74 SURFACE GEOLOGY. 



nologically, we might divide this formation into the following periods ; which, 

 however, must not be understood as completely isolated from one another, but 

 only as marking the times when certain phenomena predominated. 



1. The Period of unmodified Drift. 



2. The Period of Beaches, Osars, Submarine Ridges, and Sea Bottoms. 



3. The Period of Terraces. 



4. The Historic Period, or the Period of Deltas and Dunes. 

 Lithologically, Alluvium may be subdivided as follows : 



1. Drift unmodified, embracing angular and rounded boulders, gravel, sand, and 

 cloy. 



2. Modified Drift, embracing the following forms : 



1. Beaches, ancient and modern. 



2. Osars. 



3. Submarine Ridges. 



4. Sea Bottoms and Lake and River Bottoms. 



5. Terraces. 



6. Deltas. 



7. Dunes. 



8. Moraines. 



Such views, essentially, have been advanced by previous writers of great ability. 

 Thus, Sir Charles Lyell groups together all the strata above the tertiary, under 

 the name of Post-Pliocene, of which the Recent embraces the deposits coeval with 

 man, and Drift, those anterior to man. We find, also, that the eminent palaeonto- 

 logist and geologist, M. Alcide D'Orbigny, in his Cours Elementaire de Palcebntologie 

 et Oeologie, comprehends in his Terrains Gontemporains, ou Epoque actuelle, every 

 thing above the tertiary. Still more specifically like my own, are the views 

 expressed by William C. Redfield, Esq., at the meeting of the American Associa- 

 tion for the advancement of Science, at Cambridge, in 1850. He remarked, " that 

 the phenomena of the boulders and drift should be attributed to mixed causes, and 

 that the theories which refer these phenomena to the several agencies of glaciers, 

 icebergs, and packed ice, are, in truth, more nearly coincident than is commonly 

 imagined." I understand M. Desor, also, who has had opportunities for examining 

 drift phenomena, not inferior to those of any man living, as inclined to similar 

 views. He supposes that " the surface boulders, like many of those buried in the 

 drift, clay, and sand, have been transported by the floating ice :" and he says that 

 since " glaciers in our days occur chiefly in the valleys of the highest mountain 

 chains, it is difficult to conceive how they could exist and move in a wide and 

 level country like the northern parts of the United States and Canada." (Foster 

 and Whitney's Report, p. 215.) 



45. Such are the results to which I have been conducted by the facts respecting 

 surface geology which have fallen under my notice. I am aware that these are 

 subjects of great difficulty, and that I am in conflict with the views of eminent 

 geologists on several points; as I am, indeed, with my own opinions, as held 

 several years ago. And yet for a long time I have stood chiefly aloof from the 



