On Nov. 12, 1891, Professor Geikie made his presidential address before the Edinburgh Geological Society, the subject being “Supposed Causes of the Glacial Period.”
Many of his views advanced in this lecture were so much in accordance with my own that I am induced to repeat them. He said that the glacial period was a general phenomenon due to some widely acting1 cause, and that where we now have the greatest rain-fall the greatest snow-fall took place, and that the Pleistocene period was characterized by great oscillations of climate, extremely cold and very genial2 conditions alternating. He also said that in glacial and post-glacial times changes in the relative level of the land and sea had taken place, and any suggested explanation which did not fully3 account for these various climatic and geographical4 conditions could not be satisfactory. And, while examining the earth-movement hypothesis, he pointed5 out that in the first place there was not the least evidence of great continental6 elevations8 and depressions in the northern hemisphere, such as the hypothesis postulated9. Next he showed that, even if the diserrated earth-movements were admitted, they would not account for the phenomena10.
Such changes, no doubt, would profoundly affect the maritime11 regions of North America and Europe; but they would not bring about the conditions that obtained at the climax12 of the ice age.
Another objection to the earth-movement hypothesis was this: it did not account for interglacial conditions. The advocates of that hypothesis imagined that these conditions would supervene when the highly elevated northern regions were depressed13 to their present level. But these were the conditions77 that obtained at the present time; and yet in spite of them the climate was neither so equable nor so genial as that which obtained in interglacial times and during the mild stage of the necessary post-glacial period.
Therefore, he said that the earth-movement hypothesis should be rejected, not only because it was highly improbable that such wonderfully rhythmic14 elevations and depressions of northern lands could have taken place, but chiefly because it did not explain the conditions of the glacial periods and interglacial times.
Still, Professor Geikie says that in glacial and in post-glacial times changes in the relative level of the land and sea had taken place; and it is reasonable to suppose that such changes were obtained in the high latitudes15 of both hemispheres during the breaking up of the last ice age.
We have previously17 pointed out that much of the ice of the glacial period in the southern hemisphere was melted away, and its waters warmed sufficiently18 to assist the Gulf19 Stream and Japanese current to bring about a mild period in the northern hemisphere; for without such assistance they would be unable to disperse20 the vast ice-sheets of the northern latitudes.
Still, the attraction of the southern ocean waters into the northern seas must have commenced as soon as the growing ice-sheets of the large continents and islands of the high northern latitudes surpassed the growth and weight of the glaciers22 on the smaller lands of the southern hemisphere.
Hence the attraction of the ocean waters northward23 overcomes the force of the prevailing24 winds from moving an undue25 portion of the ocean’s surface waters southward. Consequently, the movement of water from the southern seas into the northern latitudes continued so long as the vast northern ice-sheets increased in weight greater than the glaciers of the southern hemisphere. Therefore, at the perfection of a frigid26 age straits and channels situated27 so far southward as the Magellan and Cape28 Horn channels were much diminished in width and depth78 or entirely29 deprived of their waters. Through this cause such reduced channels were readily filled with glaciers in a region of great snow-fall. The depth of water on the submerged northern lands at the close of the glacial period is not known.
According to Professor Dawson, in the township of Montague in Ontario the skeleton of a whale was found in post-glacial deposits 440 feet above tide-water, and marine30 shells are known to occur on Montreal mountain at an elevation7 of 520 feet above the ocean; and it is said that there are traces of submergence of over one thousand feet in the higher latitudes, including the islands of Great Britain.
According to the researches of Dr. J. W. Spencer, one great sheet of water covered most of the great lake region about the close of the ice age; and the lower strands31 of these inland seas are known to be connected with old marine shore lines. The probable reason why so few sea-shells collected on the glacial drift during such times was because of so much marine life having been exterminated32 in the high northern latitudes during the frigid age. Therefore, the sea, in the short period of northern submergence, left but few traces on the glacial drift it once flowed.
Thus it will be seen that, if the ocean waters were attracted northward through the preponderance of northern ice-sheets, they not only assisted in melting the northern ice, but also served to greatly reduce the waters in the Cape Horn channel, and so largely prevented the independent circulation of the southern ocean, thus furthering a mild climate in the southern hemisphere until the prevailing winds, after the northern ice-sheets were melted, were able to move more of the ocean waters southward than they could move northward, owing to the ocean currents setting southward being less obstructed33 than the lesser35 currents setting northward. This tendency of the ocean waters to move southward I have before explained in the preceding pages.
But I will say in addition that, on further consideration, it79 seems that one of the main causes of the waters of the augmented37 northern oceans moving southward so soon after the melting of the ice from the northern lands was on account of so much water being attracted southward to the great low sea-level east of Cape Horn. This vast low sea-level remained a great area of attraction for the northern seas until so much northern water was moved into the southern ocean as to reduce the seas of the northern hemisphere and augment36 the southern ocean sufficiently to enlarge the Cape Horn channel, thus causing the extinction38 of the vast low sea-level that furnished such great attraction for the waters of the more northern latitudes.
