GEOLOGICAL HISTORY OF THE LA URENTIAN BASIN. 405 



the occurrence of living marine species in the present lakes is 

 found to be widely at variance with the history of the basin as 

 determined from physical evidence, an inquiry should be made 

 in reference to the manner in which the species discovered might 

 migrate. 



7. Changes i?L elevation. One of the most difficult problems 

 in connection with .the history of an inland region is the deter- 

 mination of changes of level. By leveling along an ancient 

 beach, post- lacustral changes in the relative elevations of various 

 points may be readily ascertained. Pre -lacustral changes, how- 

 ever, by which ancient valleys have been obstructed, are much 

 more difficult of direct observation, but might appear from the 

 study of the hard -rock topography, as has already been sug- 

 gested. This branch of the investigation, however, should more 

 properly begin at the coast and be extended inland. 



8. Former outlets. Several localities where the waters of the 

 Laurentian basin have overflowed during former high -water 

 stages have been pointed out, but some confusion has arisen in 

 this connection, for the reason that the channels formed by streams 

 issuing from the margin of the ice during the closing stages of 

 the Glacial epoch have, in some instances, been mistaken for evi- 

 dence of former lake outlets. The old outlets which seem to 

 have been well determined are situated at different levels, and 

 show that the entire basin could not have been occupied by a 

 single great water -body, unless, as has been supposed by some, 

 it was in direct communication with the sea. This hypothesis 

 will be considered below. It has sometimes been assumed that 

 all of the basin below the level of some ancient outlet was once 

 flooded, so as to form a great lake in all of the basin now situated 

 at a lower level ; but, in making suth generalizations, the possi 

 bility of places in the rim of the basin being at a lower level than 

 the outlet discovered, thus necessitating a special explanation, 

 such as the partial occupation of the basin by glacial ice, or 

 changes in elevation of such a character as to raise the locality 

 of former overflow or to depress other regions, have to be con 

 sidered. 



