650 THE AMERICAN NATURALIST.  [Vou. XXXII. 
gradually raised to a height of 1400 feet, and again the slug- 
gish rivers of the Cretaceous times were revivified, cutting 
through the harder strata forming the walls of the longitudinal 
valleys, and, widening into broad estuaries, emptied into the 
Atlantic. y : 
In the Eocene Tertiary, as Willis tells us, “the swelling of 
the Appalachian dome began again. It rose 200 feet in New 
Jersey, 600 feet in Pennsylvania, 1700 feet in southern Virginia, 
and thence southward sloped to the Gulf of Mexico.” In con- 
sequence of the renewed elevation, the streams were revived ; 
and Willis adds: “Once more falling swiftly they have sawed. 
and are sawing, their channels down, and are preparing for the 
development of a future base-level.”’ 1 
We can in imagination see, as the result of these widespread 
physical changes, inducing as they must have done the forma- 
tion of separate basins or areas enclosed by mountain ranges, 
with different climates and zones on land, however uniform 
might have been the general temperature of the world at that 
time and the other physical conditions of the sea,—we can 
imagine the profound and deep-seated influence thus exerted “ 
on the life-forms peopling the uneven surface of the land. 
The vegetation of the lowlands was rich and luxuriant, as 
the Triassic (Newark) coal deposits near Richmond testify, and 
while the uplands and hills were probably clad with dense 
forests of conifers, on the drier desert areas of the peneplain 
the trees may have been more scanty, like the scattered pines 
of the drier elevated region of the southwest, and of the Great 
Basin at the present day. The distribution of the animal life 
must have corresponded ; one assemblage, especially the amphi- 
bians, characterizing the hot and humid lowlands, another the 
cooler uplands, while already perhaps a few forms became 
adapted to the more arid desert areas, as is the case now in 
Australia, which is in a sense a Mesozoic continent. 
Similar subsidences and elevations changed the Jurassic map 
in Eurasia. This continent was already a land mass of great 
extent, and fresh-water lakes extended across Siberia, and in 
China were extensive swamps and submerged lands, now repre- 
1 Quoted from Scott’s Jutrvduction to Geology, p. 342- 
