SURFACE GEOLOGY. 201 
dian side, now filled up, into Lake Ontario, and thence found 
its way to the sea either by the St. Lawrence or by the Mo- 
hawk and Hudson. Another glacier oceupied the bed of 
Lake Michigan, having an outlet southward through a chan- 
nel—now concealed by the heavy beds of drift which occupy 
the surface about the south end of the lake — passing near 
Bloomington, Illinois, and by some route yet unknown 
reaching the trough of the Mississippi, which was then much 
deeper than at present. 
3D. — At this period the continent must have been several 
hundred feet higher than now, as is proved by the deeply ex- 
cavated channels of the Columbia, Golden Gate, Mississippi, 
Hudson, etc., which could never have been cut by the 
Streams that now occupy them, unless flowing with greater 
rapidity and at a lower level than they now do. 
The depth of the trough of the Hudson is not known, but 
it is plainly a channel of erosion, now submerged and be- 
come an arm of the sea. As has been before stated this 
channel is marked on the sea-bottom for a long distance from 
the coast and far beyond a point where the present river 
could exert any erosive action, and hence it is a record of a 
period when the Atlantie coast was several hundred feet 
higher than now. 
The lower Mississippi bears unmistakable evidence of be- 
ing—if one may be permitted the paradox—a half-drowned 
river; that is, its old channel is deeply submerged and silted 
up, so that the “father of waters,” lifted above the walls that 
formerly restrained him, now wanders, lawless and ungov- 
ernable, whither he will in the broad valley. 
The thickness of the delta deposits at New Orleans is va- 
riously reported from fifteen hundred feet upwards, the dis- 
crepancies being due to the difficulty of distinguishing the 
alluvial elays from those of the underlying Cretaceous and 
Tertiary formations. It is certain, however, that the bottom 
of the ancient channel of the Mississippi has never been 
reached between New Orleans and Cairo; the instances cited 
AMER. NATURALIST, VOL. IV. 26 
