Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/931

 10. Itifliarsuk in about N. lat. 70 52

11. Innerit „ 70 56

12. Great Kangerdlursoak „ 71 25

13. Upernivik „ 72 57 1

We have bow sketched the ice-field with the glacier and the iceberg. Are there no other defluents of the "inland ice"? This leads us to speak of : —

4. The Subglacial Stream. — What is under the inland ice is, I fear, a question we shall never be able to answer. No doubt the country is undulating ; for, I believe, this immense glaciation overspread the country after the close of the Tertiary period, perhaps about the same period when Scotland lay under the ice cap. Continuously grinding over these rocks, a creamy mud must be formed, which mud must now be of considerable thickness if not swept into hollows or washed out from beneath the ice. In the Alps the glacier is said to wear for itself a muddy bed, which Agassiz 2 calls la couche de boue or la boue glaciaire, and other authors la moraine profonde (fig. 1, b) ; so that, I think, there can be little doubt that the Greenland inland ice has triturated down a similar clayey bed. However, another instrument in the arrangement and, if I may use the term, "utilization" of this mud, this moraine profonde, comes into play. Rink 3 has calculated the yearly amount of precipitation in Greenland in the form of snow and rain at 12 inches, and that of the outpour of ice by its glaciers at 2 inches. He considers that only a small part of the remaining 10 inches is disposed of by evaporation, and that the remainder must be carried to the sea in the form of subglacial rivers. These subglacial rivers are familiar in all alpine countries, and in Greenland pour out from beneath the glacier, whether it lies at the sea or in a valley, and in summer and winter. He also mentions a lake adjacent to the outfall of a glacier into the sea, which has an irregularly intermittent rise and fall. " Whenever it rises, the glacier-river disappears ; but when it sinks, the spring bursts out afresh," — showing, as he thinks, a direct connexion between the two. Arguing from what has been observed in the Alps, he concludes that an amount of glacier- water equivalent to 10 inches of precipitation on the whole surface of Greenland is not an extravagant hypothesis ; and he accounts for its presence partly by the transmission of terrestrial heat to the lowest layer of ice, and partly by the fact that the summer heats are conveyed into the body of the glacier, while the winter cold never reaches it. The heat melts the surface-snow into water, which percolates the ice, while the cold penetrates a very inconsiderable portion of the glacier, whose thickness exceeds 2000 feet. As in the alpine glaciers, these subglacial rivers are thickly loaded

1 Rink : Om den geographiske beskaffenhir af den Danske Handels districter,

i Nord Gronland (Afskrf. af Vidensk. Selskab. Skr. 5 B-3 b), et lib. cit.

2 Etudes sur lea Glaciers et Systeme Glaciaire, p. 574.

3 Naturhistorisk Tidsskrift, 3rd series, vol. i. part 2 (1862), and Proc. Roy. Geogr. Soc. vii. 76.

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