THE ERRATIC PHENOMENA. 411 



ice around the north pole, having, no doubt, extended their influence 

 over the temperate zone, and probably produced, in high mountain 

 chains, as the Alps, the Pyrenees, the Black Forest, and the Vosges, 

 such accumulations of snow and ice, as may have produced the erratic 

 phenomena of those districts. But extensive changes must have 

 taken place in the appearance of the continents over which we trace 

 erratic phenomena, since we observe in the Old World, as well as in 

 North America, extensive stratified deposits containing fossils which 

 rest upon the erratics ; and as we have all possible good reasons and 

 satisfactory evidence for admitting that the erratics were transpoi-ted 

 by the agency of terrestrial glaciers, and that therefore the tracts of 

 land over which they occur, stood at that time above the level of the 

 sea, Ave are led to the conclusion that these continents have subsided 

 since that period below the level of the sea, and that over their 

 inundated portions animal life has spread, remains of organized beings 

 have been accumulated, which are now found in a fossil state in the 

 deposits formed under those sheets of water. 



Such deposits occur at various levels in different parts of North 

 America. They have been noticed about Montreal, on the shores of 

 Lake Champlain, in Maine and also in Sweden and Russia ; and, 

 what is most important, they are not everywhere at the same absolute 

 level above the surface of the ocean, showing that both the subsidence, 

 and the subsequent upheaval which has again brought them above 

 the level of the sea, have been unequal ; and that we should there- 

 fore be very cautious in our inferences respecting both the continental 

 circumstg,nces under which the ancient glaciers were formed, and also 

 the extent of the sea afterward, as compared with its present limits. 



The contrast between the unstratified drift and the subsequently 

 stratified deposits is so great, that they rest everywhere unconform- 

 ably upon each other, showing distinctly the difference of the agency 

 under which they were accumulated. This unconformable superposi- 

 tion of marine drift upon glacial drift is also beautifully shown at the 

 above mentioned locality near Cambridge. (See Diagram.) In this 

 case the action of tides in the accumulation of the stratified materials 

 is plainly seen. 



The various heights at which these stratified deposits occur, above 

 the level of the sea, show plainly, that since their accumulation, the 



