement, was exposed to the rays of a tropical sun for a much longer time, after being replenished54 by the cold waters of the high latitudes, than the tropical currents of the Atlantic; and it is probable that, on account of tropical America possessing a colder climate than the tropical lands of the eastern continent during the frigid epoch, the cold of the western continent was more destructive to its fauna55 and flora than was the case in the tropical regions of the eastern continent. Professor Wright, in his valuable work on “The Ice Age of North America,” gives a good description of the “flight of plants and animals during the glacial epoch,” and also of the extermination56 of many superior species because of the frigid climate.
The high lands of tropical Africa, above the altitude of three thousand feet, and situated in places of great precipitation, were probably covered with snow and ice during the glacial age. Travellers have reported that islands composed partly of granite57 bowlders are found in the lakes at the head-waters of the Nile. But the glaciers that invaded the tropical latitudes were of short duration compared with the ice-sheets that burdened the lands of the temperate zones. Besides, such tropical ice as flowed to the low lands was so near a melting condition that it made small impression on the rocks; but on steep mountain slopes, where the movement of the ice was comparatively rapid, it possessed considerable eroding58 power.60 The climate of the tropical zone on both continents during the perfection of an ice period was so cold that such animals as could not endure a low temperature retreated into the warmest regions of the equatorial latitudes, while many species who failed to reach such places perished. And especially was this the case with the pre-glacial fauna of the western continent. Mr. W. B. M. Davidson, in his treatise59 on Florida phosphates, says: “The great mammal hordes60 of the glacial epoch were driven into Florida in their flight southward for life and warmth, and there perished because of the deadly cold which ever moved southward. The Florida waters grew so icy cold, fishes, reptiles61, and mammoth62 animals died, and added their frames and teeth to the valley of bones now found in that southern region.”
Such species of the tropical fauna of the ocean as survived the ice age could have existed only in torrid seas with small connection with the cold oceans during the frigid epochs. For, with the diminished oceans of a cold period, it seems that the conditions were favorable for the maintenance of such seas in the region of the East India Islands.
Such parts of Southern Europe and Northern Africa as bordered on the Mediterranean63 Sea probably possessed a milder climate during the ice age than regions in the same latitudes on the Atlantic coast, for the reason that the North Atlantic was proportionally a greater receptacle for icebergs which were launched into it from the numerous glaciers of North-eastern America, Greenland, Iceland, and North-western Europe than the great inland sea obtained from its less frigid shores. And it may have happened that during such times the tropical waters of the Indian Ocean had some connection with the Mediterranean through the Red Sea and Suez, and so during portions of the year the waters of the tropical Indian Ocean were forced by the periodical winds into the inland sea. It is the opinion of several writers that man, along with other species of animal life, existed previous to the glacial period;61 for, since the seas and lands of the globe were chilled, the conditions seem to have been less favorable for the spontaneous generation of animate64 bodies than during the previous warm ages. Therefore, it appears that the generative ages should be ascribed to the long genial65 eras prior to the glacial epochs. For it is probable that the lower parts of the ocean, which now possess a low temperature even in the tropical latitudes, were, during the warm eras, wholly composed of warm water, because the surface waters of the antarctic seas of that age, which supply the great under-currents of the ocean, would possess a high temperature; and it is probable that the temperature of a large portion of the seas of the torrid zone was for a long time maintained at blood heat. For it should be considered that the waters which moved from the torrid seas, after making their journey through the warm regions of the high latitudes, would on their return to the tropics retain a large portion of the heat they acquired in the torrid zone before making their journey to the mild polar regions.
And, when we reflect how the heat of the sun’s rays was conserved66 by the ocean waters, and that their circulation during such times was almost wholly performed by the winds, as the difference of temperature between the polar latitudes and the equator was small, it appears that during the eras previous to the glacial age the oceans must have obtained a higher temperature than possessed by the warmest seas of to-day.
According to the discoveries of Professor Wright and others, ancient stone implements67 have been found beneath the glacial drift, as well as the bones of animals whose descendants are now living, which goes to prove that man, with other species of fauna which now inhabit the earth, existed anterior68 to the glacial epoch.
And on consideration it seems unreasonable69 to suppose that any of the superior species of animals could have been brought into existence since the waters and lands of the earth were chilled by the cold of a glacial age. And it appears that many62 species of animals which are known to have survived the cold periods were indebted for such survivals to the slow process through which a frigid period is brought about, thus affording time for evolutionary70 inurement71 to the slow increase of cold which at length perfects a glacial epoch.
The inurement to cold acquired by animals during the glacial age is still an attribute possessed by many species of fauna to-day. For, when a warm climate took possession of the tropical zone, it was deserted72 by a large portion of the animals that found refuge there during the glacial age.
Thus, while the seas and shores of the cooler latitudes swarm73 with animate bodies, the torrid latitudes seem comparatively lonely to the voyagers on the tropical oceans.
