150 



lands as late as 1890, but which have now almost entirely disap- 

 peared. These circular ponds were rarely more than 18 inches 

 deep and generally contained water during the entire year. 



The remaining associations are very local, or are very much 

 circumscribed by the encroachments of agricultural operations, 

 but even now in the aggregate covering many square miles of 

 Jo Daviess county. So scattered are they that they show but 

 little influence on the general tone of the forest features, and 

 without reference to the order given above each may be briefly 

 considered. 



While it is true that occasional bur oaks may be found widely 

 scattered, as isolated specimens here and there, and in all 

 probobility squirrel or pigeon sown, it is equally true that 

 genuine associations of this species occur as fringes or limited 

 patches, always denoting good strong soil, well drained, and in 

 many places designated by the term "bur oak openings." The 

 origin of such a term is obscure and I make no attempt to solve 

 it. In general, while the bur oak is the dominant species, this 

 association is not a pure growth, but a number of different 

 trees is almost certain to be found. The oaks are often very 

 large, with wide-spreading branches, and have a trunk diameter 

 of 2 to 4 feet, and height of 80 feet, and in a typical wood- 

 land will stand 50 or 60 feet apart. 



Just what the cause may be for these limited areas devoted 

 to a particular species is not positively determined, but the con- 

 jecture is made that where the underlying lime rocks have 

 become disintegrated sufficiently and are near enough the sur- 

 face for good drainage, and where finally much humus has ac- 

 cumulated, such places seem to be the ideals for bur oak de- 

 velopment. In nearly all of the best examples that are known 

 to the writer, this association occupies the highest horizon of 

 the Galena limestone at the point where it is overlaid with 

 crumbling shales of the Cincinnati formation. .Such a region 

 has an abundant moisture from the innumerable springs that 

 have their origin in the sloping steeps of the latter strata, 

 rising in disintegrated masses to meet the weather worn es- 

 carpments of the Niagara. 



