THE CAUSE OF THE GLACIAL PERIOD. 321 



and Eocky Mountains and to deposit them in snow upon 

 the Laurentian highlands. An ingenious supplement to 

 the theory, therefore, has been brought forward by Pro- 

 fessor Carpenter, who suggests that the immense Tertiary 

 and post-Tertiary lava-flows which cover so much of the 

 area west of the Eocky Mountains were the cause of the 

 accumulations of snow which formed the Laurentide 

 Glacier. This statement, which at first seems so para- 

 doxical as to be absurd, appears less so upon close exami- 

 nation. 



The extent of the outflows of lava west of the Eocky 

 Mountains is almost beyond comprehension. Literally, 

 hundreds of thousands of square miles have been covered 

 by them to a depth in many places of thousands of 

 feet. These volcanic eruptions are mostly of late date, 

 beginning in the middle of the Tertiary and culminating 

 probably about the time of the maximum extent of the 

 Laurentide Glacier. Indeed, so nearly contemporaneous 

 was the growth of the Laurentide Glacier with these out- 

 flows that Professor Alexander "Winch ell had, with a 

 good deal of plausibility, suggested that the outflows of 

 the eruptions of lava were caused by the accumulation of 

 ice over eastern British America. His theory was that 

 the three million cubic miles of ice which is proved to 

 have been abstracted from the ocean and piled up over that 

 area was so serious a disturbance of the equilibrium of 

 the earth's crust that it caused great fissures to be opened 

 along the lines of weakness west of the Eocky Mountains, 

 and pressed the liquid lava out, as the juice is pressed 

 out of an orange in one place by pressing upon the rind 

 in another. 



Professor Carpenter's view is the exact reverse of Pro- 

 fessor Winchell's. Going back to those orographic changes 

 which produced the lava-flows and the elevation of the 

 northern part of British America, he thinks the problem 

 of getting the moisture transferred from the Pacific 