If the earth-movement hypothesis, so wholly rejected by Professor Geikie, fails to explain the cause or causes of a northern ice age, it seems to be still more inadequate39 for explaining the occurrence of ice periods extending over both hemispheres. For it is not probable that portions of continents and large islands rose above the snow-line in both temperate40 zones during the same period of time, and then again obtained their present level with the occurrence of a mild era.
Those who maintain that the continents of North America and Europe rose to great elevations during the ice age, in order to prove their assertions, point to the fiords which indent41 the eastern and western coasts of North America, and also to the fiords of Norway, as having been eroded42 by streams of ice that flowed along the bottom of such gorges43 when they were above the sea.
But it appears that such erosion could be performed by heavy glaciers with the lands at their present level. A glacier21 three thousand feet thick would fill and press heavily on the bottom of a gorge44 fifteen hundred feet in depth. Therefore, should the bottom of a fiord sink hundreds of feet below the sea-level, a glacier several thousand feet thick flowing through and over it into a sea of much greater depth, the erosion at the bottom of the sunken channel would be greater than on the land above the sea, where the ice possessed45 less weight.
Therefore, it is not necessary that lands pierced by deep80 fiords should have acquired a higher level during the ice age than they now maintain. And it is probable that on the antarctic continent ice erosion may be going on at much greater depths below the sea-level than the deepest channels in the high northern latitudes. For it is likely that the temperature of a glacier is so low in such frigid regions that it holds firmly in its freezing grasp such bowlders as may become detached from the rocks, thus giving it great erosive power.
But this great eroding46 ability could not be maintained by glaciers in the lower latitudes, where a higher temperature would largely deprive the ice of its abrading47 properties except on the steep slopes of mountainous lands.
There are deposits of ice on the North American coast bordering the arctic shores, and also on Northern Siberia, that are supposed to have existed since the last frigid period, and are likely to be preserved into a future cold age, which now appears to have made considerable progress on Greenland and other ice-clad arctic shores on account of the independent circulation of the Arctic Ocean waters, which largely excludes the Gulf Stream from the polar seas; and it is for this reason that the glaciers on the elevated lands of Iceland are being enlarged and rapidly advancing. Yet, notwithstanding the gathering48 of ice and increasing coldness of lands largely removed from the warm Gulf currents, there are still mountain regions where glaciers may have been preserved through post-glacial times, although directly to the leeward49 and under the influence of the Gulf Stream and Japanese currents. These glaciers are situated in the Alpine50 districts of Europe and on the mountain ranges of Alaska. It would appear that, were the climate growing gradually colder in the northern temperate zones, such glaciers should be increasing in size.
Yet it is said that such is not always the case. This is probably owing to their being subject to the genial influence of the tropical currents. For, although the climate of Europe and Alaska may have been slowly growing colder for centuries,81 still the slow shrinkage of these once immense glaciers may still be going on, although at a much slower rate than formerly51, even if the tender plants of these latitudes, because of the growing coldness, have gradually moved southward.
As to the Alpine glaciers, M. Forel reports from data he has collected that there have been several enlargements and diminutions during the last century. And since 1875 enlargements have taken place, their shrinkage being caused by warm and dry weather, while their enlargement was brought about during cold and rainy seasons. The glaciers of Alaska cannot attain52 much extension until the waters of the great Japanese stream acquire a lower temperature. There is at this date a small current setting down through the eastern side of Bering Strait, bearing field-ice in the spring season down to Anadyr Gulf. The Okhotsk Sea in the spring season furnishes considerable field-ice to cool the north Pacific waters, and the wintry winds which sweep down from the high lands of Northern Asia also serve to chill the Pacific seas; but all such sources of cold combined at this age have but little general effect on the vast Japanese current, which still has warmth sufficient to prevent the increase of glaciers on Alaska.
This great ocean stream in its impact against the shores of Oregon causes a high sea-level, which is mostly turned southward by the prevailing north-west winds. Still, a comparatively small stream sets along the shore of the Alaska Gulf, and also through the island passages toward a slight low sea-level, to the leeward of the Alaska peninsula; and it is probable that this current which warms these in-shore waters is favored by the difference of temperature and density53 between the waters abreast54 Oregon and the Gulf of Alaska, and it may be owing to the same cause that a small stream is sent along the eastern shore of Bering Strait into the deep portions of the Arctic Ocean. Thus because of the warm waters that proceed from the great Japanese current the glaciers of Alaska are prevented from increasing their bulk.
