548 



CENOZOIC TIME — MAMMALIAN AGE. 



nessee, so that it may be said to have a continental distribution. 

 It is not yet ascertained whether it is distinguishable by greater 

 elevation from the present river-flats in the valleys of the States 

 bordering on the Gulf of Mexico. 



It generally accompanies the whole course of a stream from its 

 mouth to its source in the mountains. It follows even all the tribu- 

 taries, and fails only where the stream is a steep mountain-torrent 

 or is bounded by lofty walls of rock. A map showing the distribu- 

 tion of the formation will hence be like that of the rivers, with the 



Fig. 830. 



Terraces on the Connecticut River, south of Hanover, N.H. 



same variations of course for each, excepting the minor irregulari- 

 ties and a much greater breadth. 



These valley alluvial beds contain no traces of marine relics, and 

 no evidence whatever of any but a fresh-water origin. 



(b.) Lake-border and other lacustrine formations. — Formations similar 

 to those along river-valleys exist about lakes in the same latitudes. 

 They are often called beaches, or lacustrine formations. Where a river 

 flows' into a lake, the elevated plain of the river-alluvium is gene- 

 rally continuous with that bordering the lake. 



It is not safe to conclude that all the upper lake-border forma- 

 tions belong to the Champlain epoch, any more than that all elevated 

 sea-beaches containing recent shells are of the same. Yet in North 



