Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/939

 vity it contains 5-3/4 oz. (avoird.) of salt to every gallon of 231 cubic inches, and freezes at 28-1/2 Fahr. The specific gravity of this ice is about 0.873. To enter upon this subject, of which the above is only the summary of a long series of experiments, is foreign to the object of this paper. From this bay-ice is formed the floe, from the floe the pack-ice and other forms familiar to arctic navigators. In the summer the ice in Davis Strait on either side breaks up sooner than that in the middle of the strait, which remains for a considerable time, forming the " middle ice " of the whalers. Still, however, a narrow belt remains attached to the shore during a considerable portion of the summer. This is called by the Danes in Greenland the " iis fod," and by the English navigators the " ice- foot." As the spring and summer thaws proceed, land-slips occur, and earth, gravel, and avalanches of stones come thundering down on the ice-foot, there to remain until it breaks off from the coast and floats out to sea with its raft-like load of land-debris. As the summer's long sunlight goes on, the ice, worn by the sea, parts with its load ; and this may be shortly after its leaving the land, or it may float tolerably far south. The ice-foot, however, rarely carries its load as far south as the mouth of Davis Strait ; and sea-ice is seldom seen far out of the arctic regions, while, as we all know, bergs often float far into the Atlantic. Often fields of ice will float along and, like icebergs, graze the surface of rocks only a wash at low tides ; and therefore its action might be mistaken for that of icebergs or land-ice. In other cases I have known the ice-foot, laden with debris, to be driven up by the wind and high tides on to low-lying islands, spits, and shores, piling them with the load thus carried from distant localities, so that blocks of trap from the shores of Disco or Waygatz might be drifted up on the beach at Cumberland Sound or on the syenitic shores of South Greenland.

It has even been found that in shallowish water the ice will freeze to the bottom of the sea ; and in such situations the gravel, blocks, &c, there lying will freeze in and be carried out to sea, to be deposited in course of time in a manner similar to the superincumbent loads of the ice-foot, though more speedy. The same phenomenon holds good of the Baltic. In the Sound, the Great Belt, &c. the ground-ice often rises to the surface laden with sand, gravel, stones, and sea-weed. Sheets of ice, with included boulders, are driven up on the coasts during storms and " packed " to a height of 50 feet. How easily such sheets of ice, with included sand, gravel, or boulders, may furrow and streak rocks beneath may be imagined 1. The patches of gravel on the pack-ice are owing, I think, to portions of the gravel-laden ice-foot having got among the ordinary materials of the pack ; for I do not think that ice formed in deep water, unless when it passes over rocks, and therefore may take up fragments of stone or earth, has any geological significance.

The conclusions which we are forced to draw from what I have said regarding the depositing-power of glacier-streams, bergs, and sea- ice must be : — 1. That the bottom of Davis Strait must be composed

1 Forchhammer in Lyell's ' Principles,' pp. 231-232.