Geology, 3rd Ed., page 381. Shaler and Davis, Glaciers, 

 page 70.) 



After an exhaustive study and review of all the prin- 

 cipal theories, Dr. James Geikie says: 



"The primary cause of these remarkable changes is 

 an extremely perplexing question, and it must be con- 

 fessed that a complete solution of the problem has not 

 been found. CrolFs Theory has undoubtedly thrown a 

 flood of light upon our difficulties and it may be that 

 some modifications of his views will eventually clear up 

 the mystery. But for the present, we must be content 

 to work and wait." (The Great Ice Age, 3rd Edition, 

 1894, page 816.) 



After reviewing the principal theories as to the cause 

 of the Ice Age, Dr. Jos. LeConte remarks of Dr. Wal- 

 lace's modification of CrolFs Theory (() " This seems to 

 be by far the most probable yet presented." (Elements 

 of Geology, 2nd Edition, page 578.) 



After a similar review, Dr. T. G. Bonney says: 



" It follows from what has been said above that the 

 low temperature which undoubtedly prevailed during 

 the Glacial Epoch has not yet received any satisfactory 

 explanation. Each one that has been proposed is either 

 inadequate or is attended by grave difficulties. It is 

 therefore probable that some factor which is essential for 

 the complete solution of the problem is as yet undis- 

 covered, or at any rate, the importance of one which is 

 already known has not been duly recognized." (Ice 

 Work, Present and Past, p. 260.) 



It is generally admitted, therefore, that no satisfactory 

 solution is offered for the problem before us, and that the 

 most distinguished scientists who have investigated the 

 subject have no considerable number of followers, that 

 equally distinguished co-workers accept in part only, 



op- -»■.,_ ' 