点击
收听单词发音
收听单词发音 |
1
glaciers
|
|
| 冰河，冰川( glacier的名词复数 ) | |
参考例句： |
|
|
|
|
2
epoch
|
|
| n.(新)时代；历元 | |
参考例句： |
|
|
|
|
3
concurrent
|
|
| adj.同时发生的，一致的 | |
参考例句： |
|
|
|
|
4
gathering
|
|
| n.集会，聚会，聚集 | |
参考例句： |
|
|
|
|
5
temperate
|
|
| adj.温和的，温带的，自我克制的，不过分的 | |
参考例句： |
|
|
|
|
6
latitudes
|
|
| 纬度 | |
参考例句： |
|
|
|
|
7
latitude
|
|
| n.纬度，行动或言论的自由(范围)，(pl.)地区 | |
参考例句： |
|
|
|
|
8
denudation
|
|
| n.剥下；裸露；滥伐；剥蚀 | |
参考例句： |
|
|
|
|
9
alpine
|
|
| adj.高山的；n.高山植物 | |
参考例句： |
|
|
|
|
10
situated
|
|
| adj.坐落在...的，处于某种境地的 | |
参考例句： |
|
|
|
|
11
frigid
|
|
| adj.寒冷的，凛冽的；冷淡的；拘禁的 | |
参考例句： |
|
|
|
|
12
penetrate
|
|
| v.透(渗)入；刺入，刺穿；洞察，了解 | |
参考例句： |
|
|
|
|
13
volcanic
|
|
| adj.火山的；象火山的；由火山引起的 | |
参考例句： |
|
|
|
|
14
scanty
|
|
| adj.缺乏的，仅有的，节省的，狭小的，不够的 | |
参考例句： |
|
|
|
|
15
scattered
|
|
| adj.分散的，稀疏的；散步的；疏疏落落的 | |
参考例句： |
|
|
|
|
16
obliteration
|
|
| n.涂去，删除；管腔闭合 | |
参考例句： |
|
|
|
|
17
craters
|
|
| n.火山口( crater的名词复数 )；弹坑等 | |
参考例句： |
|
|
|
|
18
crater
|
|
| n.火山口，弹坑 | |
参考例句： |
|
|
|
|
19
sketch
|
|
| n.草图；梗概；素描；v.素描；概述 | |
参考例句： |
|
|
|
|
20
slough
|
|
| v.蜕皮，脱落，抛弃 | |
参考例句： |
|
|
|
|
21
rims
|
|
| n.（圆形物体的）边( rim的名词复数 )；缘；轮辋；轮圈 | |
参考例句： |
|
|
|
|
22
entirely
|
|
| ad.全部地，完整地；完全地，彻底地 | |
参考例句： |
|
|
|
|
23
owls
|
|
| n.猫头鹰( owl的名词复数 ) | |
参考例句： |
|
|
|
|
24
lizards
|
|
| n.蜥蜴( lizard的名词复数 ) | |
参考例句： |
|
|
|
|
25
strata
|
|
| n.地层（复数）；社会阶层 | |
参考例句： |
|
|
|
|
26
lava
|
|
| n.熔岩，火山岩 | |
参考例句： |
|
|
|
|
27
eruptions
|
|
| n.喷发，爆发( eruption的名词复数 ) | |
参考例句： |
|
|
|
|
28
shrubs
|
|
| 灌木( shrub的名词复数 ) | |
参考例句： |
|
|
|
|
29
abruptly
|
|
| adv.突然地，出其不意地 | |
参考例句： |
|
|
|
|
30
penetrated
|
|
| adj. 击穿的，鞭辟入里的 动词penetrate的过去式和过去分词形式 | |
参考例句： |
|
|
|
|
31
eroded
|
|
| adj. 被侵蚀的，有蚀痕的 动词erode的过去式和过去分词形式 | |
参考例句： |
|
|
|
|
32
crumbling
|
|
| adj.摇摇欲坠的 | |
参考例句： |
|
|
|
|
33
testimony
|
|
| n.证词；见证，证明 | |
参考例句： |
|
|
|
|
34
formerly
|
|
| adv.从前，以前 | |
参考例句： |
|
|
|
|
35
possessed
|
|
| adj.疯狂的；拥有的，占有的 | |
参考例句： |
|
|
|
|
36
flora
|
|
| n.(某一地区的)植物群 | |
参考例句： |
|
|
|
|
37
northward
|
|
| adv.向北；n.北方的地区 | |
参考例句： |
|
|
|
|
38
icebergs
|
|
| n.冰山，流冰( iceberg的名词复数 ) | |
参考例句： |
|
|
|
|
39
frigidity
|
|
| n.寒冷；冷淡；索然无味；（尤指妇女的）性感缺失 | |
参考例句： |
|
|
|
|
40
advent
