46 The Americafi Geologist. January, i899 
ification of Croll's theory (6) "This seems to be by far the 
most probable yet presented." (Elements of Geolog}', 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 re- 
ceived any satisfactory explanation. Each one that has been proposed 
is either inadequate or is attended by grave difificulties. It is there- 
fore probable that some factor which is essential for the complete 
solution of the problem is as yet undiscovered, or at any rate, the im- 
portance 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 so- 
lution 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 distin- 
guished co-workers accept in part only, or reject entirely the 
conclusions reached by their fellows. 
The causes which have produced and are yet influencing 
the climatic evolution of our planet are so fundamental and 
far-reaching in their consequences that a failure to explain 
these causes and their mode of action constitutes a serious de- 
fect in those branches of science to which a study of these 
causes appertains. In the present state of physical geography 
and geology, the student is offered a mass of facts interpreted 
along various lines, each interpretation disputed by high au- 
thorities, and finally the results are summed up in the broad 
conclusion that "we must be content to work and wait." 
Scope of the Problem. The problem of explaining this 
succession of climatic variations is so attractive that there is 
perhaps none other to which deeper thought has been direct- 
ed, nor upon which such diverse views are held. In its en- 
tirety it constitutes one of the most far-reaching and grandest 
problems of terrestrial physics. Nor is the scope of this prob- 
>em bounded by its relations to the earth. The principles and 
law^s involved in its solution must be general. 
The development now reached by each one of the planets 
can not be the same, and we find each in that particular phase 
which its mass, environment and exposure have permitted it 
to reach. When we trace the climatic history of the earth 
backward, the line of research must lead into conditions- now 
