BIG SPRING PRAIRIE. 45 
seemingly contradictory plant societies. Upon closer 
examination, it was ascertained that, although the sur- 
face level of the muck differed but moderately, the drift 
clay beneath is not nearly so uniform in level. Conse- 
quently, while at some places the clay bed lies one or 
two feet beneath the surface, at others it lies at a depth 
unknown at present. On account of the dip of the rock 
strata of the ridges adjacent to the prairie, the lower 
strata of muck are surcharged with water, which pre- 
vents excavating a hole deep enough to determine depth 
of muck. Along iron pipe or wooden pole might be 
driven into the muck, but it would be impossible to de- 
termine, whether the entire distance of easy penetra- 
tion were muck, quick sand, or a plastic mixture of 
sand and clay. Fora conservative estimate we may 
state that the muck in some portions is at least eight 
or ten feet deep. As there is this difference in the na- 
ture and depth of the soil and sub-soil, and asin the 
neighborhood of the old lake remnant the soil is more 
sandy than elsewhere, and as the plant societies vary 
according to these factors, we shall deal jointly of the 
nature of the soilin the various portions and the charac- 
teristic plant societies thereon. 
The first division of plant societies on Big Spring 
Prairie would be the wooded portions, and the Prairie 
proper. The wooded portions may be classified under 
the following divisions: 
1 The Sand Dune Forest. 
2 The Outcrop Forest. 
3 The Clay Island Forest. 
4 The Clay Bank Forest. 
5 The Burned Area Thicket. 
These divisions will be more fully treated of in the 
division of Forest Encroachment or Introduction upon 
Prairie, the Order and Cause. 
The prairie proper supports various plant societies 
according to its elevation, proximity to ditches, and ef- 