|
|
| n.(重要事件等的)到来，来临 | |
参考例句： |
|
|
|
|
41
ridges
|
|
| n.脊( ridge的名词复数 )；山脊；脊状突起；大气层的)高压脊 | |
参考例句： |
|
|
|
|
42
lateral
|
|
| adj.侧面的，旁边的 | |
参考例句： |
|
|
|
|
43
excavated
|
|
| v.挖掘( excavate的过去式和过去分词 )；开凿；挖出；发掘 | |
参考例句： |
|
|
|
|
44
excavations
|
|
| n.挖掘( excavation的名词复数 )；开凿；开凿的洞穴（或山路等）；（发掘出来的）古迹 | |
参考例句： |
|
|
|
|
45
continental
|
|
| adj.大陆的，大陆性的，欧洲大陆的 | |
参考例句： |
|
|
|
|
46
advancement
|
|
| n.前进，促进，提升 | |
参考例句： |
|
|
|
|
47
apparently
|
|
| adv.显然地；表面上，似乎 | |
参考例句： |
|
|
|
|
48
spoke
|
|
| n.（车轮的）辐条；轮辐；破坏某人的计划；阻挠某人的行动 v.讲，谈（speak的过去式）；说；演说；从某种观点来说 | |
参考例句： |
|
|
|
|
49
gravels
|
|
| 沙砾( gravel的名词复数 )； 砾石； 石子； 结石 | |
参考例句： |
|
|
|
|
50
scooped
|
|
| v.抢先报道( scoop的过去式和过去分词 )；（敏捷地）抱起；抢先获得；用铲[勺]等挖（洞等） | |
参考例句： |
|
|
|
|
51
grooved
|
|
| v.沟( groove的过去式和过去分词 )；槽；老一套；（某种）音乐节奏 | |
参考例句： |
|
|
|
|
52
monstrous
|
|
| adj.巨大的；恐怖的；可耻的，丢脸的 | |
参考例句： |
|
|
|
|
53
avalanches
|
|
| n.雪崩( avalanche的名词复数 ) | |
参考例句： |
|
|
|
|
54
replenished
|
|
| 补充( replenish的过去式和过去分词 )； 重新装满 | |
参考例句： |
|
|
|
|
55
fauna
|
|
| n.(一个地区或时代的)所有动物，动物区系 | |
参考例句： |
|
|
|
|
56
extermination
|
|
| n.消灭，根绝 | |
参考例句： |
|
|
|
|
57
granite
|
|
| adj.花岗岩，花岗石 | |
参考例句： |
|
|
|
|
58
eroding
|
|
| 侵蚀，腐蚀( erode的现在分词 )； 逐渐毁坏，削弱，损害 | |
参考例句： |
|
|
|
|
59
treatise
|
|
| n.专著；(专题)论文 | |
参考例句： |
|
|
|
|
60
hordes
|
|
| n.移动着的一大群( horde的名词复数 )；部落 | |
参考例句： |
|
|
|
|
61
reptiles
|
|
| n.爬行动物，爬虫( reptile的名词复数 ) | |
参考例句： |
|
|
|
|
62
mammoth
|
|
| n.长毛象；adj.长毛象似的，巨大的 | |
参考例句： |
|
|
|
|
63
Mediterranean
|
|
| adj.地中海的；地中海沿岸的 | |
参考例句： |
|
|
|
|
64
animate
|
|
| v.赋于生命，鼓励；adj.有生命的，有生气的 | |
参考例句： |
|
|
|
|
65
genial
|
|
| adj.亲切的，和蔼的，愉快的，脾气好的 | |
参考例句： |
|
|
|
|
66
conserved
|
|
| v.保护，保藏，保存( conserve的过去式和过去分词 ) | |
参考例句： |
|
|
|
|
67
implements
|
|
| n.工具( implement的名词复数 )；家具；手段；[法律]履行（契约等）v.实现( implement的第三人称单数 )；执行；贯彻；使生效 | |
参考例句： |
|
|
|
|
68
anterior
|
|
| adj.较早的；在前的 | |
参考例句： |
|
|
|
|
69
unreasonable
|
|
| adj.不讲道理的，不合情理的，过度的 | |
参考例句： |
|
|
|
|
70
evolutionary
|
|
| adj.进化的；演化的，演变的；[生]进化论的 | |
参考例句： |
|
|
|
|
71
inurement
|
|
| n.习惯 | |
参考例句： |
|
|
|
|
|
72
deserted
|
|
| adj.荒芜的，荒废的，无人的，被遗弃的 | |
参考例句： |
|
|
|
|
73
swarm
|
|
| n.(昆虫)等一大群；vi.成群飞舞；蜂拥而入 | |
参考例句： |
|
|
|
| 欢迎访问英文小说网 |