104 LAKE SUPERIOR. 



of the globe, and always having the same general direction. Its limits in 

 elevation, as ascertained on the sides of mountains, is about five thousand 

 feet above the sea. At about this height on Ben Nevis in Scotland, and on 

 Mt. Washington in New Hampshire, the grooving and polishing ends. Be- 

 low this level the whole northern surface of the earth as a general thing 

 shows the marks of this agency. Some geologists attribute these effects to 

 the action of currents. But currents extending over such a vast extent of 

 the earth's surface must necessarily have been ocean currents, and these 

 must have brought with them marine animals, of the existence of which no 

 traces have been found. Moreover such extensive currents in one direction 

 could not have existed : there would necessarily have been refluxes and 

 counter-currents. 



These and other difficulties have led me to attribute these effects to an- 

 other cause. It has been ascertained that the glaciers of Switzerland formerly 

 extended much farther than at present, reaching, without interruption, to 

 the vicinity of Paris, and, near their origin, to the height of nine thousand 

 feet above the sea. Similar indications are to be found in all the mountain 

 chains of Great Britain, and in various parts of Europe. Now at the time 

 when such glaciers existed in Europe, the temperature must have been 

 much lower than at present. The mean annual temperature of Switzerland 

 must have been 15° Fah. below the present. That such a depression of 

 temperature actually took place is also indicated by other facts. Thus the 

 fossils found in the glacial moraines are of an arctic character, and shells of 

 the German Ocean are found in the moraine gravels of Sicily. This, how- 

 ever, is inconceivable without a corresponding depression all over the globe. 

 Now if we suppose the mean annual temperature of this country to be 

 reduced to 26° Fah., it would naturally be covered to a considerable depth 

 with ice, which would move from north to south. Such a mass of ice mov- 

 ing over the country would produce these effects of rounding and scratching 

 the rocks, and would remove the soil, except from the depressions. It is 

 sometimes objected to this theory that we have here no slope which should 

 cause such a mass of ice to move onward. But it is not necessary that 

 there should be any slope in order that a glacier should move. In the 

 Swiss glaciers the motion is often slowest on the steepest part of the slope, 

 and some glaciers of 7° inclination move faster than others with a slope of 

 40°. The great motive force is not tlie gravitation of the mass, but the 

 pressure of the water infiltrated into it. Then supposing the country to have 

 been subsequently depressed, (as we see has been the case in Sweden and 

 Norway, where marine shells have been found at the height of three or four 

 hundred feet above the level of the sea,) and afterwards raised again, these 