82 The only way to furnish the Japanese stream with colder water, and so cause glaciers to increase on the north-west coast of America, is through the great Humboldt current, which has its rise in the southern ocean west of Patagonia and the Cape Horn channel, where a moderate but vast high sea-level is formed on account of the great drift current of the southern ocean being somewhat obstructed on its passage through the Cape Horn channel, which is about one-third the breadth of the westerly wind-belt.
Therefore, the northern portion of the waters of the high sea-level so caused are attracted northward to the low sea-level abreast Peru, from whence they are moved by the south-east trade winds as a drift current to the equatorial latitudes, thus meeting and mingling55 with the returning Japanese current abreast Central America, and so giving head to the great equatorial stream which moves westward56 over the Pacific Ocean, partly impelled58 by the trade winds, and, on gaining the western side of the ocean, sends off from a moderate high sea-level a large stream to the low sea-level caused by the westerly winds abreast Japan, from whence it is drifted by the same winds over to the north-west coast of America, thus forming the great Japanese current.
Meanwhile the temperature of the Humboldt current, being governed by the temperature of the southern ocean from which it takes its rise, is cooling at a slow rate through the enlargement of ice-sheets in the antarctic regions, while the increase of glaciers on Patagonia will in time greatly add to its coolness, and so lower the temperature of the equatorial current from which the Japanese current branches, the latter current being made cooler through the increase of coldness of the former streams. Therefore, the temperature of Alaska, which is governed by the Japanese current, will slowly acquire a colder climate; and, consequently, its glaciers will increase in size sufficient to launch icebergs59 into the Pacific to be currented southward, and so still further lower the temperature of the83 Eastern Pacific waters, and consequently the equatorial current from which the Japanese stream branches, and so eventually, under the above conditions, cause heavy ice-sheets to spread widely over the north-west coast of North America.
It will be seen from the above explanations how an increase of cold in the southern hemisphere is necessary to cause a wider spread of ice-sheets on lands in the northern hemisphere.
Especially is this the case to promote the gathering of glaciers on the west coast of North America. The great equatorial current while on its way to the Indian Ocean not only sends off the Japanese stream, but also the East Australian current, which is like the Japanese current, having its temperature lowered in proportion as the equatorial stream is cooled. Therefore, the southern ocean is slowly being deprived of equatorial heat from this source.
I have explained how the increasing coldness of the superior oceans of the southern hemisphere affects more or less the temperature of the Gulf Stream, which meanwhile is only able to enter a small portion of its waters into the Arctic Ocean after undergoing a long cooling process as a drift current; and, while thus mingling with the arctic waters, it is not able to prevent the gathering of ice-sheets on Greenland, where glaciers are launching bergs to float southward as far as the latitude16 of 40° north. Consequently, the northern seas are now being cooled as well as the seas of the southern hemisphere.
Yet this cooling process is so slow there is a lack of data to show that the temperature of the high latitudes is lowering. Our thermometrical observations are of such recent date they cannot be used to determine climatic changes which requires centuries to bring about. Still, it is generally known that the climate of Northern Europe has been accused of growing colder. The vine no longer flourishes on the shores of Bristol Channel or in Flanders or Brittany; and vineyards are no longer planted on the elevated shores of France where84 they flourished three hundred years ago. Arago did not refuse to believe that the laws regulating the temperature of Western Europe had notably60 altered. This is proved, he said, by the general retrogradation of the vineyards southward.
The recent deadly freezing of the orange groves61 of Florida makes it uncertain whether the cultivation62 of the orange can again be successful in the counties where during this generation it has been very profitable.
Travellers visiting Iceland say that the old accounts of its prosperity seem strange to those who now visit its shores; and it is narrated63 in the Sagas64 that in early times sheep could shift for themselves during winter, and that there were large forests and that corn ripened65. Several years ago a correspondent of the Spectator, writing from Northern Russia where the Volga is locked with ice for six months in the year, stated that “the people were beginning to show increased resentment66 at the climate, and that there was reason to believe that the northern government of Russia would be abandoned to the desert. The people silently glide67 south by the tens of thousands every year, so the life of Russia was concentrating in the south.”
It is now the opinion of travellers in arctic lands that the inhabitants of the Esquimaux regions are decreasing, as are also the inhabitants of Northern Siberia.
A writer in the North China Herald68, of Shanghai, says that “the climate of Asia is becoming colder than it formerly was, and its tropical animals and plants are retreating southward at a slow rate. In the time of Confucius elephants were in use on the Yangtse River. A hundred and fifty years after this Mencius speaks of the tiger, the leopard69, the rhinoceros70, and the elephant as being in many parts of China.
“It is also said that the ferocious71 alligator72, that formerly infested73 the rivers of South China, has retreated southward.
“The flora74 of the country is also affected75 by the increasing coldness of the climate. The bamboo is not found in the forests85 of North China, where it grew naturally two thousand years ago, but is still grown in Pekin, with the aid of good shelter, as a sort of garden plant only.”
