RELATIONSHIPS OF METASPERMAE. 607 



variety of conditions. It becomes, therefore, both a condition 

 and, to some extent, a cause of the diversity of plant-popula- 

 tion. From the bare gneissic rocks of the Granite Falls 

 district one may find in the valley all sorts of variations to the 

 peat-bog soil of the Mankato and Kasota districts. Here sands 

 or gravels in various proportions, there clays of different 

 compositions or mixtures of sands and clays give a great diver- 

 sity to the soil-surface of the basin. As, through water agency, 

 the soil becomes more and more analysed and segregated as 

 one leaves the general prairie level in passing down some 

 channel, so in the main gorge at different places where different 

 kinds of selective action have progressed and where the cumu- 

 lative selective action of tributary streams is felt, may be 

 found the maximum of variety in passing from one area to 

 another. It is possibly due in part to this relative homogeneity 

 of the prairie soils and relative heterogeneity of the bottom- 

 land soils that the prairie itself supports relatively a 

 more homogeneous plant-population than the bottom- 

 lands. The grasses, composites, pulses, polygalas, phlox and 

 sedges of the rolling prairie constitute after all a rather small 

 percentage of the total population of the valley. To contribute 

 to this result not only past glacial actions, but present, con- 

 tinuous activities of rain, wind and temperature have played 

 their part. It is, however, not incorrect to attribute, in part, 

 the difference between the upland and lowland characters, to 

 the glacial invasion. 



The topography of the valley is evidently in great part due 

 to the subsequent action of the various forces of nature upon 

 the general mass of till which was deposited in a sheet averag- 

 ing more than two hundred feet in thickness over the older 

 surfaces of the basin. The original aspect of this drift -sheet 

 was doubtless somewhat undulating and under the weathering 

 and erosion of the last 7,800 years it has come to present its 

 modern aspect. The hills have become rounded, the streams 

 have cut their gorges and deposited their silt in the form of 

 alluvium, the lakes, formed by the disturbance of the old 

 drainage, have sometimes persisted, with, however, reductions 

 of original size in varying degree, and have sometimes disap- 

 peared through the cutting and draining action of their ontlets 

 or through the silting up of their inlets. Thus many level 

 meadows have been formed and the production of such 

 meadows from older lakes may be seen going on to-day. In 

 this way, habitats are provided for aquatic plant-immigrants, 



