206 
TERTIARY SYSTEM. 
The cause which operated so extensively, and which resulted in the total extinction of 
these vigorous races, is only to be conjectured. We have no data on which to found a 
rational hypothesis concerning it. Whatever it may have been, it was one affecting the 
same races over an immensely extended territory; one which operated over the whole of 
the northern part of this continent, as well as in that of Europe. 
