120 



This fact, together with the pure yellow sand of which the substratum 

 is composed, indicates that it never has been forested. 



The boundary between the forest and prairie differs from the 

 usual forest margin in the absence of a tension zone and a definite 

 vegetation. Thickets of hazel, of sassafras, or of sumach, which 

 surround the typical Illinois forests, are absent. There is no sharp 

 distinction of flora within and without the forest edge, and no mass- 

 ing of a large number of species near the margin. The whole suc- 

 cession is of a type rarely mentioned or described, in which there 

 is at first no essential change in the environment. 



There is no first-hand evidence concerning- the rate at which the 

 extension of the forest is proceeding. The first settlements were 

 usually made near the edge of the forest, where clearing and culti- 

 vation at once stopped any advance. Historical evidence is not al- 

 ways of value, because complete dependence can not be placed on 

 statements of a scientific nature made by travelers or casual observ- 

 ers. A note by Patrick Kennedy (Imlay, ij^l'. 508), however, is 

 suggestive, and probably at least partially correct. 



"About sun-set we passed the river Demi-Quian.* It comes in on 

 the western side of the Illinois river (165 miles from the Missis- 

 sippi) ; is 50 yards wide, and navigable 120 miles. We encamped on 

 the south-eastern side of the Illinois river, opposite to a large 

 savanna, belonging to, and called, the Demi-Quian swamp. The 

 lands on the southeastern side are high and thinly timbered ; but at 

 the place of our encampment are fine meadows, extending- farther 

 than the eye can reach, and affording" a delightful prospect. The low 

 lands on the western side of the Illinois river extend so far back from 

 it, that no high grounds can be seen. Here is plenty of buffalo, deer, 

 elk, turkies, etc." 



Kennedy's whole narrative seems reliable, and we may believe 

 that at least in some directions his camp commanded an uninter- 

 rupted view of the prairie. At the present time, however, the mar- 

 ginal belt of timber along the river in the vicinity of Havana is 

 from 100 yards to a quarter of a mile (100 to 500 m.) wide, while 

 the fringing woods along Quiver creek and large tracts of black 

 oak completely cut off a view of the prairies. If Kennedy's state- 

 ment is correct, then large areas of timber have developed within 

 the last century. 



It is difficult to explain the migration of the oaks. Their normal 

 method is by gravity, which tends to scatter the acorns to a little 

 distance as they fall from the trees. By this method alone the mi- 



* The Spoon river, which empties into the IHinois opposite Havana. 



