PHYSICAL AND CLIMATAL CONDITIONS. 85 



By many writers on this subject it is apparently main- 

 tained that in North America a continental glacier 

 extended in temperate latitudes from sea to sea, and this 

 glacier must, in many places at least, have exceeded a 

 mile in thickness. Independently of the physical diffi- 

 culties attending the movement of such a mass without 

 any adequate slope, it is obvious, from the considerations 

 above stated, that the amount of snow necessary to the 

 production of such a glacier could not possibly be obtained. 

 With a depression such as we know to have existed, 

 admitting the arctic currents along the St. Lawrence 

 valley, through gaps in the Laurentian watershed, and 

 down the great plains between the Laurentian areas and 

 the Eocky mountains, we can easily understand the 

 covering of the hills of eastern Canada and New England 

 with ice and snow, and a similar covering of the moun- 

 tains of the west coast ; more especially when we take 

 into account the probability of an elevation of the 

 mountains along with the depression of the plains, and of 

 the southern part of the continent not having been 

 depressed, and so blocking the exit of the ice to the south, 

 along with the escape of the equatorial current through 

 the isthmus of Panama, then submerged. The sea also in 

 this case might be ice-laden and boulder-bearing as far 

 south as 40, while there might still be low islands far to 

 the north, on which vegetation and animals continued to 

 exist. We should thus have the conditions necessary to 

 explain all the anomalies of the glacial deposits. 



Whatever difficulties may attend such a supposition, 

 they are small compared with those attendant on the 

 belief of a continental glacier, moving without the aid of 

 gravity, and depending for its material on the precipitation 

 taking place on the interior plains of a great continent. 



