25 G RECORD OF SCIENCE FOR 1887 AND 1888. 



matters, President Cbamberlin.* These American observations agree 

 with those made ou the other side of the northern hikes and the forty- 

 ninth parallel 5 and it is now possible to ma|> approximately the area 

 glaciated during the Pleistocene, and to represent moreover the form 

 and direction of movement of each principal lobe and tongue of the 

 Mer de Glace during some i)eriod of its existence. 



The last-named geologist has also recently described the phenomena 

 of the Driftless area of the Upper Mississippi — a tract of over 15,000 

 square miles, which was completely encompassed, though never over- 

 swept, by the ice, and atone time partly or wholly submerged beneath 

 the waters of an ice-bound lake; and from moraines, a<j[ueo-glacial grav- 

 els, and other products of the combined action of water and ice found 

 about the periphery of the tract, he has deduced a history of the Pleisto- 

 cene more extended and refined than any previously recorded. The 

 epochs in this history are: (1) The Tiansition Epoch, not yet satisfac- 

 torily distinguished from the Pliocene ; (2) the Earlier Glacial Epocli, 

 comjirising an episode of glaciation, an ei)isodeofdegiaciatiou and veg- 

 etal accumulation, and a second episode of glaciation accompanied by 

 deposition of the loess ; (3) the Chief Interglacial Epoch, with vegetal 

 accumulation, etc.; (4) the Later Glacial Epoch, comprising at least 

 three episodes of glaciation and moraine-building and two of deglacia- 

 tion and vegetal accumulation ; (o) the Champlain Epoch of marine 

 and lacustral sedimentation ; and (G) the Terrace Epoch, which grad- 

 uates into the present, t 



The history of the Pleistocene and of its relations to the Tertiary has 

 recently been read in a slightly different way from a different series of 

 dej)Osits (including the Columbia formation) by McGee: The geologic 

 history recorded in the Columbia deposits and terraces and in the ero- 

 sion and alteration which both have suffered is almost wholly supple- 

 mentary to that read by most geologists in the later glacial deposits, 

 and multiplies many times the length of the Pleistocene as commonly 

 conceived. Collectively the two series of deposits indicate that the 

 Pleistocene consisted of two, and only two, great epochs of cold (the 

 later comprising two or more sub-epochs); that these epochs were sej)- 

 arated by an interval three, five, or ten times as long as the post-glacial 

 interval ; that the earlier cold endured much the longer; that the ear- 

 lier cold was the less intense, and the resulting ice sheet stopped short 

 (in the Atlantic slope) of the limit reached by the later; that the ear- 

 lier glaciation was accompanied by much the greater submergence, ex- 

 ceeding 400 feet at the mouth of the Hudson and extending 500 miles 

 southward, while that of the later reached but a tithe of that depth or 

 southing; and that during the long interglacial interval the condition 

 of land and sea was juuch as at present. | 



* 7tli Ann. Rep. U. S. Geol. Surv. 1888, pp. 155-248. 



t 6th Ann. Rep. U. S. Geol. Survey, p. 21-2. 



t Am. Jour. Sci., 1888, 3d series, vol. xxxv, p. 465. 



