3M POPULAR SCIENCE MONTHLY 



cient, and to produce such as were would necessitate a period of cold 

 sufficient for his hypothetical polar ice-cap. He pointed out that the 

 northern erratics were rounded and widespread; that the highest hills 

 were scratched and polished to their summits, while to the south the 

 mountain tops had protruded above the ice-sheet and supplied the 

 glaciers with their load of angular boulders. He also called attention 

 to the absence of marine or fresh-water shells from the ground moraine 

 deposits, showing that it was not subaqueous. 



Eef erring to the stratified deposits overlying the drift, he wrote: 



The various heights at which these stratified deposits occur above the level 

 of the sea show plainly that since their accumulation the mainland has been 

 lifted above the ocean at different rates in different parts of the country; fur- 

 ther, it must be at once obvious that the various kinds of loose material all 

 over the northern hemisphere have been accumulated, not only under different 

 conditions, but during long-continued subsequent periods. To the first, or ice, 

 period belong all phenomena connected with the transportation of erratic 

 boulders, polishing, scratching, etc., during which the land stood at a higher 

 level. To the second period belongs the stratified drift such as indicates a 

 depression of the continent. 



In 1856 Dr. Edward Hitchcock came once more to the front, 

 through the medium of the ' Smithsonian Contributions to Knowledge,' 

 with a paper of some 150 royal quarto pages and 12 plates, in which 

 he considered the changes which had taken place in the earth's surface 

 since the close of the Tertiary period. The products of these changes 

 he classed as, first, drift unmodified, and second, drift modified, in- 

 cluding under the latter such deposits as ancient and modern beaches, 

 submarine ridges, sea bottoms, osars, dunes, terraces, deltas and mo- 

 raines. The drift proper he regarded, as before, as a product of sev- 

 eral agencies, including icebergs, glaciers, land slips and waves of 

 translation, which, though more active in the past than now, are still 

 in operation. 



To account for the drift accumulations at various altitudes he con- 

 ceived that the water must have stood some 2,500 feet above its present 

 level and, further, that all the northern part of the continent — at least 

 all east of the Mississippi — had been covered by the ocean since the 

 drift period. 



As to the origin of the material of the irregular coarse deposit 

 beneath the modified beaches and terraces (ground moraine), he agreed 

 essentially with Naumann in supposing that, first, the eroding ma- 

 terials must have been comminuted stone ; second, they must have been 

 borne along under heavy pressure; third, the moving force must have 

 operated slowly and with prodigious energy; and fourth, moving in a 

 nearly uniform direction, though liable to local divergence; fifth, the 

 vehicle of the eroded material could not have been water alone; but, 

 sixth, a firm and heavy mass, somewhat plastic. The exact period of 

 operation of the drift agency he naturally found difficulty in deter- 



