132 



tion of soil. A slow change in the water-level, for such it must 

 have been along- these comparatively sluggish streams, would per- 

 mit the long-continued existence of swamps and the consequent ac- 

 cumulation of large deposits of muck. In the Winnebago area, on 

 the other hand, the deposits lie many feet above the Rock, Sugar, 

 and Pecatonica rivers ; swamps would be of short duration and the 

 accumulation of muck would not take place. Consequently, the de- 

 pressions have been occupied almost entirely with the bur oak asso- 

 ciation, with the exception of the single pond already mentioned, 

 of the hydrophytic extreme is, as usual, chiefly due to changes in the 

 water factor, while that of the xerophytic extreme is in this case 



The development of the bur oak association represents, therefore, 

 another case of the interpolation of a mesophytic mean association 

 between a hydrophytic and a xerophytic extreme. The succession 

 caused primarily by the development of humus. The whole probable 

 successional histoiy is indicated upon the diagram ( Fig\ 6.) showing 

 the relation of the various associations. 



The preceding statements concerning the development of the 

 l)ur oak association following the swamp vegetation does not imply 

 that it was the first type of forest to appear. On the other hand, 

 there is some fragmentary evidence that an entirely different forest 

 association preceded it. 



Within the bur oak association (PI. XVI, Fig. i), Qiiercus macro- 

 carpa is everywhere the prevailing tree. In the Amboy area, it was 

 the only arborescent species in the small areas examined. In the 

 Winnebago area it is mixed with white oak, Qucvcus alba, and shell- 

 bark hickory, Carya ovata. The former composes 25-50 per cent, of 

 the whole, while there is seldom over 2 per cent, of hickory. The 

 trees are larger and straighter than those in the black oak association, 

 but still much inferior to those of their own species growing on a more 

 fertile soil. A large number of shrubs form a second layer beneath 

 them, and are often aggregated into dense thickets. Fniuus scrofiiia, 

 Pninits virginiana, and Coryhis americana are by far the most 

 abundant, with several other occasional species. Lianes are not com- 

 mon, and consist of scattered individuals of Vitis vnlpina, Smilax 

 liispida, and Rluis Toxicodendron, with a few other species of less 

 importance. 



The forest cover is dense and the light diffuse. This prevents 

 the growth of most grasses, except where the forest has been partially 

 cleared. The ground cover is composed chiefly of a dense luxuriant 

 growth of herbaceous plants. They are of a larger average size and 

 much more mesophytic appearance than those of the black oak ridges. 



