38 BIG SPRING PRAIRIE. 
4th. The erosion of surface soil in the vicinity of 
ditches. 
The above four causes are direct results of artificial 
drainage. 
This settling of marsh soil after drainage becomes 
of economic importance. For, if there is just enough 
difference in level to secure proper drainage when the 
first ditches are dug, the area in the course of years, is 
bound to revert to the semi-marsh type, if the muck is 
sufficiently thick. 
If, however, the fall in outlet ditch is sufficiently 
great, the ditches must be deepened from time to time, 
until the first two causes have approximately accom- 
plished their work. ‘The third force or cause does not 
manifest itself on all drained marsh areas. But wherever 
it is liable to occur, provision must be made both for 
prevention, if possible; and for staying its destructive 
progress. 
Figure 5 illustrates a method employed on Big 
Spring Prairie to stay the ravages of prairie fires. The 
dry prairie grass was set on fire by sparks from a pass- 
ing engine. The sedgy or grassy hummocks project- 
ing above the general level are quite dry, and hence are 
set on fire by the burning grass. The heat from this 
burning portion expels the moisture from the soil in 
immediate contact with it and raises it above the ignit- 
ing point and the firespreads. Before the real value of 
this marsh land was learned, little effort was made to 
check its progress unless it threatened to encroach upon 
fences or the land adjoining prairie. But, after Mr. 
W. C. Johnson proved the value of this marsh land by 
developing a celery and truck farm upon it, the land 
rose in value from $10 an acre to $75 or even more. 
At present if a prairie fire originates, which threat- 
ens the burning of the soil, a trench is dug around the 
burning area. Ifthe trench is deep enough to reach 
very wet soil, it effectually prevents its further ravages 