A letter from Hong Kong, published in the London Standard, reports that on the 15th of January, 1893, the temperature of Hong Kong, a tropical seaport76 of China, was below freezing for three days, and was colder than ever before known. The rocks and also vegetation were covered with a coating of ice. The thermometer at times stood at 23° and 26° Fahrenheit77.
I have previously explained how the slow increasing coldness of the northern temperate zone is also being carried out in the southern hemisphere. The meteorological records for the lofty table lands of Ecuador, although very incomplete, furnish strong evidence to show that the mean temperature of that region is gradually lowering.
Observations made by Boussingault at Quito in 1831, compared with those from 1878 to 1881, showed a decrease from 15.2° Centigrade to 13.27° Centigrade.
Records made by Hall from 1825 to 1827 give averages of 16.1° Centigrade, 15.52° Centigrade, and 15.6° Centigrade. This decrease holds good for all points in the inter-Andean region where records have been kept.
Yet we know that the falling temperature in the northern temperate latitudes is not brought about by a yearly increase of cold, because, when the arctic channels are somewhat obstructed with icebergs, the movement of arctic waters through them is lessened78; and, therefore, during such times the Gulf Stream, meeting with less opposition79 from arctic currents while flowing northward, is able to move a larger volume of its waters into the arctic seas, thus warming their waters sufficiently in a few seasons to clear the obstructed channels, and also somewhat soften80 for several successive years the temperature of such lands as border on the seas of that region.
And in this way we account for the mild seasons which at86 times follow those of lower temperature in high northern latitudes.
But, when the detained icebergs are set adrift, and currented into the temperate North Atlantic, the heat consumed while melting such numerous bodies of ice is able to more than overcome the warmth gained during the temporary detention81 of ice in the northern seas. Thus, under such considerations, it appears that the conditions are favorable for the growth of glaciers in the high northern latitudes.
I have pointed out the manner in which the superior oceans in the southern hemisphere are obtaining a lower temperature, and how they impart their coldness to the tropical currents, and in this way slowly cool the waters of all oceans. Thus it appears that the northern temperate zone, with all other parts of the earth, is slowly approaching a cold epoch82.
Several writers on climatic changes have expressed their views as to the number of glacial and mild periods that have been perfected since the conditions have been favorable for their appearance on the globe. According to my views, while considering the reasons for the occurrence of the great glacial periods which have left such extensive traces on the land, it seems certain that two very cold epochs have possessed the earth, separated by a warm period; and, possibly, other preceding cold epochs of less intensity83 have possessed the high latitudes, with intervening periods of mildness. But the earlier cold periods, if they ever existed, were comparatively short, because the Cape Horn channel during such times possessed less capacity than in the later periods, and, therefore, was more easily and quickly obstructed by the natural methods previously explained.
Consequently, the independent circulation of the southern ocean was sooner arrested than during the later epoch, when the channel had become enlarged by erosion from heavy glaciers and icebergs; and meanwhile the same conditions may have governed the arctic channels which give an independent87 circulation to the arctic waters which surround Greenland, and thus, in connection with cold epochs in the southern hemisphere, have caused periods of cold of small intensity to occur in the high northern latitudes, and it may happen in the future that more ice periods will be perfected than the one now progressing.
Still, it is well to bear in mind that the Cape Horn channel, which is the real cause of glacial periods having occurred in both the northern and southern hemispheres, in the manner previously explained, is being made wider and deeper during each succeeding ice age. For this reason the latest cold epoch will require a longer continuance of cold to obstruct34 the channel than the cold period preceding. Therefore, it appears that the time will come when there will be such great accumulations of ice stored on the land and in the sea before the enlarged Cape Horn channel can be closed that, when it is closed, there will not be sufficient warmth remaining in the tropical seas to unite with the sun’s rays to subdue84 the intense cold stored in the immense gatherings85 of ice. And thus the earth, which began its career with a warm temperature, and so continued for long ages, will finally terminate in an endless glacial age.
The statements made by General Cowell in Science of Nov. 25, 1892, in reference to the alleged86 discovery of the second rotation87 of the earth by Major-general Drayson, represents the discovery as affording a new solution for the cause or causes of an ice age.
