2 9 o THE POPULAR SCIENCE MONTHLY. 



with the glacier theory of M. Agassiz, Mr. Lyell's eminent authority 

 would not permit the total extinguishment of the iceberg theory, and 

 his generalizations have, perhaps, had more influence in directing the 

 efforts of others in such reconciliation than the writings of any other 

 man. It required but very slight changes in Mr. Lyell's method of 

 dividing the history of the drift to evolve, in its present aspect, the 

 latest theory of geologists touching the origin of the drift-deposits. 

 Mr. Murchison, of England, coincides with Mr. Lyell in the sub- 

 mergence, or iceberg theory. Mr. J. D. Dana advocates the glacier 

 theory in its fullest extent ; but, although adopting also the term 

 Champlain, he is far from admitting the recent enlargement of that 

 epoch, so as to bring the continent beneath the water of the ocean as 

 required by the supporters of the combination theory. 



Professors E. W. Hilgard, of Mississippi, and J. S. Newberry, of 

 Ohio, are among the most prominent advocates in this country of this 

 new theory, resulting from the combination of the glacier and the ice- 

 berg theories. Dr. A. Winchell, of New York, also advocates the same. 

 It is as follows : 



First. The glacier epoch proper. 



During this epoch the continent was considerably elevated above 

 its present level, especially in the north. This either produced, or was 

 accompanied by, a greater degree of cold, the effect of which was to 

 bring over the continent the vast sheets of ice in the form of con- 

 tinental glaciers, required by the hypothesis of Prof. Agassiz. During 

 this epoch the rocks were scored, and many deep valleys were exca- 

 vated. Large bowlders were transported to regions farther south. 



Second. The submergence of the continent, attended by an ame- 

 lioration of the climate and the disappearance of the glaciers, or their 

 retreat to the far north. The assortment and stratification of the drift, 

 produced by the glaciers, and the deposition of the great mass known 

 as Erie clay, and other clayey portions of the drift-sheet. This con- 

 dition of the continent was attended by the appearance of numerous 

 icebergs which floated over the submerged land, and aided to trans- 

 port the coarse drift, according to the hypothesis of Peter Dobson. 



Third. The emergence of the continent with a halting progress, pro- 

 ducing terraces and ridges marking the ancient levels of the ocean. 



These three steps have been named by Prof. Dana, in their order, 

 the Glacial Epoch, the Champlain Epoch, and the Terrace Epoch. 



The studies of Professors Agassiz and Tyndall on the glaciers of 

 the Alps, and of Dr. Kane on those of Greenland, have so fully demon- 

 strated the adequacy of glaciers to produce all the effects attributed 

 to them by the theory of Agassiz, that it is now very generally ad- 

 mitted that, wherever those phenomena are seen, glaciers must have 

 existed. 



Geology, having demonstrated thus the necessity for a period of 

 cold, to account for the phenomena of the surface of the earth,, labored 



