254 ILLINOIS ACADEMY OF SCIENCE 



mining the forested area. Generally these trees are lim- 

 ited to a single row and are often imported species such as 

 Norway spruce and Lombardy poplar. 



In general the strip of forest fringing the streams hav- 

 ing a north and south course is decidedly more extensive 

 upon the east than upon the west side. The explanation 

 commonly current for this inequality of distribution is 

 connected with the theory that the prairies are caused by 

 the prevalence of fires which destroy the trees. Streams 

 naturally act as barriers to the advance of prairie fires, 

 this advance usually being in the direction of the prevail- 

 ing wind from west to east. This would lead to a greater 

 destruction of forest trees upon the west bank of north and 

 south streams. This explanation might have been accepted 

 as a probable one had not the soil maps shown the close 

 coincidence of the distribution of the yellow-grey silt 

 loam (1134) and that of the upland oak forest. And since 

 an examination of the soil maps reveals the fact that the 

 expanse of the yellow-grey silt loam (1134) fringing the 

 Little Vermilion, Big Indian Creek, Little Indian Creek 

 and other streams is decidedly greater on the east than 

 upon the west bank we are led to conclude that, in this 

 county at least, soil distribution and not fire is the con- 

 trolling factor of forest distribution. Upon a subsequent 

 page an attempt will be made to explain the irregular 

 distribution of the soil in question. 



Mendota Township ( Map 1 ) . 



An examination of the maps of the soil reports already 

 mentioned shows that the entire surface of this township 

 is covered with prairie soils. Over 95 per cent of the area 

 is the brown silt loam (1126), the remainder being black 

 silt loam (1125). The altitude of the northern portion of 

 the township is slightly over 900 feet, sloping gently to 

 720 feet at the southeast. The surface is very gently roll- 

 ing and several branches of the Little Vermilion have 

 slightly entrenched themselves ( Map I ) , forming channels 

 that are without water during part of the summer. 

 Only along these streams is there the slightest indication 

 of any natural tree growth and this consists of a very 

 scanty and much interrupted fringe of black willow which 

 in one locality only is more than a single row of trees, in- 



