74 STUDIES, SCIENTIFIC AND SOCIAL chap. 



We may take it, therefore, that the views of Charpentier, 

 Agassiz, and Sir Charles Lyell as to the extent and thick- 

 ness of the great Rhone glacier are admitted to be correct, 

 or, at least, not to be exaggerated, by the most strenuous 

 opponents of the extreme glacialists. We may, therefore, 

 use this as a fixed datum in our further investigations, and 

 I think it will be found to lead us irresistibly to conclu- 

 sions which in other cases these Avriters declare to be in- 

 admissible. 



Erratic Blocks in North America. 



We must now consider briefly the distribution of erratics 

 in North America, because they present some peculiar 

 features and teach us much concerning the possibilities of 

 glacier motion. 



An immense area of the North-eastern States, extending 

 south to New York, and then westward in an irregular line 

 to Cincinnati and St. Louis, is almost wholly covered with 

 a deposit of drift material, in which rocks of various sizes 

 are embedded, while other rocks, often of enormous size, 

 lie upon the surface. These blocks have been carefully 

 studied by the American geologists, and they present us 

 with some very interesting facts. Not only are the dis- 

 tances from which they have been transported very great, 

 but in many cases they are found at a greater elevation 

 than the place from which they must have come. Pro- 

 fessor G. F. Wright found an enormous accumulation of 

 boulders on a sandstone plateau in Monroe County, Penn- 

 sylvania. Many of these boulders were of granite, and 

 must have come either from the Adirondack Mountains 

 200 miles to the north, or from the Canadian Highlands 

 still farther away. This accumulation of boulders was 70 

 or 80 feet high, and it extended many miles, descending 

 into a deep valley 1,000 feet below the plateau in a nearly 

 continuous line forming part of the southern moraine of 

 the great American ice-sheet. 



On the Kentucky hills, about twelve miles south of Cin- 

 cinnati, conglomerate boulders containing pebbles of red 

 jasper can be traced to a limited outcrop of the same rock 