The second rotation as defined consists in the pole of the heavens describing a circle around a point which is ascertained88 to be situated six degrees distant from the pole of the ecliptic. And it is asserted that by a knowledge of the second rotation it is proved that a variation of twelve degrees in the extent of the arctic circle and the tropics occurred not later than 13,500 B.C., “the tropics varying in distance from the equator from the minimum of 23° 25′ 47″ to the maximum of 35° 25′ 47″, thus extending the torrid zone during its widest expansion from Cape Hatteras to the river Plate.... It is calculated that88 at this date we are about 403 years distant from the time when the pole of the heavens in its revolution, the pole of the ecliptic and that of the second rotation, will be in the same colure,—that is, in the year 2,295 A.D.; and then the least differences in temperature between summer and winter will be experienced. From that time forward this difference will increase, and about 6,000 years later, or about the year 8,300 A.D., the earth will enter the next glacial period, and attain its greatest severity about the year 18,136 of our era.” General Cowell does not state how the widening of the tropical zone, as above set forth89, would bring about a glacial period. The winters of the temperate zones would evidently be colder than now; but, on the other hand, the summers would be proportionally warmer, while the westerly winds above the latitudes of 40° would prevail the same as now.
Therefore, their general effect on the surface waters of the ocean in the high latitudes would not be changed with such an extension of the tropical zone, neither would the trade winds change their general direction with a wider torrid zone; yet the boundaries of the trade winds and also the westerly winds would be more shifting according to the declination of the sun, such winds being governed as now by the position of the sun during the summer and winter solstice. Yet the natural process for moving tropical water into the high latitudes, or excluding it therefrom, would not be greatly changed.
Consequently, the expansion of the torrid zone to the latitudes named by General Drayson would not affect the climate of the hemispheres sufficiently to cause a frigid epoch. On the contrary, the summer monsoons90, which now blow from the north-east, along the shores of Eastern Africa, and also along the coast of Southern Brazil, would be much stronger with a vertical91 sun in midsummer as far south as river Plate, thus forcing the surface waters of the tropical oceans into the higher latitudes with greater facility than at this age.
Moreover, according to the statements of General Cowell, the89 present period of mildness should be on the increase, and obtain perfection in the year 2,295, or about 400 years hence; while, on the contrary, according to the explanations we have given in the preceding pages, there is much to show that an ice age is advancing, and has made considerable progress in the high latitudes of both hemispheres. Furthermore, if the second rotation, as claimed by General Cowell, is able to perfect a glacial period at regular intervals92 of 31,600 years, it seems that traces of frigid epochs should not be confined to late geological records, as there appear to be little or no traces of glacial work prior to the Quaternary or Post-tertiary periods.
It appears that explanations so far given, which depend on the astronomical93 theory to account for the ice age, are not in harmony with well-known geographical facts. The explainers neglect the attention due to the great prevailing winds which since the earlier geological ages have, in connection with continents, moved the surface waters of the ocean from torrid latitudes to colder zones, and from the colder zones to the warmer latitudes.
This exchange of ocean waters between the zones is as old as the continents which shape their courses. The important change wrought94 in the ocean currents sufficient to have caused the glacial age which ended the early warm epochs was brought about through the action of the prevailing winds, which, in connection with the form of continents, became able to move the ocean waters from the northern hemisphere into the southern sufficient to submerge the low lands of the southern hemisphere, causing a great diversion of the tropical currents from the high southern latitudes, such as I have pointed out in preceding chapters.
Those writers who believe that ocean currents have been the cause of great climatic changes have suggested that the existence of an ancient channel through the isthmus95 of Panama would have caused a frigid period on lands bordering on the northern shores of the Atlantic by turning the head-waters of the Gulf Stream into the Pacific Ocean.
90 Professor Agassiz thinks that such a channel existed during some remote geological age, judging from the semblance96 of the fauna97 pertaining98 to the Caribbean Sea and Pacific Ocean.
Yet it may be said that an open channel through Central America would have connected two high sea-levels.
For this reason there would be little or no exchange of water between the Caribbean Sea and the Pacific Ocean.
The high sea-level on the Pacific side is caused by the prevailing north-west winds which blow down the North American coast past California as far south as Central America; while, on the other hand, the south-east trade winds impel57 the surface waters of the South Pacific along the coast of Peru down to the equator, and so onward99 5° to 8° north latitude. Thus the space between the ending of the two ocean winds obtains a high sea-level, corresponding to the high level of the Caribbean Sea. This has been proved from levellings for the Nicaragua ship canal.
Consequently, the Atlantic waters would not run into the Pacific Ocean, even if a channel opened through Central America.
Therefore, the Gulf Stream has never been turned away from the North Atlantic.
Writers, while seeking a cause for the mild climate of ages preceding the glacial epochs, have thought that during such times channels opening through Asia from the Indian Ocean by the way of the Persian Gulf into the arctic seas would be the means of furnishing the Arctic Ocean with warm water. But it is evident that such a movement of water could not be brought about, because the winds would not be favorable for it. For, when we reflect that the prevailing winds would blow in the same direction as now, and that the seas of Eastern Europe and Western Asia were enlarged during the warm epochs, it seems that they would obtain high levels superior to the high level seas of the Indian Ocean.
