THE ICE AGE AND ITS WORK. 687 



barriers, the tenacity with which it held its armature, and withal the pressure 

 that both forced it into compliance with its tortuous channel, and pressed it 

 relentlessly forward. * 



Kelly's Island is at the western end of Lake Erie, and in the 

 direction of the striae to the northeast there is no high ground for 

 about four hundred miles. Looking at these facts, I can not give 

 any weight to the opinions of those who, from observations of 

 existing glaciers, declare positively that ice can not go up-hill, and 

 can exert no grinding power on level ground. 



4. Erratic blocks were among the phenomena that first attract- 

 ed the attention of men of science. Large masses of granite and 

 hard metamorphic rock, which can be traced to Scandinavia, are 

 found scattered over the plains of Denmark, Prussia, and northern 

 Germany, where they rest either on drift or on quite different 

 formations of the Secondary or Tertiary periods. One of these 

 blocks, estimated at fifteen hundred tons weight, lay in a marshy 

 plain near St. Petersburg, and a portion of it was used for the 

 pedestal of the statue of Peter the Great. In parts of North 

 Germany they are so abundant as to hide the surface of the 

 ground, being piled up in irregular masses forming hills of granite 

 bowlders, which are often covered with forests of pine, birch, and 

 juniper. Far south, at Fiirstenwalde, southeast of Berlin, there 

 was a huge block of Swedish red granite, from one half of which 

 the gigantic basin was wrought which stands before the New 

 Museum in that city. In Holstein there is a block of granite 

 twenty feet in diameter; and it was noticed by De Luc that the 

 largest blocks were often found at the greatest distance from the 

 parent rock, and that this fact was conclusive against their hav- 

 ing been brought to their present position by the action of floods. 



It is, however, in Switzerland that we find erratic blocks which 

 furnish us with the most conclusive testimony to the former 

 enormous extension of glaciers ; and as these have been examined 

 with the greatest care, and the facts, as well as the main induc- 

 tions from the facts, are generally admitted by all modern writers, 

 it will be well to consider them somewhat in detail. It will be 

 found that they give us most valuable information both as to the 

 depth and extension of ancient glaciers, and also to the possibili- 

 ties of motion in extensive ice-sheets. 



The most important of these facts relate to the erratic blocks 

 from the higher Alps, which are found on the flanks of the Jura 

 Mountains wholly formed of limestone, on which it is therefore 

 easy to recognize the granites, slates, and old metamorphic rocks 

 of the Alpine chain. These erratic blocks extend along the Jura 



* Seventh Annual Keport of the United States Geological Survey, p. 179. Arrange- 

 ments have now been made for the preservation of these remarkable examples of ice-work. 



