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: 



1st. 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. 



