America, on her way to Liverpool, passed some icebergs 600 feet in height, and of great length.

April, 1864.-The Royal Standard came in collision with an iceberg 600 feet in height.

Dec., 1856.-Four large icebergs, one of them 700 feet high, and another 500 feet, were met with in lat. 50° 14′ S., long. 42° 54' E.

Dec. 25th, 1861.-The Queen of Nations fell in with an iceberg in lat. 53° 45′ S., long. 170° 0' W., 720 feet high.

Dec., 1856.-Captain P. Wakem, ship Ellen Radford, found, in lat. 52° 31' S., long. 43° 43′ W., two large icebergs, one at least 800 feet high.

Mr. Towson states that one of our most celebrated and talented naval surveyors informed him that he had seen icebergs in the southern regions 800 feet high.

March 23rd, 1855.-The Agneta passed an iceberg in lat. 53° 14′ S., long. 14° 41′ E., 960 feet in height.

Aug. 16th, 1840.-The Dutch ship, General Baron von Geen, passed an iceberg 1,000 feet high in lat. 37° 32′ S., long. 14° 10' E.

May 15th, 1859.-The Roseworth found in lat. 53° 40′ S., long. 123° 17′ W., an iceberg as large as as large as "Tristan d'Acunha." In the regions where most of these icebergs were met with, the mean density of the sea is about 10256. The density of ice is 92. The density of icebergs to that of the sea is therefore as 1 to 1.115; consequently every foot of ice above water indicates 8.7 feet below water. It therefore follows that those icebergs 400 feet high had 3,480 feet under water,—3,880 feet would consequently be the total thickness of the ice. The icebergs which were 500 feet high would be 4,850 feet thick, those 600 feet high would have a total thickness of 5,820 feet, and those 700 feet high would be no less than 6,790 feet thick, which is more than a mile and a quarter. The iceberg 960 feet high, sighted by the Agneta, would be actually 9,312 feet thick, which is upwards of a mile and three-quarters.

Although the mass of an iceberg below water compared to

that above may be taken to be about 87 to 1, yet it would not be always safe to conclude that the thickness of the ice below water bears the same proportion to its height above. If the berg, for example, be much broader at its base than at its top, the thickness of the ice below water would bear a less proportion to the height above water than as 8.7 to 1. But a berg such as that recorded by Captain Clark, 500 feet high and three miles long, must have had only 1-8-7 of its total thickness above water. The same remark applies also to the one seen by Captain Smithers, which was 580 feet high, and so large that it was taken for an island. This berg must have been 5,628 feet in thickness. The enormous berg which came in collision with the Royal Standard must have been 5,820 feet thick. It is not stated what length the icebergs 730, 960, and 1,000 feet high respectively were; but supposing that we make considerable allowance for the possibility that the proportionate thickness of ice below water to that above may have been less than as 8.7 to 1, still we can hardly avoid the conclusion that the icebergs were considerably above a mile in thickness. But if there are icebergs above a mile in thickness, then there must be land-ice somewhere on the southern hemisphere of that thickness. In short, the great antarctic ice-cap must in some places be over a mile in thickness at its edge.

Inadequate Conceptions regarding the Magnitude of Continental Ice. Few things have tended more to mislead geologists in the interpretation of glacial phenomena than inadequate conceptions regarding the magnitude of continental ice. Without the conception of continental ice the known facts connected with glaciation would be perfectly inexplicable. It was only when it was found that the accumulated facts refused to be explained by any other conception, that belief in the very existence of such a thing as continental ice became common. But although most geologists now admit the existence of continental ice, yet, nevertheless, adequate conceptions of its real magnitude are by no means so common. Year by year, as the outstanding facts connected with glaciation accumulate, we are compelled to

extend our conceptions of the magnitude of land-ice. Take the following as an example. It was found that the transport of the Wastdale Crag blocks, the direction of the striæ on the islands of the Baltic, on Caithness and on the Orkney, Shetland, and Faroe, islands, the boulder clay with broken shells in Caithness, Holderness, and other places, were inexplicable on the theory of land-ice. But it was so only in consequence of the inadequacy of our conceptions of the magnitude of the ice; for a slight extension of our ideas of its thickness has explained not only these phenomena,* but others of an equally remarkable character, such as the striation of the Long Island and the submerged rock-basins around our coasts described by Mr. James Geikie. In like manner, if we admit the theory of the glacial epoch propounded in former chapters, all that is really neces sary to account for the submergence of the land is a slight extension of our hitherto pre-conceived estimate of the thickness of the ice on the antarctic continent. If we simply admit a conclusion to which all physical considerations, as we have seen, necessarily lead us, viz., that the antarctic continent is covered with a mantle of ice at least two miles in thickness, we have then a complete explanation of the cause of the submergence of the land during the glacial epoch.