Besides, we should consider that there is a continuous range91 of high land separating the Persian Gulf from the northern seas, which probably existed anterior100 to the ice age. Still, during later periods, while the ice-sheets were being melted from the northern hemisphere and also on the ending of the last ice age, the Isthmus of Suez was submerged, as were all other low lands in that latitude; but it is probable that the waters of the high sea-level of the Indian Ocean abreast tropical Africa did not flow largely into the Mediterranean101 Sea for the reason that the enlarged European seas, being within the westerly wind-belt, maintained a high sea-level, while at the same time the high level tropical Indian Ocean waters were strongly attracted into the southern oceans through the Mozambique and Agulhas currents in the manner I have previously explained. Yet the waters of the high sea-level of the southern European seas must have been strongly attracted to the low sea-level abreast the Canary Islands.
While considering the causes which brought about the glacial periods, it is well to reflect that the natural mode of action which could have produced a frigid age was as extensive as the surface of the globe; and, therefore, any geographical change that would affect only a comparatively small portion of the earth cannot serve to account for ages of warmth which extended over the globe, or for glacial epochs which were separated by warm periods of time, which seem to have affected all lands and seas.
And it appears from the geographical explanations given in preceding pages of the general movements of the winds and currents of the sea how impossible it is for heat to be conveyed to the antarctic latitudes sufficient to prevent the growth of glaciers on their lands while the Cape Horn channel is in possession of its present capacity.
For, as has been shown, this channel furnishes opportunity for the westerly winds to impel the surface waters of the great southern ocean constantly around the globe, and so largely turns away the tropical currents from the high southern latitudes.
92 Consequently, there seems to be no method yet devised through nature’s mode of action that can carry sufficient heat into the antarctic latitudes to melt the ice-sheets from the southern continent, or even arrest their growth, while the Cape Horn channel maintains its present width and depth.
Therefore, the increase of glaciers and icebergs will slowly continue until a glacial epoch is perfected.
And it seems that this arrangement for bringing about a frigid age made slower progress in its early stage than at this date, owing to there having been a lack of glacial ice in the polar regions to produce icebergs for cooling the ocean waters. But the independent circulation of the great southern ocean, after turning away the tropical currents from the high southern latitudes for thousands of years, did at length cause glaciers to form on the antarctic lands, which have been slowly, but constantly increasing; and, consequently, the cooling of the ocean has been accelerated proportionate to the increase of ice-sheets. Therefore, with the cooling process so well advanced as it now appears to be, it seems that more than half of the time required to bring a frigid age to perfection has been expended102 since ice-sheets began to gather on the antarctic shores. For, when we realise how the facilities for making ice have advanced through the increase of glaciers in both hemispheres, and how large a portion of the ocean waters have been cooled below a temperate or tropical temperature even in the torrid latitudes where the warm upper waters of the ocean have been reduced to a comparatively thin stratum103 when compared to the vast bulk of the cooled under-waters, it appears that the cold will increase at a faster rate for the next thousand years than was the case during the last ten centuries. Therefore, the climate will be less favorable for plants and animals existing on lands in the high latitudes for the next thousand years than during the ten centuries preceding; and, when we take into consideration the accelerative growth of a frigid epoch, it seems that the increasing cold will in a few thousand years drive the greater93 portion of both plants and animals from the now temperate latitudes to maintain an existence in the tropical zone, where a large part of the existing species of such life must have taken refuge during the last ice period.
And, from what can be learned from the relics104 of man’s prehistoric105 life, it seems to point to the lands of the tropical latitudes as having been his home during the frigid ages; and, because of his long undisturbed residence in favored portions of the tropics, he there attained106 his earliest civilization. For it appears that the tropical zone was not only less burdened with ice in glacial times than the higher latitudes of the globe, but was also more exempt107 from the great flooding of lands which obtained in the more northern latitudes through the shifting of the ocean waters, from causes set forth in the preceding pages. Yet it may be said that the low lands of the tropical zone south of the equator during cold epochs were much more extensive than at this age, on account of the shrinkage of the sea, because of the great amount of water evaporated from its surface, and stored in ice-sheets on the great continents and islands. Hence the reefs and shallows which surround such tropical islands as include the Seychelles Archipelago, and also the extensive banks covered with shoal water in that portion of the Indian Ocean, were during the glacial period elevated above the surface of the sea, possessing a climate favorable for vegetable and animal life. But, owing to the great rain-fall of that region, it is probable that the highest lands were glaciated, as it is reported that granite108 bowlders still rest on the mountain slopes of the highest island. The numerous islands and shoals of the south-western tropical Pacific must also have afforded wide land areas, with a temperate climate, owing to their having been situated on one of the warmest regions of the earth during the ice age.
