THE SUCCESSION OF DEPOSITS. 



69 



preceded the re-elevation of the Saxicava sand,* and it is 

 even possible that some may have been placed in their 

 present positions in the post-glacial subsidence, of which 

 there is evidence on both sides of the Atlantic. Some 

 belong to lake margins of post-glacial date. Thus no 

 general statement can safely be made respecting these 

 erratics, but each group or belt must be studied with 

 reference to its local associations and the source of the 

 material, as well as with reference to the probable stage 

 in the various continental subsidences and elevations to 



Fig. 3. Terraces at L' Arise a Loup, near Tadousac. Lower terrace, clay ; upper, sand. 



which it belongs. The assumption that all boulder-drift 

 may belong to one period is a fertile source of error, and 

 though many important observations on the subject have 

 been made by Spencer, Dr. Gr. M. Dawson, Chalmers and 

 others, there is an almost unlimited field for detailed work 

 in this direction. 



A still farther complication arises here from the pro- 

 bability of differential elevation, whereby the relative 

 levels have been changed in different parts of the Pleisto- 



* McGee refers to this in connection with the 

 of the Appalachians. 



Columbia " deposits 