Although of no great importance to the question under consideration, it may be remarked that, except during the severest part of the glacial epoch, we have no reason to believe that the total quantity of ice on the globe was much greater than at present, only it would then be all on one hemisphere. Remove two miles of ice from the antarctic continent, and place it on the northern hemisphere, and this, along with the ice that now exists on this hemisphere, would equal, in all probability, the quantity existing on our hemisphere during the glacial epoch; at least, before it reached its maximum severity.

See Chapter XXVII., and also Geol. Mag. for May and June, 1870, and January, 1871.

CHAPTER XXIV.

THE PHYSICAL CAUSE OF THE SUBMERGENCE AND EMERGENCE OF

THE LAND DURING THE GLACIAL EPOCH.-Continued.

Extent of Submergence from Displacement of Earth's Centre of Gravity.Circumstances which show that the Glacial Submergence resulted from Displacement of the Earth's Centre of Gravity.-Agreement between Theory and observed Facts.-Sir Charles Lyell on submerged Areas during Tertiary Period.-Oscillations of Sea-level in Relation to Distribution.-Extent of Submergence on the Hypothesis that the Earth is fluid in the Interior.

Extent of Submergence from Displacement of Earth's Centre of Gravity.-How much, then, would the transference of the two miles of ice from the southern to the northern hemisphere raise the level of the ocean on the latter hemisphere? This mass, be it observed, is equal to only one-half that represented in our section. A considerable amount of discussion has arisen in regard to the method of determining this point. According to the method already detailed, which supposes the rise at the pole to be equal to the extent of the displacement of the earth's centre of gravity, the rise at the North Pole would be about 380 feet, taking into account the effect produced by the displaced water; and the rise in the latitude of Edinburgh would be 132 feet. The fail of level on the southern hemisphere would, of course, be equal to the rise of level on the northern. According to the method advanced by Mr. D. D. Heath,* the rise of level at the North Pole would be about 650 feet. Archdeacon Pratt's method† makes the rise still greater; while according to Rev. O. Fisher's method the rise would be no

Phil. Mag. for April, 1866, p. 323. + Ibid., for March, 1866, p. 172. Reader, February 10, 1866.

less than 2,000 feet. There is, however, another circumstance which must be taken into account, which will give an additional rise of upwards of one hundred feet.

The greatest extent of the displacement of the earth's centre of gravity, and consequently the greatest rise of the ocean resulting from that displacement, would of course occur at the time of maximum glaciation, when the ice was all on one hemisphere. But owing to the following circumstance, a still greater rise than that resulting from the displacement of the earth's centre of gravity alone might take place at some considerable time, either before or after the period of maximum glaciation.

It is not at all probable that the ice would melt on the warm hemisphere at exactly the same rate as it would form on the cold hemisphere. It is probable that the ice would melt more rapidly on the warm hemisphere than it would form on the cold. Suppose that during the glacial epoch, at a time when the cold was gradually increasing on the northern and the warmth on the southern hemisphere, the ice should melt more rapidly off the antarctic continent than it was being formed on the arctic and sub-arctic regions; suppose also that, by the time a quantity of ice, equal to one-half what exists at present on the antarctic continent, had accumulated on the northern hemisphere, the whole of the antarctic ice had been melted away, the sea would then be fuller than at present by the amount of water resulting from the one mile of melted ice. The height to which this would raise the general level of the sea would be as follows:

The antarctic ice-cap is equal in area to of that covered by the ocean. The density of ice to that of water being taken at 92 to 1, it follows that 25 feet 6 inches of ice melted off the cap would raise the general level of the ocean one foot, and the one mile of ice melted off would raise the level 200 feet. This 200 feet of rise resulting from the melted ice we must add to the rise resulting from the displacement of the earth's centre of gravity. The removal of the two miles of ice from the antarctic