PRINCIPLES OF DISCRIMINATION OF AQUEOUS DEPOSITS. 479 



terraces are radically different from shore terraces. Again, the terraces of 

 static water must necessarily be horizontal, and must remain so except as 

 crust flexures distort them. Now, in the Monongahela valley, as was said 

 many years ago by Professor Stevenson, the terraces decline from the south 

 towards the north. The terraces on the Allegheny river slope south towards 

 Pittsburgh. Those of the Monongahela slope north towards Pittsburgh. Now. 

 this is just what we should expect in the case of rivers, but not in the case 

 of lakes. Some of these terraces are rocky shelves, as long ago shown by 

 Professor Stevenson and Professor Chance, who have put correct interpreta- 

 tions upon the phenomena. These rocky shelves extend sometimes nearly 

 half a mile back. Below Pittsburgh one is described by Professor Wright, 

 in exemplification of the submergence theory just mentioned. He states that 

 the shelf is cut back half a mile in the rock. Now, imagine the time requisite 

 for the cutting back of half a mile on one side of the river yet practically 

 nothing on the other side! Again, take the case where a valley passes off 

 among the hills and returns, forming an " ox-bow." Here we have phe- 

 nomena that do not lend themselves at all to the lacustrine hypothesis. And 

 so, again, if you turn to the material it will be found to be of the kind 

 produced by onward-moving water rolling the pebbles over and over again, • 

 rather than by a to-and-fro action which slides 'the pebbles and gives a dif- 

 ferent form. The discrimination is not as sharp and clear as in the other 

 case, but is still capable of being made. There are other facts lying in the 

 same line. 



I am unable to discuss the evidences of the submergence about Belvidere, 

 because I did not see such evidences. Some of the later terraces are made 

 up of well-rounded fresh gravel, without any depth of silt upon it. Now, 

 if these had been submerged, the greater part of the silt would have been 

 on these gravels and the moraine itself. All of these terraces are evidences, 

 it seems to me, of land conditions since the formation of the later glacial 

 deposits. 



Professor White : I agree perfectly with President Chamberlin that these 

 benches which slope downward were the result of erosion, but I claim that 

 subsequent to the erosion of these benches all of them were covered with 

 lacustrine deposits. The proof of this is found in the fact that along the 

 Monongahela and its tributaries there is at the summit of this lacustrine 

 level a deposit of clays and bowlders and erosion debris of every description, 

 beginning at 1,100 feet above sea level and extending down to the present 

 flood-plain. Now, on the Baltimore and Ohio railway there is, it seems to 

 me, an absolute proof of this submergence, because the old valley slopes there 

 on the one hand into the Ohio, and on the other into the Monongahela, and 

 yet the the summit has thirty feet of a clay deposit; and on this summit, 

 and on other tributaries of the Monongahela, these clay deposits cease at 



