290 G. F. WRIGHT POSTGLACIAL EROSION AND OXIDATION 



innumerable cases where water-worn Canadian pebbles had evidently 

 been oxidized inwards until a core of unoxidized material remained in 

 the middle, when afterwards they had been taken up by the ice and in 

 the process of transportation glaciated on one side until the unoxidized 

 core was almost or entirely exposed on the glaciated side, while the oxi- 

 dized portion retained its original thickness on the other side. These 

 pebbles therefore indicate that the oxidation had taken place mainly in 

 pre-Glacial times, proving that it is no criterion touching the time of its 

 transj^ortation by the glacier. 



Again, the amount of postglacial oxidation of the glaciated surfaces, 

 even over the area covered by the Kansas invasion, is in many instances 

 so small as to forbid the supposition of the enormous lapses of time 

 which are currently made concerning the date of that invasion. In my 

 original observations upon the glacial deposits in Saint Louis, Missouri, 

 which are certainly as old as any, I found extensive limestone areas 

 freshly uncovered near the Botanic Gardens which still retained glacial 

 markings on their surface where not deeply eroded by chemical action. 

 In the enormous lengths of time usually supposed to have elapsed since 

 this glacial action the thin beds of limestone would have entirely disap- 

 peared. 



Again, Professor Williams' observations upon the mammoth coal vein 

 near Pottstowm, Pennsylvania, where its surface had been glaciated in 

 close proximity to the unglaciated area, disclosed the fact that, though 

 the superincumbent soil was easily permeable to water and its eroding 

 acids, the glaciated surface had not been eaten away to any appreciable 

 extent, whereas a short distance beyond, in the unglaciated area, the coal 

 was rotted to a depth of several feet. 



I quote from a private communication, giving unpublished results of 

 Professor Williams' observations on the attenuated border in Pennsyl- 

 vania : 



"The outcrop of Oriskniiy sandstone north of the Blue Ridge shows the 

 coarse-grained variety cemented by calcite and carrying t^tjirifcr arcnosu and 

 the cherty band. 



"[But] the pebbles of the former can be traced through the moraine of 

 Lewis and Wright and across the great valley of Pennsylvania ; thence over 

 the South Mountain, being carried from 25'0 above tide over the summit at 900 

 above tide and down the south slope to and half way across the Saucon Valley 

 (average elevation. 400 above tide). These pebbles are mingled with local 

 material, mostly angular. The local stuff is weathered and rotten; but tJtere 

 is absolutely no (liffercncc in color, iccathcrinfh or decree of (lisintvijnition 

 'beticeen the pebbles found on and south of South Mountain and those found 

 in the moraine and even in the Lehigh at the foot of the outcrcops. 



