STUDIES FOR STUDENTS. 723 



The true theory of the drift must involve the action of an 

 agent or of agents which, under normal conditions were capable 

 of planing and beveling and striating many stones, especially 

 the softer ones of the unstratified drift, while rounding and leav- 

 ing unstriated most of those of the stratified. But the agents 

 must have been such that under certain circumstances their 

 activities failed, on the one hand, to leave more than a very 

 small percentage of the stones of the unstratified drift beveled 

 and striated, while, on the other hand, they sometimes permitted 

 the stratification of gravels containing many sub-angular, plane- 

 faced, striated stones, varying in size from pebbles to bowlders. 



The Sources of the Drift. 



Direction and distance of transportation. By tracing identifiable 

 constituents of the drift back to their sources, it has been found 

 that the general direction of drift transportation in the United 

 States was from north to south. From this general direction 

 there are many and considerable local deviations, both to the 

 the east and west. Generally speaking, it has been found that 

 the larger part of the drift of any locality has not been trans- 

 ported any great distance, though a small part of it has usually 

 traveled far. Even at the southern margin of the drift, bowl- 

 ders are sometimes found which have come from territory north 

 of our national boundary. Those materials which have come 

 farthest are generally of hard rock. These general statements 

 are not without local and striking exceptions. 



The forces which are responsible for the drift, therefore, 

 must have moved, in .general, from north to south, and must 

 have been capable of local and considerable deviation both to 

 the east and west. They must have been active over an exten- 

 sive area. They must have been forces capable of gathering 

 and carrying materials in such wise that, at the time of their 

 deposition, the larger part had been transported but a few miles, 

 and in such wise that the contributions of various formations to 

 the drift of any locality are, in a rough sort of way, inversely 

 proportional to their distance. The drift forces must have been 



