J. D. Dana on the Origin of Prairies. 303 
do not destroy the grass, while they do the seedlings of the 
trees; and when, under such conditions, the old trees die, they 
die without successors. The encroachment is the slow work of 
has its chance for encroachment. 
4. If moistness, then, is especialiy favorable to the growth of for- 
ests, a change in the moistness of a region occasioned by geological 
events would be attended by a change in the adaptedness to such 
growth.—The Champlain epoch of the Post-tertiary, when por- 
tions of the continent over the higher latitudes were much de- 
pressed, (in many parts 300 to 400 feet), and the more southern 
auch less so, and when the great upper terrace flats of our lakes 
and rivers, often many miles in width, were made, was a time 
of warmer climate over the continents than the present, as the 
distribution of the terrestrial animal life of the era proves." 
It was, also, as the same terraces and the raised beaches prove, 
an era of wider expanse of lakes and rivers over the land. It 
nently favorable for the gr 
would have taken possession of the 
up toward its present level, causing a change of climat 
of greater coolness and dryness, drainiig extensive regions that 
had been under water, and drying moist areas. Consequently, 
there would have been, from the beginning of this change, a 
tendency to a narrowing of the forest regions ; and, with such a 
tendency, an actual narrowing would, in one region or another, 
a . 
pas Id have differed hy- 
in the same way as 
t, on this side of the 
m eS 
ually stretched their bare surfaces : 
* See the author's Manual of Geology, pages 547 to 667. : = ; 
