250 Prof. C. H. Hitchcock — Glacial Period in E. America. 



fossiliferous deposit, bringing a load of coarse ferric material, but no 

 ground moraine. The ice soon melted, and thus the upper till was 

 precipitated upon the previously formed floor. In the Champlain 

 and St. Lawrence valleys no occurrence of till overlying the marine 

 beds has yet been observed ; so that the recurring glacial conditions 

 were too insignificant to cause a re-advance of the ice-sheet less 

 than 200 miles to the north of Portland. 



5. All the facts of striation and drift dispersion from the Gulf of 

 St. Lawrence to the Rocky Mountains are best explained by the 

 existence of a central mer de glace in the Labrador peninsula, or 

 somewhat farther west. Torell and Dana have maintained that the 

 starting-point of the glacier was in Greenland ; but we find (a) the 

 glacial movements from the Labrador peninsula have been toivards 

 Greenland instead of away from it; (b) the Greenland ice must 

 have been twenty-two miles high to flow to the Saskatchewan 

 region ; (c) the bridging of Davis Straits could be hardly possible. 



6. The Labrador plateau is lower than the New England hills. 

 To overcome this difficulty Professor Dana supposed there had been 

 an elevation of the northern Laurentian watershed, sufficiently 

 gigantic to allow of a south-east sliding of the ice. There is no 

 evidence of such elevation of the land beyond the requirements of 

 the theory. It will be easier to accept Dr. Croll's view that the ice- 

 sheet itself became miles in thickness, by the gradual retention of 

 frozen moisture, and moved then as readily as if a less amount 

 capped high mountains. It fii'st moved south-westerly as far as 

 Manitoba, Dakota, and Missouri ; then when the St. Lawrence Valley 

 became filled to the brim, the surplusage moved towards the Atlantic 

 in a south-east direction. Either of these theories is extreme ; but 

 there seems to be no escape from accepting one of the two horns of 

 the dilemma. 



7. The south edge of the American ice-sheet is well marked by 

 the presence of enormous terminal moraines, coupled with extensive 

 sand and gravel plains washed from the glacial debris by the waters 

 derived from the melting ice. The eastern extremity is at Cape 

 Cod, Massachusetts, and Long Island, New York. These long drift 

 hills, consisting largely of gravel and sand, are isolated from all present 

 appearance of connexion with any possible higher land from whence 

 the depositing waters could have flowed. By supposing that they 

 formed the front of the glacier just before its decadence, it is easy to 

 understand both how an immense supply of drift material was 

 furnished, and the origin of the freshwater streams which formed 

 plains and hummocks of stratified sand far above the present water- 

 level. A study of the writings of all the State geological reports 

 enables us to discover more than one continuous moraine line, from 

 Massachusetts to the Eed Eiver country west of Manitoba, save in a 

 portion of Pennsylvania, which needs further study. 



