658 Plain, Prairie, and Forest. [ November, 
not a single pebble can be found ; children are born and grow up 
without ever having seen a fragment of stone, a bowlder, or 
even a pebble large enough to throw at a dog. 
If, then, this extreme fineness of the soil is the cause of the 
absence of forest growth, we ought. to be able to explain, when 
looking at the facts from this point of view, that which from any 
other theoretical stand-point has seemed entirely inexplicable. 
The apparently eccentric distribution of the timbered tracts 
within the prairie, and of forest-covered patches in the midst of 
great treeless regions, — these conditions, which are evidently so 
little connected with absence or presence of moisture, and which 
seem so obscure, how clear they become when we examine the 
soil itself, instead of interrogating the skies and the rain-tables ! 
ow, then, are the wooded tracts distributed in the prairie 
region? An examination of the maps before us, on which the 
prairies of Illinois, Wisconsin, Iowa, and Minnesota are desig- 
nated, from materials collected at the General Land Office, shows 
clearly that, as a general rule, it is the higher portion of the 
country which is destitute of timber. All are probably some- 
what familiar with the terms commonly in use at the West, 
“ riyer-bottom,” “ bluff,” and “ prairie upland.” * All are aware 
that the prairie country has, in general, a moderately undulating 
surface, and that the streams, which are very numerous, have sun 
their beds to a depth of from a few feet up to two hundred or 
three hundred below the general level; that these valleys are 
often very wide in comparison with the size of the streams whic! 
meander through them, and that the ascent on to the uplands 1s 
not a gradual one, but usually rather steep, such steep ascents 
being universally known as “ bluffs.” These bluffs often, €s- 
pecially in Wisconsin, Northern Iowa, and Northern Illinois, €x- 
hibit outcropping edges of rocks, forming low, nearly perpendicu- 
lar ledges, the geological formations lying almost everywhere in 
the prairie region in a nearly horizontal position, and consisting 
of nearly, if not quite, unaltered limestones, shales, and argilla- 
ceous sandstones. 
As a general rule, the timbered tracts are found in one of two 
positions: either they stretch along the bluffs which border the 
river valleys, or they occupy patches, called groves, high up P 
the uplands, at a level of a few feet — rarely as muc 
dred — above the surrounding prairies. The river bottoms them- 
1 For a careful description of the surface in the prairie region, by the present 
writer, see Hall and Whitney’s Geology of Iowa, 1858, vol. i., chapter l. 
