Very early in the history of our science it became evident that the earth had passed through great climatic changes, and the effort to find an explanation of these changes which should be adequate and satisfactory has never ceased till this day. There has of late been a revival of interest in this problem, and many new works on the subject have appeared in this country, as well as in England and Germany.
One great obstacle in the way of finding convincing explanations for past climatic changes was the fact that, since weather records have been kept, no definite changes of climate could be detected, though it was admitted that those records covered too short a period of observation to be at all decisive. Some historians, notably. Gibbon, in his famous Decline and Fall of the Roman Empire, have attempted to prove the reality of climatic changes within . . .