REMARKS ON THEORIES ADVANCED FOR EXPLAINING ICE PERIODS.  
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 acting 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 genial 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 fully account for these various climatic and geographical conditions could not be satisfactory. And, while examining the earth-movement hypothesis, he pointed out that in the first place there was not the least evidence of great continental elevations and depressions in the northern hemisphere, such as the hypothesis postulated. Next he showed that, even if the diserrated earth-movements were admitted, they would not account for the phenomena.
Such changes, no doubt, would profoundly affect the maritime regions of North America and Europe; but they would not bring about the conditions that obtained at the climax 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 depressed 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 rhythmic 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 latitudes of both hemispheres during the breaking up of the last ice age.
We have previously pointed out that much of the ice of the glacial period in the southern hemisphere was melted away, and its waters warmed sufficiently to assist the Gulf Stream and Japanese current to bring about a mild period in the northern hemisphere; for without such assistance they would be unable to disperse 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 glaciers on the smaller lands of the southern hemisphere.
Hence the attraction of the ocean waters northward overcomes the force of the prevailing winds from moving an undue 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 frigid age straits and channels situated so far southward as the Magellan and Cape Horn channels were much diminished in width and depth78 or entirely 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 marine shells are known to occur on Montreal mountain at an elevation 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 strands 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 exterminated 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 obstructed than the lesser 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 augmented 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 augment the southern ocean sufficiently to enlarge the Cape Horn channel, thus causing the extinction 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 inadequate 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 temperate 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 indent the eastern and western coasts of North America, and also to the fiords of Norway, as having been eroded by streams of ice that flowed along the bottom of such gorges 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 glacier three thousand feet thick would fill and press heavily on the bottom of a gorge 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 possessed 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 eroding ability could not be maintained by glaciers in the lower latitudes, where a higher temperature would largely deprive the ice of its abrading 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 gathering 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 leeward and under the influence of the Gulf Stream and Japanese currents. These glaciers are situated in the Alpine 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 formerly, 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 attain 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 density between the waters abreast 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 mingling with the returning Japanese current abreast Central America, and so giving head to the great equatorial stream which moves westward over the Pacific Ocean, partly impelled 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 icebergs 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 latitude 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 notably altered. This is proved, he said, by the general retrogradation of the vineyards southward.
The recent deadly freezing of the orange groves of Florida makes it uncertain whether the cultivation 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 narrated in the Sagas that in early times sheep could shift for themselves during winter, and that there were large forests and that corn ripened. 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 resentment 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 glide 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 Herald, 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 leopard, the rhinoceros, and the elephant as being in many parts of China.
“It is also said that the ferocious alligator, that formerly infested the rivers of South China, has retreated southward.
“The flora of the country is also affected 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 seaport of China, was below freezing for three days, and was colder than ever before known. The ............