Moreover, it is probable that these tropical lands afforded space for numerous lagoons109 which had little connection with the surrounding oceans, and consequently were able to maintain,94 in their secluded110 shallow basins, a warmer temperature than obtained in the open seas; and at the same time, owing to the great rainfall in such tropical portions of the Indian and Pacific regions, the waters of the lagoons were rendered less salt than the briny111 depths of the shrunken oceans of a cold period. Hence because of such conditions the fauna of the tropical seas were preserved from the destructive rigor112 which beset113 the earth during the frigid epochs.
The End
The End
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| n.高度；海拔；高地；上升；提高 | |
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elevations
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| （水平或数量）提高( elevation的名词复数 )； 高地； 海拔； 提升 | |
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postulated
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| v.假定，假设( postulate的过去式和过去分词 ) | |
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phenomena
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| n.现象 | |
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maritime
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| adj.海的，海事的，航海的，近海的，沿海的 | |
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climax
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| n.顶点；高潮；v.(使)达到顶点 | |
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depressed
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| adj.沮丧的，抑郁的，不景气的，萧条的 | |
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rhythmic
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| adj.有节奏的，有韵律的 | |
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latitudes
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| 纬度 | |
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latitude
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| n.纬度，行动或言论的自由(范围)，(pl.)地区 | |
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previously
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| adv.以前，先前（地） | |
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sufficiently
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| adv.足够地，充分地 | |
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gulf
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| n.海湾；深渊，鸿沟；分歧，隔阂 | |
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20
disperse
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| vi.使分散；使消失；vt.分散；驱散 | |
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glacier
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| n.冰川，冰河 | |
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glaciers
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| 冰河，冰川( glacier的名词复数 ) | |
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23
northward
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| adv.向北；n.北方的地区 | |
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prevailing
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| adj.盛行的；占优势的；主要的 | |
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undue
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| adj.过分的；不适当的；未到期的 | |
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frigid
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| adj.寒冷的，凛冽的；冷淡的；拘禁的 | |
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situated
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| adj.坐落在...的，处于某种境地的 | |
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28
cape
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| n.海角，岬；披肩，短披风 | |
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entirely
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| ad.全部地，完整地；完全地，彻底地 | |
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marine
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| adj.海的；海生的；航海的；海事的；n.水兵 | |
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strands
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| n.（线、绳、金属线、毛发等的）股( strand的名词复数 )；缕；海洋、湖或河的)岸；（观点、计划、故事等的）部份v.使滞留，使搁浅( strand的第三人称单数 ) | |
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exterminated
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| v.消灭，根绝( exterminate的过去式和过去分词 ) | |
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obstructed
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| 阻塞( obstruct的过去式和过去分词 )； 堵塞； 阻碍； 阻止 | |
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obstruct
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| v.阻隔，阻塞(道路、通道等)；n.阻碍物，障碍物 | |
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lesser
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| adj.次要的，较小的；adv.较小地，较少地 | |
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augment
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| vt.(使)增大，增加，增长，扩张 | |
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Augmented
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| adj.增音的 动词augment的过去式和过去分词形式 | |
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extinction
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| n.熄灭，消亡，消灭，灭绝，绝种 | |
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inadequate
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| adj.(for,to)不充足的，不适当的 | |
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temperate
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| adj.温和的，温带的，自我克制的，不过分的 | |
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41
indent
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| n.订单，委托采购，国外商品订货单，代购订单 | |
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eroded
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| adj. 被侵蚀的，有蚀痕的 动词erode的过去式和过去分词形式 | |
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43
gorges
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| n.山峡，峡谷( gorge的名词复数 )；咽喉v.（用食物把自己）塞饱，填饱( gorge的第三人称单数 )；作呕 | |
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44
gorge
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| n.咽喉，胃，暴食，山峡；v.塞饱，狼吞虎咽地吃 | |
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45
possessed
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| adj.疯狂的；拥有的，占有的 | |
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46
eroding
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| 侵蚀，腐蚀( erode的现在分词 )； 逐渐毁坏，削弱，损害 | |
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47
abrading
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| v.刮擦( abrade的现在分词 )；（在精神方面）折磨（人）；消磨（意志、精神等）；使精疲力尽 | |
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48
gathering
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| n.集会，聚会，聚集 | |
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49
leeward
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| adj.背风的；下风的 | |
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50
alpine
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| adj.高山的；n.高山植物 | |
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51
formerly
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| adv.从前，以前 | |
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52
attain
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| vt.达到，获得，完成 | |
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53
density
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| n.密集，密度，浓度 | |
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54
abreast
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| adv.并排地；跟上(时代)的步伐，与…并进地 | |
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mingling
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| adj.混合的 | |
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56
westward
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| n.西方，西部；adj.西方的，向西的；adv.向西 | |
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57
impel
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| v.推动；激励，迫使 | |
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58
impelled
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| v.推动、推进或敦促某人做某事( impel的过去式和过去分词 ) | |
