Review of Hall and Whitney’s Geology of Iowa. 105 
€ prairies, those wonderful treeless and fertile plains, 
the most marked feature of the Northwest, are described and 
dominating cause of this peculiar condition of the vegetation, 
and some facts are stated which go 
red; “that the whole region now occupied by the prairies of the 
Northwest was once an immense e, in whose basin sediment of 
aided perhaps by a more rapid rise of th 
ficent velocity to wear down through the finer material on the 
surface, wash away a portion of it altogether, and mix the rest 
80 effectually with the underlying drift materials, or with abra- 
tion, and containing a larger proportion of coarse materials than 
that of the uplands, seems to have been adapted to the growth 
of forest vegetation; and in consequence of this, we find such 
localities covered with an abundant growth of timber.” 
_ Wherever there has been a variation from the usual condi- 
tons of soil, on the prairie or in the river-bottom, there is a cor- 
the prairies we sometimes meet with ridges of coarse material, 
*pparently deposits of drift, on which from some local cause, 
ere has never been an accumulation of fine sediment: in such 
localities We invariably find a growth of timber. This is the 
mal 
SECO 
- 
