458 T. F. Jamieson — Oscillation of Land in Glacial Period. 



tlie sarae train of events. First a great covering of ice, then a 

 submergence beneath the sea, during which the glacier vanishes, and 

 after that the land conies up again. 



North Abierica. 



The lower part of the Mississippi basin, as described by Hilgard 

 (American Journ. of Science, vol. xlvii. p. 77, 1869), seems to afford 

 good evidence of a depression having taken place in the northern 

 part of that valley after the commencement of the Glacial Period. 

 The lowest bed he calls the Orange Sand, which he says bears 

 evidence of having been formed by a rapid current of water flowing- 

 down the valley. The mineral character of the pebbles which it 

 contains shows that the material came from the northward. Above 

 tlie Orange Sand there are swamp and lagoon deposits, and heavy 

 beds of fine silt or loess, indicating, according to Hilgard, a period 

 of slow secular subsidence. In these beds there are no marine 

 fossils ; the organic remains are of terrestrial and freshwater animals. 

 Now these features are quite intelligible on the supposition that the 

 weight of the great glacier which lay on the region to the north- 

 ward caused a slow and gradual depression of the area beneath it, 

 and thus lowered the gradient of the river and led to a more sluggish 

 flow of the water, so that it was no longer able to carry along gravel 

 and pebbles, but stagnated in pools and marshes. Lastly the 

 denudation of the Loess shows that when the ice disappeared, the 

 northern area I'ose again, but not to so gi'eat a height as it had before, 

 during the time of the Orange Sand. 



American geologists also seem to think that it is impossible to 

 account for the freshwater drift and terraces along the great lakes 

 without supposing a depression of that region to have taken place 

 after the commencement of the Grlacial period ; a depression which 

 seems to have been greatest on the northern side of the lakes and to 

 have diminished to the south of them.' It is evident Dr. Croll's 

 theory can give us no help here nor in the Mississippi Valley, for 

 there is no trace of the presence of the sea. 



Along the borders of the lakes there are terraces of finely strati- 

 fied clay and sand, many of which lie at a much greater height than 

 the wide stretch of low ground to the southward, which divides 

 these lake-basins from the head-waters of the Mississippi and Ohio. 

 The country fi"om Lake Erie round the southern border of Lake 

 Michigan consists of a wide extent of low land or plain opening into 

 the basin of the Mississippi and Ohio, so that were we to dam up 

 the present outlet of these lakes to the eastward, and raise the water 

 to the level of these upper terraces, it would flow freely away by 

 these wide openings down to the Gulf of Mexico. At first it was 

 supposed by Desor, and other geologists who led the way in explor- 

 ing the district, that these beds were marine, and that the waters of 

 the Gulf of Mexico had formerly extended up the valley of the 

 Mississippi into the heart of the continent ; but the progress of 



1 Dana says the terraces oa the north side of Lake Ontario are very much higher 

 than those on the south side. See his " Manual of Geology," 2nd ed. p. 552. 