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59
icebergs
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| n.冰山，流冰( iceberg的名词复数 ) | |
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60
notably
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| adv.值得注意地，显著地，尤其地，特别地 | |
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61
groves
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| 树丛，小树林( grove的名词复数 ) | |
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62
cultivation
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| n.耕作，培养，栽培（法），养成 | |
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63
narrated
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| v.故事( narrate的过去式和过去分词 ) | |
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64
sagas
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| n.萨迦（尤指古代挪威或冰岛讲述冒险经历和英雄业绩的长篇故事）( saga的名词复数 )；（讲述许多年间发生的事情的）长篇故事；一连串的事件（或经历）；一连串经历的讲述（或记述） | |
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65
ripened
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| v.成熟，使熟( ripen的过去式和过去分词 ) | |
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66
resentment
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| n.怨愤，忿恨 | |
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67
glide
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| n./v.溜，滑行；(时间)消逝 | |
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68
herald
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| vt.预示...的来临，预告，宣布，欢迎 | |
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leopard
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| n.豹 | |
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rhinoceros
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| n.犀牛 | |
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71
ferocious
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| adj.凶猛的，残暴的，极度的，十分强烈的 | |
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72
alligator
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| n.短吻鳄(一种鳄鱼) | |
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73
infested
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| adj.为患的，大批滋生的（常与with搭配）v.害虫、野兽大批出没于( infest的过去式和过去分词 )；遍布于 | |
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74
flora
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| n.(某一地区的)植物群 | |
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75
affected
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| adj.不自然的，假装的 | |
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76
seaport
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| n.海港，港口，港市 | |
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77
Fahrenheit
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| n./adj.华氏温度；华氏温度计(的) | |
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78
lessened
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| 减少的，减弱的 | |
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79
opposition
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| n.反对，敌对 | |
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80
soften
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| v.(使)变柔软；(使)变柔和 | |
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81
detention
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| n.滞留，停留；拘留，扣留；(教育)留下 | |
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82
epoch
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| n.(新)时代；历元 | |
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83
intensity
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| n.强烈，剧烈；强度；烈度 | |
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84
subdue
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| vt.制服，使顺从，征服；抑制，克制 | |
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85
gatherings
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| 聚集( gathering的名词复数 )； 收集； 采集； 搜集 | |
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86
alleged
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| a.被指控的，嫌疑的 | |
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87
rotation
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| n.旋转；循环，轮流 | |
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88
ascertained
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| v.弄清，确定，查明( ascertain的过去式和过去分词 ) | |
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forth
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| adv.向前；向外，往外 | |
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90
monsoons
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| n.（南亚、尤指印度洋的）季风( monsoon的名词复数 )；（与季风相伴的）雨季；（南亚地区的）雨季 | |
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91
vertical
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| adj.垂直的，顶点的，纵向的；n.垂直物，垂直的位置 | |
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92
intervals
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| n.[军事]间隔( interval的名词复数 )；间隔时间；[数学]区间；（戏剧、电影或音乐会的）幕间休息 | |
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93
astronomical
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| adj.天文学的，（数字）极大的 | |
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94
wrought
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| v.引起；以…原料制作；运转；adj.制造的 | |
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95
isthmus
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| n.地峡 | |
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96
semblance
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| n.外貌，外表 | |
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97
fauna
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| n.(一个地区或时代的)所有动物，动物区系 | |
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98
pertaining
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| 与…有关系的，附属…的，为…固有的(to) | |
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99
onward
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| adj.向前的，前进的；adv.向前，前进，在先 | |
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100
anterior
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| adj.较早的；在前的 | |
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101
Mediterranean
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| adj.地中海的；地中海沿岸的 | |
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102
expended
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| v.花费( expend的过去式和过去分词 )；使用（钱等）做某事；用光；耗尽 | |
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103
stratum
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| n.地层，社会阶层 | |
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104
relics
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| [pl.］n.遗物，遗迹，遗产；遗体，尸骸 | |
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105
prehistoric
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| adj.（有记载的）历史以前的，史前的，古老的 | |
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attained
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| （通常经过努力）实现( attain的过去式和过去分词 )； 达到； 获得； 达到（某年龄、水平、状况） | |
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107
exempt
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| adj.免除的；v.使免除；n.免税者，被免除义务者 | |
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108
granite
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| adj.花岗岩，花岗石 | |
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109
lagoons
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| n.污水池( lagoon的名词复数 )；潟湖；（大湖或江河附近的）小而浅的淡水湖；温泉形成的池塘 | |
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110
secluded
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| adj.与世隔绝的；隐退的；偏僻的v.使隔开，使隐退( seclude的过去式和过去分词) | |
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111
briny
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| adj.盐水的；很咸的；n.海洋 | |
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112
rigor
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| n.严酷，严格，严厉 | |
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113
beset
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| v.镶嵌；困扰，包围 | |
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