66 BIG SPRING PRAIRIE. 
theories which have been proposed in regard to either 
are made so general that there are nearly or fully as 
many exceptions to the rule as there are conformities 
to it. In the course of time, the forest formation is 
certain to encroach upon the eastern type of prairie or 
natural meadow, which, in general, can be more cor- 
rectly designated as a semi-marsh. The problem be- 
fore us is to determine the order, cause, and rate of 
encroachment. 
The factors which determine order of tree en- 
croachment upon natural meadow are relative elevation 
of the various portions, drainage, nature of the soil and 
the occurrence of suitable bare spots for germination of 
the seeds. 
Drainage may be either natural or atificial. The 
natural drainage of wet meadows is secured either by 
the gradual elevation of one portion above another by 
the more rapid accumulations of plant remains on the 
former than on the latter, or by the erosion of some ad- 
jacent portion. The proper drainage of a portion of 
prairie remote from an artificial ditch may be secured 
indirectly by the settling of the soil in the space inter- 
vening between the ditch and the remote area. The 
suitable places for germination and growth of tree 
seedlings are furnished by sandy beaches, established 
sand dunes, or exposures of bare soil, caused by either 
prairie fires or artificial drainage. 
On Big Spring Prairie the following order of en- 
croachment has been observed: 
Ist. On the limestone or outcrop island indicated 
on map I. 
2nd. On the sand beaches and sand dunes of the 
old lake. 
3rd. On the clay banks along margins and on the 
clay islands of the old lake. 
4th. On the bare soil thrown out of drainage 
ditches, or on the exposed slopes. 
