No. 1.] DAWSON — POST-PLIOCENfi. 31 



It is further to be observed that oscillations of land must be 

 taken into account in explaining these phenomena. Elevations 

 increasing the height and area of land might increase the space 

 occupied by snow and land ice. Depressions, on the other hand, 

 would brino; laro-er areas under the influence of water-borne ice 

 and marine deposits, and these might take place either in a 

 shallow sea loaded with field and coast ice, or in deeper water in 

 which lar^e icebergs miaht float or eround. There is reason to 

 believe that such alternations were not infrequent in the Post- 

 pliocene, and that their occurrence will explain many of the com- 

 plexities of these deposits. 



If we adopt the iceberg hypothesis, we must be prepared to 

 consider in connection with this subject a subsidence so great as 

 to place the Laurentides and all but the highest summits of the 

 Appalachians under water. In this case a vast volume of Arctic 

 ice and water would pour over the country of the great lakes to 

 the S.W., while any obstruction occurring to the south would 

 throw lateral currents over the Appalachians to the eastward. If 

 we adopt the glacier hypothesis, we may on the other hand 

 imao;ine a o-reat movement of land ice to the S.W., westward of 

 the Appalachians, and a separate outward movement eastwiird 

 from these hills and down the Atlantic slope of America. On 

 either hypothesis there are difficulties in accounting for some sets 

 of striae, but on that last-mentioned I believe them to be insuper- 

 able. 



It is evident from the descriptions of Smith, Geikie, Jameson, 

 Crosskey, and others, that the Boulder-clay of Scotland and Scan- 

 dinavia corresponds precisely in character with that of Canada, 

 and there, as in America, the theory of a continental glacier has 

 been resorted to for its explanation. The objections to this hpyo- 

 thesis are very ably stated by Mr. Milne Home in a paper on the 

 " Boulder-clay of Europe," in the Transactions of the Royal 

 Society of Edinburgh, 1869. 



To this period and these causes must also be assigned the ex- 

 cavation of the basins of the great American lakes. These 

 have been cut out of the softer members of the Silurian and De- 

 vonian Formations; but the mode of this excavation has been 

 regarded as very mysterious ; and like other mysteries has been 

 referred to glaciers. Its real cause was obviously the flowing of 

 cold currents over the American land during its submergence. 

 The lake basins are thus of the same nature with the deep hollows 



