582 THE ICE AGE IN NORTH AMERICA. 



2. The till over this attenuated border is a mixture of 

 fresh and oxidized material at all levels, showing that most 

 of the oxidization preceded the glaciation, and that not suffi- 

 cient time has elapsed since for the oxidization of the fresh 

 material picked up by the glacier. This statement is based 

 upon the examination of sections miles in length; when it 

 everywhere appears that there is such a mixture of fresh 

 material with oxidized material that the conclusion is irre- 

 sistible that it was one movement which brought both. 



For example, in the vicinity of Warren and south of Oil 

 City at an elevation of 380 feet above the Allegheny River 

 numerous pebbles were found, both of sedimentary and of 

 granitic rocks, which had evidently been oxidized nearly to 

 their center before starting on their journey from Canada, 

 but had been planed down on one side so as almost to expose 

 the core on that side, \v hile leaving the oxidized layers undis- 

 turbed on the other. Some of these described were five inches 

 or more in diameter and had been rotted so that only ao 

 inch or more of fresh nucleus remained; while in some cases 

 the unoxidized core was exposed through the glacial erosion 

 of one side. These instances were numerous. Mr. Williams 

 informs me that "one striking peculiarity in those with joint 

 planes through which the water could readily reach the 

 center, was that the relative permeability of the mass from 

 its different sides did not have the slightest influence on the 

 position of the fresh nucleus. It was as often nearer the side 

 whence water could most easily enter than to any other side. 

 This impressed me greatly," he says, "as an indication of the 

 extreme recency of the final shaping as with time the relative 

 porosity of the various sides would tend to bring the re- 

 maining nucleus under the usual law as to position." 



I am permitted, also, to use the following extract from 

 Professor William's unpublished notes, in which he sums up 

 some most significant facts concerning the Kansan advance : 



"The glaciated outcrops in the east [in Pennsylvania] are 



