Page:Popular Science Monthly Volume 44.djvu/703

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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 attracted 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 Fürstenwalde, 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 having 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 inductions 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 possibilities 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