Ch. XI] 



ROCKS DRIFTED BY ICE. 



127 



These stones themselves also are often furrowed and scratched on more 

 than one side. 



In explanation of such phenomena I may refer the student to what was 

 said of the action of glaciers and icebergs in the Principles of Geolooy 

 (ch. xv.). It is ascertained that hard stones, frozen into a moving mass of 

 ice, and pushed along under the pressure of that mass, scoop out long 

 rectilinear furrows or grooves parallel to each other on the subjacent 

 solid rock. (See fig. 109.) Smaller scratches and striae are made on 



Fig. 109. 



Limestone polished, furrowed, and scratched by the glacier of Bosenlaui, in Switzerland. (Agassiz.) 



a a. White streaks or scratches, caused by small grains of flint frozen into the ice. 

 b 6. Furrows. 



the polished surface by crystals or projecting edges of the hardest min- 

 erals, just as a diamond cuts glass. The recent polishing and striation 

 of limestone by coast-ice carrying boulders even as far south as the coast 

 oi Denmark, has been observed by Dr. Forchhammer, and helps us to 

 conceive how large icebergs, running aground on the bed of the sea, may 

 produce similar furrows on a grander scale. An account was given so 

 long ago as the year 1822, by Scoresby, of icebergs seen by him drifting 

 along in latitudes 69° and 70° N., which rose above the surface from 

 100 to 200 feet, and measured from a few yards to a mile in circumfer- 

 ence. Many of them were loaded with beds of earth and rock, of such 

 thickness that the weight was conjectured to be from 50,000 to 100,000 

 tons * A similar transportation of rocks is known to be in progress in 

 the southern hemisphere, where boulders included in ice are far more 

 frequent than in the north. One of these icebergs was encountered in 

 2839, in mid-ocean, in the antarctic regions, many hundred miles from 

 any known land, sailing northwards, with a large erratic block firmly 



* Voyages in 1822, p. 233. 



