686 DYNAMICAL GEOLOGY. 



Glaciers, as these facts show, are efficient means of widening and 

 deepening valleys ; and in this work the torrents of water they beget 

 take a prominent part. The thickness of the ice in the Alps nowhere 

 exceeds 500 feet. Let it be 2,000 feet, as now in some Greenland 

 glaciers, or twice this, as in many regions during the Glacier period, 

 and the work of erosion accomplished would be vastly greater, since it 

 is directly proportioned to the thickness. 



The snow and ice of Alpine valleys often cause, indirectly, violent 

 erosion and transportation of material, by damming up streams. In 

 no other way can barriers be thrown so readily across profound val- 

 leys ; and the deluges caused by the accumulated waters, when they 

 break loose, are often very destructive. The Alps are full of examples. 

 Again, the valleys are sometimes dammed up by great moraines, mak- 

 ing lakes ; and such lakes sometimes break through their barriers, and 

 flood the valley below with tearing waters.. 



4. Icebergs. 



A glacier on a sea-coast often stretches out its icy foot into the 

 ocean ; and, when this part is finally broken off, by the movement of 

 the sea, or otherwise, it becomes an iceberg. Greenland is the great 

 region of icebergs, no less than of glaciers. They carry away the 

 stones and earth with which the glacier was covered during its land- 

 progress, and transport them often to distant regions, whither they are 

 borne by the polar oceanic currents. 



Dr. Kane describes the great pack of icebergs that occupies the 

 centre of Baffin's Bay, and mentions that some were 300 feet high, 

 and large numbers over 200 feet. There were 280 icebergs of the 

 first magnitude (the most of them over 250 feet) in sight at one time. 



In the Antarctic, Captain Wilkes observed a long ice barrier, having 

 a height above the sea of 1 50 to 200 feet ; and some of the bergs 

 were 300 feet high. The ice of the barrier was stratified ; and, ac- 

 cording to Wilkes, this was owing to the constant increase from the 

 freezing mists over it. 



As the specific gravity of ice is 0-918 (at 32° F.), the proportion in 

 weight of the mass out of water is about one-twelfth. 



The icebergs of the Atlantic melt mostly about the Banks of New- 

 foundland, or between the meridians of 44° and 52°. They have been 

 observed in this ocean as far south as 36° 10'. 



Icebergs are (1) a means of transporting stones and earth from one 

 region to another (see p. 534). (2) When grounded on rocks, they 

 may scratch the surface ; but closely-crowded and regular scratches 

 like those of glaciers, over large areas, could hardly be made. The 

 currents of Baffin's Bay flow southward on the west side, and north- 



