the Middle Atlantic Slope. 461 



contemporaneous erosion. Space would not permit discussion 

 of the data upon which the conflicting opinions rest, even if 

 discussion were desirable ; but the difference of view is simply 

 an effect of intellectual perspective which shows to each inves- 

 tigator in exaggerated proportion the phenomena which he 

 has most closely scrutinized, and will disappear with continued 

 observation. 



The juxtaposed sections are exhibited in the accompanying 

 table. 



It may be pointed out that the succession of deposit in the Mis- 

 sissippi Yalley, as interpreted in the third column of the table 

 coincides almost exactly with the sequence recognized by 

 Penck in the German Alps, where the succession in historic or- 

 der is (1) glacial drift, (2) an enormous accumulation of torren- 

 tial gravels, now commonly ferruginated, cemented and deeply 

 eroded, (3) a second glacial deposit, (4) a less accumulation of 

 torrential gravels, with alluvium, laminated clays, lignite, etc., 

 and (5) a third glacial deposit, found only at considerable alti- 

 tudes in the mountains.* 



B-ecapitulation. — In . short the Columbia formation under- 

 lies and is several times older than the moraine-fringed drift- 

 sheet of northeastern United States ; it is apparently the aqueo- 

 glacial margin of a drift-sheet largely concealed or obliterated 

 in the northern Atlantic slope ; it appears -to be equivalent to 

 the lower lake beds of the Great Basin, to the basal till and 

 " gumbo" of Missouri, to (probably) the Port Hudson of Mis- 

 sissippi, and to (perhaps) the lowest glacial deposits of the 

 Alps ; and while the vertebrates of its correlatives suggest that 

 it is Pliocene, both stratigraphy and invertebrate fossils prove 

 that it is Quaternary. 



It should be added that the conjoined phenomena of the 

 Middle Atlantic slope and the Mississippi Yalley indicate the 

 respective areas of the earlier and later ice sheets : In the east 

 the earlier extended the farther as shown by the superposition 

 of the newer moraine upon the older aqueo-glacial deposits ; 

 but whether the earlier glacier formed no moraine, whether 

 there was an earlier moraine perhaps coincident with the drum- 

 lin zone passing through central Massachusetts and New York, 

 or whether the earlier drift was obliterated by the later glacia- 

 tion, remains to be determined. In Iowa, Missouri and south- 

 ern Illinois, on the other hand, the earlier ice sheet, extended 

 fully one hundred miles farther southward than the later, and, 

 having evidently terminated in the waters of the expanded 

 Gulf or of an .inland lake, its limit is not marked by a terminal 

 moraine. A hypothetic explanation of this discrepance, based 

 * Die Vergletscherung der Deutschen Alpen, 1882, 239, Tabelle II, etc. 



