CHICAGO AND VICINITT. 39 



Sisyrinchium angustifolium (Blue-eyed grass), Hypoxys erecta (Yellow- 

 eyed grass), Xyris flexuosa, Triglochin maritima. The shrubs in such 

 places are chiefly Salix glaucophylla (Glaucous willow), Cornvs stolo- 

 nifera (Osier dogwood), Potentilla fruticosa (Shrubby cinquefoil), Hyper- 

 icum Kalmianum. The conditions that determine this type of bog, as 

 contrasted with the Cassandra type, are not clear. The soil is hard, 

 compact, shallow, and usually sandy; it may be that this type develops 

 in shallow depressions, while the- type with spongy, quaking ground 

 develops in deeper depressions. This second type much more closely 

 resembles the half-drained swamps in its flora than does the Cassandra 

 type, although so far as drainage is concerned it agrees with the Cas- 

 sandra bogs. 



There is yet a third type of swamp which still more closely resem- 

 bles the half-drained swamp in its flora. It is found along the edge of 

 the Calumet valley near Dune Park, also at West Pullman. In this 

 case the soil is rather deep and rich, in which respects there is agree- 

 ment with the first type rather than the second. Grasses and sedges, 

 but of a more luxuriant type, dominate here also, and with them are 

 found such plants as Cephalanthus occidentalis (Button bush), Aspidium 

 Thelypteris, Onoclea sensibilis (Sensitive fern), Saxifraga Pennsylvanica, 

 Caltha palustris (Marsh marigold), Viola blanda, Polygala sanguinea. 

 Sphagnum occasionally occurs here, as it never does in the second 

 type. Here again there is doubt as to the determining conditions, but 

 it may be that things can be explained by the difference in the drain- 

 age. The ultimate fate of the second and third swamp types is not 

 known. The relative absence of trees and shrubs is certainly natural 

 and in no wise due to man. Possibly local prairies will be the final 

 type, or it may be that the forest will come in. Fig. 21, which shows 

 pines encroaching upon the grassy areas, favors the latter view. So do 

 some of" the facts seen in the Calumet valley. 



All of the peat bog types have a characteristic marginal flora, i. e., 

 the vegetation at the margin of the original lake is essentially alike in 

 all cases. These plants, as well as those of Cassandra bogs, are the 

 same over wide areas. The most common members of the bog margin 

 flora are the sour gum {^Nyssa sylvatica,) the aspen (Populus iremuloi- 

 des), Ilex verticillata, Pyrus arbutifolia (including var. melanocarpd). 

 Spiraea salicifolia and S. tomentosa, Rubus hispidus, Gaultheria procum- 

 bens (Wintergreen), Osmunda cinnamomea, O. Claytoniana, O. regalis, 

 Betuia papyri/era (Paper birch), and Polytrichum commune. This vege- 



